GHXHJVJGJ
GHXHJVJGJ
GHXHJVJGJ
A. FAHN
Professor of Botany
The Hebrew University, Jerusalem, Israel
Translated from the Hebrew by SYBIL BROmO·ALTMAN
Mature Tissues
4 PARENCHYMA ............................. 73
5 COLLENCHYMA ....... .
..................... 80
6 SCLERENCHYMA ..........•....•..............
85
7 XYLEM ••.•.••.• ,.,.. . • • . • . . . . . •. . . . . • . . •. . •. . . . .• 102
I I .
8 PHLOEM •.•••••••....••••••.••.•.•.•••.•...•.••. 118
!
9 LATICIFERS .•...•..•.....•.•.....••.•.••..••••. 130
10 EPIDERMIS...................................... 137
I •
Copyright © J 967
A. FAHN
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PREFACE
Reproductive Organs
19
I
THE FLOWER . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . • . 360
20 THE FRUIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439
SUBJECT INDEX
viii Preface
PLANTS that bear seeds are termed spermatophytes. These plants pro-
duce spores (newly formed embryo sacs and pollen grains) and therefore
they are sporophytes. These plants develop from a zygote which results
'v
CotyJeEion
-Hypocotyl
-Root
2
from the fertilization of an egg cell by a male gamete. At the start the
zygote divides into two cells which themselves undergo further divisions
to form the embryo. The embryo usually consists of radicle, hypocotyl,
cotyledons and olumule (Pi!!. L no n TJ,p. p".,h .." ...... ~~ 0 1_..J
............ " ____
2 Plant Anatomy
and dormant within the seed which develops from the entire ovule. IUs
not always clear whether the embryo possesses a radicle ~roper or whether
it merely has a root apical meristem. It is difficult to make a definite dis-
tinction between the radicle and hypocotyl, and therefore the axis of the
embryo is called the hypocotyl-root axis.
With the germination of the seed the embryo renews its growth. The
radicle grows and penetrates deeper into the soil. In some species the
hypocotyl elongates and so raises the cotyledons above soil level where
they become green (epigeal germination). In other species the hypocotyl
does not elongate, .or it elongates only very slightly, and the cotyledons
remain below soil level where they eventually rot (hypogeal germination).
The plumule, which is situated above the junction of the cotyledons to the
hypocotyl, elongates and gives rise to the stem and leaves (Fig. 1, nos.
2, 3) ..
That part of the stem to which the leaf is attached is ter~ed the node and
that part of the stem between two nodes, the internode. The number of
nodes and internodes ihcreases with the continued growth of the stem.
At the start of germination all the cells of the embryo divide, but later
cell division is restricted to certain areas of the seedling-usually in the
apices of the axis. /' '
The morphology of the various organs of the spermatophytes is extrem-
ely varied. The nature of the different. organs, such as the stem, leaf, root,
flower and fruit, and the differences in their external and internal structure
have been variously interpreted (De Bary, 1877; Strasburger, 1891, 1923;
Haberlandt, 1918; Goebel, 1928-33; Troll, 1935, 1937,1938, 1939, 195+-
57; Eames and MacDaniels, 1947; Foster, 1950; McLean and Ivimey-
'Cook, 1951, 1956; Esau, 1953; Eames, 1961). The stem (which bears the
leaves) together with the leaves forms a single ont9genetic and, apparent-
ly, also evolutionary unit, and so these organs together are termed the shoot.
The shoot and root, together with their branches, form .an organic con-
tinuation of the embryo as their development results form the activity of
the apical meristems, which are tissues directly descended from those of
the embryo.
In those spermatophytes in which the apical meristems of the main
shoot remain active throughout the life of the. plant, the shoots developing
from the axillary buds remain secondary and the extent of their gro,wth is
regulated by the apex of the main shoot. Such branching of the stem is
termed monopodia! (Fig. 2, no. 1). The main axis and the successive axial
branches do not always have the ability to grow indefinitely. In many
plants the shoot apex becomes reproductive or aborts, and then further
growth is carried out by lateral buds. Such branching is termed sympodial
(Fig. 2, no. 2).
In many cases buds and roots may develop from portions of the plant
distant from the apical meristems; such organs are termed adventitious
General Structure of Higher Plants 3
organs. Examples of such organs are the fibrous roots which are common
among the monocotyledons and develop from the hypocotyl or from the
basal internodes of the stem,· Adventitious roots sometimes develop from
aerial portions or Trom old roots of woody plants. Adventitious shoots
are known. to develop on roots and on stems from·places in which no dor-
mant buds are found. The apices of the adventitious shoots and roots
contain the same meristematic tissues as the apical meristems of the ordi-
;;ary organs of the primary axis.
As the cells formed by the meristem become more distant from the
apex, they undergo gradual differentiation (Fig. 3). Near the apex of the
shoot and root three meristems of different tissues become observable:
(1) protoderm, from which the epidermis, the protective tissue, develops;
(2) procambium, from which the primary vascular tissues (primary xylem
which serves mainly to transport water and the primary phloem which
serves to transport metabolites) develop; (3) ground meristem, from which
tissues of the cortex and pith develop. These comprise parenchyma, the
basic tissue of the plant, sclerenchyma and colienchyma, the supporting
tissues of the plant.
The cells of the procambium gradually differentiate into phloem and
xylem elements, and so these elements become more numerous as seen in
consecutive cross-sections of the stem made at levels further away from
the ::Inpv Th ... _1..1 ___ _ 1 _____ "-_ '-
4 Plant Anatomy
Primary phloem
- lateral root
- Root cap
References
BOUREAU, E. 1954, 1956, 1957. Anatomie Vegetate, VQIs. 1, 2, 3, Presses Univer-
sitaires de France, Paris.
CARLQUIST, S., 1962. Comparative Plant Anatomy, Holt, Reinhart and Winston, New
York.
De BARY, A. 1877. Vergleichende Anatomie der Vegetationsorgane. W. Engelmann,
Leipzig.
EAMF<:' 4 T 10£1 .,,----, ,
Plant Anatomy
EAMES, A. 1. and MACDANIELS, L. H. 1947. An Introduction to Plant Anatomy. '2nd ed.
McGraw~Hill. New York-London. t
ESAU, K. 1953. Plant Anatomy. John Wiley, New York.
FOSTER, A. S. 1950. Practical Plant Anatomy. D. Van Nostrand, New York-London.,
GOEBEL, K. 1928-33. Organographie der Pflanzen, Vols. 1-3.' G. Fischer. Jena.
HABERLANDT, G.1918.Physi%gischePjlanzenanatomie. 5thed. W. Engelmann, Leipzig.
McLEAN, R. C. and IVIMEY-Com" W. R. 1~51. 1956. Textbook o/Theqretical Botany,
1 and 2. Longmans. London.
PALLADIN, W. I. 1914. Pjianzenanatomie. B. G. Teubner, Leipzig and Berlin.
StRASBuRGER, E. 1891. Ober den Bau und die Verrichtungen der Leitungshahnen in
den PJlanzen. Histo!ogische Dei/riige. Vol. 3. G. Fischer, Jena.
STltASBURGER, E. 1923. -Das botanische Praklikum. 7th ed. G. Fischer, Jena.
TROLL, W. 1935, 1937. 1938, 1939. Verglelchende Morph%gle der Mheren Pflanzen.
Gebr. Borntraeger, Berlin.
TROLL, W. 1948. Allgemeine Botanjk. F. Enke, Stuttgart.
TROLL, W. 1954-7. Praktische £infiihrung in die Pf/anzenmorphologie. Vols. 1, 2. G.
Fischer, Jena.
CHAPTER 2
THE CELL
THE basic. units of which organisms are constructed are the cells. The term
cellula was first used by Robert Hooke in 1665 .. Hooke gave this term to
the small cavities surrounded by walls that he saw in cork; later he ob-
served cells in other plant tissues and saw that they contained "juice"
(Matzke, 1943).
Still later the protoplasm-the substance within the cell-was discov-
ered." In 1880 Hanstein coined the term protoplast to indicate the unit of
protoplasm found in a single cell. He also suggested that the term proto-
plast should be used instead of the term cell, but his suggestion is not gener-
ally,accepted and cell is the accepted term. In plants the term cell includes
'the protoplast together with the wall.
The cell wall. was, for a long time, regarded as a non-living excretion of
the living cell matter, but recently more and more evidence has been found
that organic unity exists bhween the protoplast and the wall, especially in
young cells, and that the two together form a single biological unit.
In 1831 Robert Brown discovered the nucleus in an epidermal cell of an
.s-."cJ.h.ld 0\ttft. lcr 18# H(J)sa H7Jl' l~l(h'hl db'tiago.·l~hed (;e{~\.X'U' tire f'l'(lt<J-
plasm and the cell sap, and in 1862 Kiilliker introduced the term cyto-
plasm. From the end of the nineteenth century and during the twentieth
century research on the cell has developed so rapidly and with such enorm-
ous strides that cytology has become a science of its own.
It is customary to divide the protoplast constituents into two groups: (a)
protoplasmic components and (b) non-protoplasmic components.
To the first group belongs the cytoplasm, the "living" protoplasmic sub-
stance of the cell in which the specialized protoplasmic organelles, such as
the nucleus and plastids, are located (Fig. 4, no. 6). The nucleus carries the
information of heredity and so is of paramount importance to all the pro-
cesses in the cell. The plastids usually contain pigments, but sometimes they
are devoid of pigments and then they may store starch granules, lipid
droplets and protein crystals. Other protoplasmic organelles are the mito-
chondria, which are minute bodies concerned in the respiratory processes
and the ribosomes, still smaller organelles, which are the sites of protein
synthesis.
To the second group belong the vacuoles, which are nOllofotonl::1o;;:mi,..
inclu~i(\n<:l (,.. ... ~ .. _.J ~ ~. •
8 . Plant Anatomy
include reserve materials such as starch grains, oil droplets, and aleurone
grains, and other products of metabolism such as various, crystals.
Usually the cell contains a single nucleus, but in some lower plants the.
presence of a nucleus with a distinc.t and permanent structure. is doubtful.
Endopla smic
Pr'oplaslid reticulum
FIG. 4. 1, Three-dimensional diagram of a plant cell from which a porlion has been
removed to reveal a large central vacuole and the cytoplasm, which contains the
nucleus, lining the cell waU. 2, As above, but of a cell in which 'the nucleus is
located more or less centrally and in which the cytoplasm surrounding the nu-
cleus is connected to the peripheral cytoplasm by cytoplasmic strands. 3, An
adaxial epidermal cell from the calyx of Tropaeolum majus containing chromo-
plasts. 4, Chromoplasts in a carrot root cell. 5, Leucoplasls in a young endosperm
cell of Zea. 6, Diagram of a meristematic plant cell. (No.3, adapted from
Strasburger, J923 ; nos. 4 and 5, adapted from Eames a nd MacDaniels, 1947;'
no. 6, adapted from Sitte, 1961.)
In some cells of the higher plants, such as the sieve elements of the phloem
which a re adapted for translocation, the nucleus is absent from the mature
cell. However, there are also cells which have numerous nuclei. A .multi-
The Cell 9
nucleated cell can comprise an entire organism as in some fungi and algae,
or multi-nucleated cells may be a transitory stage in the development of a
tissue as, for example, in the endosperm of many plants and sometimes in
fibres. The accepted view in many cases is that each nucleus together with
the protoplasm surrounding it' forms a walHess cell so that the entire
multi-nucleate body comprises a group of protoplasmic units. Such a struc-
ture is called a coenocyte.
The coenocyte aroused much interest in phylogenetic and ontogenetic
studies. Two theories exist which deal with the relation of the entire organ-
ism and the single cell. According to the cell theory, which was developed
about the middle of the nineteenth century, the organism consists, both
phylogenetically and ontogenetically, of a complex of an enormous number
of cells each of which plays a role in determining the nature of the organism.
The theory contradicting the above is the organisrnal theory. This theory
gives less importance to the individual cells and mainly stresses the unity
of the protoplasmic mass of the entire organism. According to this theory
the. organism as a unit, to a large extent, determines the nature of the cells.
_. These two theories are important and, for the following reasons, atten-
.,!ion_was.paid to both of them in histological and cytological research of
. plants. ,Many aspects of ontogeny, such as the processes of cell division,
_the.· origin· of vessels and articulated laticifers, the development of idio-
blasts, etc., were investigated in the light of the cell theory. However, the
specialization of the different cells and tissues in the plant and the sites of
appearance of the various types of cells and tissues can be explained only
on the basis of the organismal theory which regards the organism as an
unit.
Tlie protoplast
PROTOPLASMIC COMPONENTS
The cytoplasm
The nucleus
The nucleus is a round or ellipsoid protoplasmic body (Fig. 5, no. I).
It is separated from the cytoplasm by the nuclear membrane. The nuclear
membrane has been observed to consist/6f double porous lamella~
(Fig. 4, no. 6). In many cases connections between these lamellae and the
endoplasmic' reticulum have been observed (De Robertis et al., 1960).
The nuclear sap or karyolymph, one or rilOr'; nucleoli, and the chromo-
somes are found within the membrane.- The chromoso'mes consist of
chromonemata. The last two terms are derived from the word chromatin
which means an intensely staining substance. The chromosomes consist of
nucleoproteins of .~hich the nucleic acid component is mainly DNA
(deoxyribonucleic acid), the carrier of genetic information. Another group
of nucleic acids is RNA (ribonucleic acid) which is concerned in the syn-
thesis of proteins. Part of the RNA is formed on the DNA template. The
prevailing nucleic acid in'the cytoplasm is RNA.
The plastids
FIG. 5. 1, Portion of an epidermal cell of a bulb scale of Allium cepa showing the
nucleus and m1\ochondria. x "830. 2, Cells of a moss leaf showing chloroplasts.
X140. 3, Chloroplasts in a su bepidermal cell of the greert fruit of Lyco-
perSiCO!1 escu/elltum .. grana can be distinguished as darker areas in the chloroplasts.
X660. 4, As above, but in a mature fruit where the chlorop!;lsts have become
changed into chromoplasts. X 66{l. (Nos. 3 a nd 4, courtesy of Y. Ben-Shaul.)
Plant Anatomy
around the nucleus (Fig. 4, no. 5; Fig. 58, no. 3). Their main function is
concerned with the development of starch grains. When the leucoplasts
become specialized to store starch in those regions where starch is stored,
they are called, amyloplasts, and similarly those_leucoplasts related to the
production of oils and fats are termed elaioplasts. The Jatter are fo und
mainly in liverworts and monocotyledons.
Chloroplasts contain aJI the enzymes respo nsible for photosynthesis and
they are fo und in tissues exposed to light. They usually have the form of
The Cell 13
flattened convex discs, plates or ellipsoids. Among the higher plants the
average diameter of chloroplasts is 3 1', but larger and smaller ones exist.
The number of chloroplasts per cell depends on the particular tissue as well
as the plant. In the higher plants there is always more than one,plastid per
cell (Fig. 5, no. 2). The chlorophylls (the green pigments) are aggregated
within the plastid entirely, or almost so, in small bodies, the grana (Fig. 6,
nos. 2, 3).The material in which the grana are dispersed is called the stro-
.. ma. The protein content of the chloroplast is high. By means of the electron
microscope a double-membrane envelope and an inner lamellar structure
which is denser and more complicated in the grana was discerned.
Chromoplasts have various shapes which are usually irregular. They may
be more or less round, elongated or angled and many of them are lobed
(Fig. 4, nos. 3, 4). Their colour is variable-from yellow tones through
orange to yellowish-red. The colour is due to xanthophylls and carotenes.
The pigments are present in the chromoplasts in various forms-diffused,
granular or crystalline. It is thought that it is the crystalline form that gives
the various angular shapes to the chromoplasts as can be seen, for example,
"in the carrot root (Fig. 6, no. 4).
ChromopJasts play an important roJe in the composition of the colours
"Of flowers and fruits, but they are also found in roots and other parts
of. plants. Many of them are chloroplasts that have undergone changes,
but they can also develop directly from proplastids (Fig. 6, no. 1).
I
Mitochondria
wbich sends complex infoldings into the lumen of the mitochondrion. The
-lumen, which is surrounded by the inner membrane, is occupied by a rela-
tively dense material, which is generally termed the mito1chondrial matrix.
Mitochondria are very sensitive to environmental influences and they are
often destroyed by the usual fixation methods, especially those involving
tbe. acids, used in cytology and histology. Mitochondria are produced by
division and are passed on from generation. to generation via the gamet~s.
Mitochondria contain enzymes that playa role in respiration:
NONPROTOPLASMIC COMPONENTS
The vacuoles
/
Pigmentation
The plant pigments are usually found in the plastids and in the cell sap.
The green colour is due to chlorophyll which is found in the chloroplasts.
In the same plastids carotenoids, the yellow to orange pigments, are also
The Cell 15
found but they are masked by the chlorophyll. Carotenes and xanthophylls
belong to the carotenoids. The latter pigments become noticeable when
there is little or no chlorophyll as is the case in the chromoplasts. Another
group of pigments is the flavoneswhich are water soluble and which colour
the cell sap. In some genera, for example Verbascum, it is the flavones that
give the yellow colour to the petals. The anthocyanins, which are the oxi-
dation products of the flavones, are also water soluble and give red, purple,
violet and blue colours to the cell sap. These pigments are responsible for
iii'; colouring of flowers, fruits, young leaves, etc. The colour of antho-
cyanins varies according to the pH of the cell sap: they are red in an acid
medium and blue in a basic one. Sometimes the visible colour is the result
of a few pigments occurring together in a single cell. For instance, chloro-
or chromoplasts can be found together with anthocyanins.
White petals are devoid of pigm,nts and the colour seen results from the
reflection of light from the petals which are opaque due to the presence of
numerous large intercellular spaces that are filled with air.
-·'The colouring of autumn leaves is the result of various processes as well
!'as'the combination of different pigments. With the gradual death of the
::-feaf the chlorophyll breaks down into colourless substances and the caro-
"tenoids become visible making the leaf appear yellow. The red and purple
colours are from pigments in the cell sap, i.e. oxidation products of the
flavones. These colours are most brilliant when formed in the presence of
sugars in leaves exposed to stro-ng light: Autumn colours, which result
from the combination of ;mall amounts of chlorophyll and carotenoids
and greater amounts of anthocyanins together with tannins and various
uncommon pigments and the browning of the cell wall, are best developed
in the cold temperate zones.
Ergastic substances
FIG. 7. 1-4, Potato starch grains. 1 and 2, Compound starc h gra ins. 3, Simple
sta r ch grain. 4 , Half-compound starch grain. 5, Cross-section of the outer por-
tion of a potato tube r. 6, Banana starch grains. 7 and 8, Starch grains' of
Triticum durum. 9, Stages in d evelopment of sta rch g ra ins in chloroplasts of
Phaius maculala. 10, Compound starch grain of Avena. 11 , As in No. 10, b ut
disintegrating . 12, Sphaerocrysta ls of inulin in ceUs of a Dahlia tuber . 13, Aleurone
gr ains. in an endosperm cell of Ricinus communis [(Om a section of material
embe dded in dilute glyce ri ne. (Most of the figures a da pte d from Strasburger ,
Palla din a nd Troll.)
The Cell 17
~_,
\~ -
-_.,_ _
r.~
1 _ >
e?
FIG. 8. 1, Two parenchyma cells from the petiole of Begonia; in the upper cell a
solitary prismatic crystal and in the lower cell. a druse. 1., Variously shaped pris-
matic crystaJs. 3, Indivjdual raphides and a bundJe of raphides. 4, A cell with
pentagonal faces. 5-8, Various types of cell shapes with 14 faces. (Nos. 1-3, adapt-
ed from Palladin, 1914; nos. 4--8, adapted from Frey-Wyssling; 1959.)
Crystals of calcium oxalate are found usually in the vacuole but some
workers (for instance, Scott, 1941) refer to crystals that develop in the
cytoplasm. Crystals can be found in cells resembling those neighbouring
them but lacking crystals, or they may be confined to special crystal-con-
taining cells, i.e. idioblasts (Foster, 1956).
Idioblasts are cells that differ distinctly from the surrounding cells in
both shape and structure. Raphides are usually found in very large cells
which, when mature, do not contain a living protoplast, but which are
The Cell 19
filled with mucilage. Idioblasts with raphides are found in many mono-
cotyledons and also in some dicotyledons as, for instance, in the petals of
Impatiens balsamina.
Silicon salts ",re often deposited in. cell walls, as is common in the
grasses, but they can also be found within the cell.
Cystoliths. These are internal outgrowths of the cell wall that are en-
crusted with calcium carbonate (Fig. 55, no. I).
Tannins. The tannins are a heterogeneous group of phenol derivatives.
in' microscopical sections of tissues tannins are usually identifiable in the
cells as yellow, red or brown substances. Tannins can be found in the differ-
ent parts of the plant, especially leaves, the periderm, galls, and in cells
associated with vascular bundles. Tannin-containing cells may be inter-
connected or tannins may be found in isolated specialized cells (idioblasts).
Within the cell the tannins may be found in the vacuole or in the form of
droplets in the cytoplasm, and sometimes they penetrate into the cell wall,
as, for instance, in cork tissue. Tannins are thought to protect the plant
against dehydration, rotting and damage by animals.
gonal and the remaining ones tetra- or hexagonal. In the apical ineristem
of Anacharis densa, Matzke.(1956) found that during Ithe interphase, the
average number of faces of the polyhedron increased from 13·85 to J6·84,
and after division the daughter cells had an average of 12·61 faces.
As result of the continued increase of cell volume during growth, the
number of wall faces increases above 14. This makes it impossible for all
the sides to remain in contact with all the sides of the neighbouring,cells
and so intercellular spaces develop. In some tissues the'intercellular spaces
reach relatively large dimensions and then they are referred to as air spaces,
ducts, etc. Such spaces can develop in two ways: (a) by the separation of
neighbouring cell walls, as in the development of the resin ducts in Pinus;
this type of development is known as schizogenous development (Fig. 33,
nos. 2-5); (b) by the disintegration of the cells in the place where the space
develops, as in the essential oil cavities in the peel of citrus fruits; this type
of development is known as lysigenous development (Fig. 34, nos. 1-6).
In some cases spaces are formed by these two methods together and then
the development is known as schizo-lysigenous I development. The inter-
cellular spaces in the protoxylem are sometimes' formed in this way.
The intercellular spaces can be irregular and variable in shape or they
may form a distinct and permanent system as' in many water plants, in the
banana leaf, and other plants (Fig. 93, ~o/ 3; Fig. 97, no. I) .
The Cell
I D~te
S''i I
21
new areas of contact are formed. In;-s-uc-h:-c-a-se-s-t:7h-e-re-:-is-n-o-g";l~id~i-n-g-.':'In-m-a-n-y-l
plants the ends of the fibres grow in this way. The-growth of the branches
of some sclereids and of the non-articulated laticifers is intrusive.
Many cells whose growth continues over a long period develop by all
three methods of growth, or by two of them.
axis of the cell (Fig. 124, nos. 3, 4). In such cells the young nuclei almost
reach the. resting state,..with a membrane and nucleoli, ;while the cell plate
has not yet reached the end walls oflhe dividing cell. When the cell plate
reaches all parts of the existing wall of the dividing cell the phragmoplast
disappears completely. At this stage the viscosity of the cell plate becomes
~,-
:I LphragmOP,ast
,.
}~'"
i}~'i<Z'''''''',,",H~W'''''_''L~~
,
-~~'~~-'~'~;~'fJii'~"M*';"
i. ""i"' " Cell piate%: '.
~-
• ·Wv .,.,/-->:_"·"'-
3 4
,Phragmoplast
'qt'"l'~'~"~(
Cell plate
/5 s
Parent-cell waIJ
Middle Newly - formed
lamell cell wall
9 Intercellular '10
space
higher, and on both sides thin lamellae are laid down by the daughter pro,
toplasts. These lamellae are the initial stage in the development of the new
walls of the daughter cells. The cell plate gradually undergoes changes to
form the intercellnlar $ubstance referred to as the middle lamella..It is not
yet known what substance comprises the cell plate; it is possible'that it is of
protoplasmic origin.
The Cell 23
Along the iine of contact of the new wall and the wall of the mother cell,
the new and old middle lamellae are separated by the primary wall of the
mother cell (Fig. 9, no. 8). According to Martens (1937, 1938) the connec-
tion between,these.middle lamellae is effected in the following manner. In
the primary wall of the mother cell a cavity; which is triangular in cross-
section, develops all along the line of contact of the new' and old walls.
This cavity continues to enlarge till it reaches the middle lamella of the
mother cell and so connection is made between the new and old middle
lamellae. If the cavity continues to grow and the intercellular substance
does not fill' it, an intercellular space lined with intercellular substance is
formed., According to Priestley and Scott (1939) the middle lamellae are
brought into contact after the stretching wall of the mother cell tears
opposite the new wall.
The secondary wall is very strongly anisotropic- and layering can be ob-
served in.it. The reason for this layering is discussed later in this chapter.
In the majority of tracheids and fibres three layers_Jthe outer layer (Sl)'
central layer (S2)' and inner layer (Sa)-can be discerned in the secondart
wall. Of these layers the central layer is usually the-thickest. In some-cells,
however, the number of layers may be ffi()re than three (Fig. 10, no. 1)·
/
FIG. 10. I, Outer portion of a cross-section of a young stem of Linum usitalissi-
mum showing maturing fibres in which the various layers of the. secondary
wall have separated from each other during sectioning. X 460. 2, Pi. dark-field
photograph of a bordered pit of Cedrus showing the fringed torus. x 700. 3,
Electron micrograph of a cross-section of a pit membrane of a living fibre of
Tamarix showing plasmodesmata. x 35,000. (No.2, from Huber in Handbuch
der Mikroskopie in der Technik, Umschau Verlag, 1951,)
J
Some authors (for example, Meier, 1957) use the term tertiary wall of
tertiary layer for the inner layer of the secondary wall. According to Frey'
Wyssling (1959) an innermost layer with a chemical c;omposition different
from that of the secondary wall may be present in addition to the innef
layer of the secondary wall . He suggests that this layer should be called
the tertiary or terminal layer.
It should be mentioned that some investigators use the term compound
mjddle lamella when dealing with wood tissue. This term is used to refef
The Cell 25
The fine structure of the cell wall, particularly that of the secondary wall,
has been intensively studied in the last century. This research was stimu-
lated because of its importance to the fibre, paper and other industries.
rhe researchers worked in two directions, i.e. from the morphological and
)hysico-chemical approaches. By combining the results of these two fields
)f research a rather clear picture of the fine structure of the cell wall has
b-een derived.
Results of the morphological line of research. With the high power magni-
"cation of the ordinary light microscope, different arrangements of the
layering in the secondary.wall can be seen in cross-sections of fibres, tra-
eheids, hairs, etc. The layers can be concentric, radial or have a compli-
cated arrangement. When the cell wall is allowed to swell under the in-
fluence of suitable reagents, fine structures can be observed with the a"id
of the light microscope (Fig. 11, no. 1). By means of such methods Bailey
and others (Bailey and Kerr, 1935; Bailey and Vestal, 1937 a, b; Bailey,
1957) found that ttle cell wall is built of a s,!stem of mtccosco>;lic tbceads-
the'microfibrils, which are grouped together in larger bundles. The layer-
ing seen in the secondary wall is often the result of the different density of
the microfibrils; this is the case in cotton fibres, for example. In the denser,
darker wall layers, the microfibrils are more numerous per unit area
and they are more tightly packed. In the less dense, lighter wall layers,
the microfibrils are looser and the capillary spaces between the fibrils are
larger. In heavily lignified walls it is possible to dissolve the cellulose and
retain the lignin only, or the lignin alone can be dissolved and the cellulose
retained. In this way the component retained gives, as it were, a nega.tive
image of the component which has been dissolved. This phenomenon not,
only proves that the lignin is found in the elongated interfibrillar 'pace~. of
the cellulose but also that these capillary spaces are continuous. Therefore
it is clear that the secondary wall consists of two continuous interpen~trat
ing systems, one of which is the cellulose microfibrils, and the,other the
continuous
. system of microcapillary spaces. These spaces / may be filled
WIth lignin, cutin, suberin, hemicelluloses, and other organic substances
and even mineral crystals, and in fresh tissue aque~us solutions. The
26 Plant Anatomy
FIG. 11. 1, Striations seen on the surface of a tangential section of the secondary
wall of a fibre-tracheid of. Siparuna bifida. x 1100, 2, A longitudimi.l Section
through the secondary wall of a tntcheid of Pinus showing the plane of mechanical
cleavage. x 460. 3, A longitudinal section in the secondary xylem of Pinus showing
the spiral arrangement of the cavities produced by enzymatic actibn of fungi on
the secondary wall. x 230. 4, Longitudinal section of tracheids of Larix showing
the orientation of iodine crystallized in the spaces between the microfibrils. x 480.
(Courtesy of I.W. Bailey.)
FlO. 12. 1, Electron micrograph showing the structure of the secondary wall of
Valonia. X 7000. 2, As above, but of the primary wall. x 8000. (From Steward
. and MUhleth~ler, 1953.) /
made possible many advances in the study of the structure of the cell wall.
The amazing photographs which were made with the electron microscope
(Fig. 12, nos. I, 2) revealed the fine microfibrils which Jannot be seen by
means of the ordinary light microscope. The results of research with the
electron microscope have in general confirmed the theories of Bailey on the
structure of the walL
The morphological structure orihe cellulose in the cell wall, as is known
today, can be summarized in the following way. Within the cell wall differ-
ently arranged lamellae are recognized, each of which consists of micro-
fibrils. According to at least some researchers the microfibrils are grouped
in bundles. These bundles as well as the microfibrils anastomose to form
a three-dimensional network. This network is interwoven with a parallel
network of microcapillary spaces occupied by noncellulotic substances.
The width of the microfibrillar bundles can be as much as 0·25 fl. and
that of the microfibrils themselves, as measured from fibres of Boehmeria,
),021' (170--200 A) approximately. However, thinner and sometimes even
_. - -in - differenLcells. and \ plants: The microfibrils
thicker microfibrils are found
have recently been found to be fasciations of elementary fibrils which are about
35 A thick.
Results or the physico-chemical. line of research. The cellulose molecule-
consists of long chains of linked glucose resid6es. The chain molecules are
arranged in bundles which are generally termed micellae. The hypothesis
of the presence of micellae was proposed by N.iigeli in the last century.
According to him the micellae are the individual units arranged in a perma-
n"nt order within an intermicellar matrix. With the aid of the polarizing
microscope the crystal-like nature of the micellae was proven. From the
res~lts of various investigations, especially those made with X-rays, inves-
tigators came to the conclusion that the micellae consist of parallel chains
of glucose residues which have characteristic and permanent distances be-
tween them. As a result of extensive research carried out in the last thirty years
by botanists, chimists and,physicists, several theories 'f.ere suggested which
attempted to explain the organization of the cellulosf-molecules in the cell
walL Frey-Wyssling ~nd Muhlethaler have recently come to 'the fo1l6wing
conclusion as regards tIIe,structure of cellulose. The thrdtd-like cellulose
molecules are regularly arranged in bundles. Each such 'bundle .Which forms
an elementary fibril, consists of about 36 cellulose molecules. The elementary
fibril is in greatest part crystalline. Only very small parts of it, which are
presumably arranged at random, may be paracrystalline. The number .of
glucose residues in cellulose molecules of fibre cells was found to vary from
500 to 10,000 and the length of these molecules varies from 2500 to 50,000 A
(Frey-Wyssling, A. and K. Muhlethaler, 1965. Ultrastructural Plant Cytology.
Elsevier Publishing Co.) (Fig. 13).
Most ofttle above is based on the results of research made on the secondary
cell wall, but recently much attention has been paid to the structure of the
The Cell 29
Primary woll
Bundles of
microfibrils ~--,=---,__
~*-
r
'"
Microfibril
I
Elementary fibril
,!
FIG. 13. Diagrammatic representation of the submicroscopic structure of the cell
wall. 1, Portion of a cell with secondary wall layers, 2, Bundles of microfibrils,
microfibrils and an elementary fibril. 3, Two unit cells of cellulose, as suggested by
Meyer and Mark. 4, Two glucose residues. .,
/'
or of other substances. The interfibrillar matrix usually contains pectic
compounds and hemicellulose, but in many cells lignin, cutin, suberin,
waxes and many other organic substances may also be present. The pri-
mary wall of manv cel1s h~", ~ 1'1""",11", .. .,. " .. _ .. ~ .... __
'T" •
30 Plant Anatomy
prisms. In these layers the crystals are parallel to the longitudinal axes ot
the tracheids. The bright layers. however, are not cobtinuous on the cir'
cumference but are interrupted in four places. The brightest sections ot
0
such a layer are those that lie at an angle of 45 to the axis of the analyser
and the polarizer of the microscope, and'the darkest areas of the same layer'
are approximately parallel to these axes. The birefringence is only apparent
I /
I /
I /
I /
--4-_ :\
when the longitudinal axes of the crystals are at an angle of 45 to the ax~,
0
of the crossed analyser and polarizer. By the study of oblique sections, cu.1
at different angles or on the basis of accurate calculations of the degree
of the birefringence, it is possible to use this method to determine the
accurate orientation of the cellulose crystals in the various layers of the
wall (Preston, 1952).
From the results obtained from investigations using X-rays, lhe orien'
tation of the cellulose crystals in the different lavers cannot nf" nptprn-o-i..,,,,,,.-I'
The Cell 33
FIG. 16. 1, Micrograpl1 of parenchyma cells from the pith of Nicotiana tabacum
showing primary pit fields. x 750. 2, Electron micrograph of a primary pit field
of Zea mays. x 24,OOO. (No.2, after Miihletbaler, 1950.)
Pits
Certain portions of the cell wall remain thin even as the secondary wall
is formed and they, therefore, consist only of primary wall material. These
areas, which are of variable shape, are called pits (Fig. 17, nos. 1-6). Some
authors use the term pit to refer only to the pit cavity together with the
primary wall, which closes the pit. Others use the term pit to refer to the
above structures together with that part of the secondary wall that surrounds
the pit cavity. Pits can develop over the primary pit fields and then one
or more pits may develop in the pit field, or .pits develop on those parts of
the primary wall devoid of pit fields. On the other hand, the primary pit
fields can become completely covered by the secondary cell wall.
The pits are apparently areas through which sUbsdnces pass from cell
to cell. The concentration of plasmodesmata,. in living cells in the region
of the pit membranes is an "additional proof of the pit being a channel of
exchange. Generally each pit has a complementary pit exactly opposite it
in the wall of the neighbouring cell. Such pits form a morphological and
functional unit called the pit-pair (Fig. 17, no. 2). The cavity formed by
the break in the secondary wall is called the pit cavity. The membrane,
built of the primary cell walls and middle lamella, that separates the two
pit cavities of the pit-pair, is called the pit membrane or closing membrane.
The opening of the pit on the inner side of the cell wall, i.e. on that side
facing the lumen of the cell, is called the pit aperture (Fig. 17, no. 2).
Two principal types of pits are recognized-simple pits and bordered
pits (Fig. 17, nos. 1-7). The main characteristic of bordered pits is that the
secondary wall develops over the pit cavity to form an overarching roof
with a narrow pore in its centre. In a simple pit no such development of
the secondary wall is present.
If the two pits of a pair are simple, a simple pit-pair is formed (Fig.
17, no. 2); if the two pits are bordered, a bordered pit-pair (Fig. 17, no. 3);
if one of the pits is simple and the other bordered, a ha!J,bordered pit-pair
(Fig. 17, no. 4). If the pit has no compiement&ry pit in the adjacen(cell or
ifit is opposite an intercellular space, it is termed a blind pit (Fig. 17, no. I).
Sometimes two or more pits are found opposite one large pit- such an
arrangement is called unilateral compound pitting.
). " ;~ .
The pit cavity of a simple pit may have the same diameter over its entire
depth, or it may widen or narrow toward the pit aperture. In places where
the secondary wall is very thick the pit cavity has the form of a canal.
Pit chamber
Outer
aperture
\
\
2 3 4 5 6
Pil
membrane
.J
Torus
-~-
1...
9 10
II
12
FIG. 17. Structure of pits. 1, Simple pit. 2, Simple pit-pair. 3, Bordered pit-pair.
4, Half-bordered pit-pair. 5 and 6, Bordered pits . 7, Three-dimensional diagram
. of a portion of the adjacent walls of fwo tracheids showing the structure of
bordered pit-pairs. 8, Diagram of pit membrane and torus of, Pinus showing the
perforations in the membrane. 9 and 10, Longitudinal sections of bordered pit·
pairs of a tracheid. Arrows indicate direction of water flow. 9, Torus and memo
brane in median position.\lO, Torus closing one of the pit apertures. 11 and 12,
Longitudi nal section·of tlie wall of adjacent vessels with vestured pits. (Nos.
8- 12 after LW. Bailey.)
38. Plant Anatomy
Sometimes this canal is branched towards the outer layers of the cell wall
and then the pit is called a branched simple pit. Such pits arise from the
fusion of several pits during the centripetal addition of liiyers to the secon-
dary wall (Fig. 38, no. 4).
Simple pits ·are usually' found in parenchyma cells with thickened walls,
in libriform fibres and scleroids. Bordered pits are found in the tiacheary
elements and in fibre-tracheids.
The bordered pit is more complicated than the simple in its structure
and is variously shaped. In the bordered pit that part of the pit cavity t!tat
is formed by overarching of the secondary wall is called the pit chamber
and the opening in the secondary wall that faces the cell lumen is called
the pit aperture. If the secondary wall is very thick a canal-the pit
canal-is formed between the cell lumen and the pit chamber. In the pit
canal two openings are distinguished-that facing the cell lumen is termed
the inner aperture and that nearest the pit chamber, the· outer aperture
(Fig. 17, no. 6).
In some plants there are bordered pit-pairs in which the pit membrane
is thickened in its central portion; this thickening, which is of a primary
nature, is disc-shaped and is termed the torus (Fig. 17, nos. 7-10). The dia-
meter of the torus is wider than that of the pi~ aperture. \
Bannan (1941) describes the occurrence,of thickenings, other than the
torus, on the pit membrane. These thickenings may be radial or tangential
in relation to the torus. The torus found in the bordered pits of Cedrus is
fringed on its circumference (Fig. 10, no. 2). This feature is a characteristic
of Cedrus, and as such it aids in the identification of the wo'od of this genus.
It\. tracheids of many conifers the ",it membrane around the torus is
porous. The presence of these pores was discovered in 1913 by Bailey in
experiments that derr,onstrated the passage of a suspension of finely divided
particles of carbon from one tracheid to anotJ:;er. This has been con- \
firmed by electron micrographs (Liese and Fahnenbrock, 1952). The pit
membrane is usually flexible and, under certain conditions, the torus can
be pushed against one of the pit apertures (Fig. 17, nos. 9, 10). When the
torus is in the median position, i.e. in the middle of a pit·pair, water can
easily pass from one tracheid to another, In a pit-pair where the torus is in\
a lateral position, i.e. pressed against one of the pit apertures, the passage .
of water is very limited. Most of the tori in late wood and all 'of them in the
heartwood are always in a lateral position and the flexibility of the pit
membrane is lost.
The presence of a torus is especially characteristic of the bordered pits
of the Gnetales, of Ginkgo, and most of the Coniferales. Tori occur only
rarely in the Ophioglossales (Bierhorst, 1960) and in the angiosperms.
In some dicotyledons thin, simple or branched sculpturings are present
on the secondary wall that forms the pit chamber, or around the pit aper-
ture. Such pits are called vestured pits (Fig. 17, nos. II, 12) and the sculp-
The Cell 39
turings may have various shapes .. Because of the special properties of light
refraction and of,staining these pits appear.as jf porous or net-like in sur-
face view an9, thus, were once , termed sieve-pits. Vestured pits are found
in the tra~heary elements of the secondary wood of certain dicotyledonous
genera and species, such as some of the Legumjnosae, Cruciferae, Myrta-
~
«~
~~
;_(,~
""*=
t~
::~
~'
,,~ =:,
~.--
2
3 .... 5
FIG. 18. 1, Portion of the common wall between two fibre-tracheids showing the
type of bordered pit characteristic of these elements. 2-5, Types of pitting. 2, Sca-
lariform pitting. 3, Transition from elongated pits in scalariform arrangement
to shorter circular pits in opposite arrangement. 4, Opposite pitting. 5, Alternate
pitting.
the walls continue to thicken the pit chamber becomes smaller and the pit
canal between the inner and outer apertures becomes longer. In such pits
the inner aperture often becomes long and narrow, as seek in surface view,
and, in very thick walls, its longitudinal axis may be longer than the dia-
meter of the pit chamber. When the inner aperture is large and linear,
narrow or elliptic, and when the outer aperture is small and circular, the
pit canal has the shape of a flattened funnel. The elongated inner apertures
of such a. bordered pit-pair may be parallel or crossed. This type of pit
occurs mainly in fibre-tracheids (Fig. 18, no. I). '
Bordered pits found in tracheary elements vary in shape and arrange-
ment. When the pits are distinctly elongated or linear and arranged in
ladder-like tiers the arrangement is termed scalariform pitting. When the
pits are circular or only slightly elongated and the outlines are elliptic
there are two possible ways in which they may be arranged on the wall:
in horizontal lines) i.e. opposite pitting, or in diagonal lines, i.e. alternate
pitting. When the pits are crowded the outline of the opposite pits becomes
rectangular or square, and that of the alternate pits hexagonal (Fig. 18,
nos. 2-5). \
Tracheary elements have especially well developed bordered pits in
those regions where they are adjacent to other tracheary elements. In the·
regions of contact with parenchymatous cell( reduced bordered pits are
sometimes found. Usually those parts of the wall adjacent to fibres are
devoid of pits.
/
Other sculpturings on the cell wall
In addition to the pits many other sculpturings exist on the cell walls.
These include, for example, the perforations in the end walls of the vessel
members, various thickenings on the inner surface lof cell walls, such as
wall thickenings in the proto xylem elements, spiral thickenings on the
inner surfaces of pitted secondary walls, casparian strips of endodermal
cells, thickenings in the walls of the endothecial cells of pollen sacs, and
external projections formed partly by the wall itself and partly by deposits,
e.g. of cuticle on epidermal cells and of external layers on spores and pollen
grains. The above features are discussed in later chapters. Here only three
structures will be discussed-crassulae, trabeculae and lvarl structures.
Crassulae are linear or crescent-shaped thickenings of the primary wall
and middle lamella which occur between bordered pits or small groups of
these pits. The crassulae may sometimes surround the pits. They represent
the borders of the primary pit fields of the young cell from which the ele-
ment developed. Crassulae are well developed. in the tracheids of certain
gymnosperms (Fig. 133, no. 4).
Trabeculae are rod-shaped thickenings of the wall which traverse the
cell lumen radially. They usually appear in radial rows in the wood ele-
The Cell 41
Cystoliths
In some dicotyledonous families, such as the Moraceae and Urticaceae,
stalked outgrowths of the wall that project into the cell lumen are present.
These outgrowths are called cystoliths. They consist of cellulose and are
Impregnated with calcium carbonate. Cystoliths are irregular in shape and
sometimes they almost completely fill the cell. Cystoliths may appear in
parenchymatous cells in various parts of the plant including even the
xylem and phloem rays, but they are usually found in the epidermis in
hairs ,?r special large cells which are termed lit/wcysts (Fig. 55, no. J).
I
References
BAILEY, 1. W. 1'~n.lhe_preservat'lve treatment 01 wood. 11. 'The structure 01 the p'ft
membranes in the tracheids of-conifers" and their relation to' the penetration of
-gases, liquids and finely divided solids into green and seasonal wood. Forestry
Quart. 11: 12-20.
BA.lLEY, I. W. 1957.',Aggregation of microfibrils and their orientations in the secondary
wall of coniferous tracheids. Amer. Jour. Bot. 44: 415--418.
BAILEY, L W. and BERKI.EY, E. E. 1942. The significance of X-rays in studying the
orientation of cellulose in the ~econdary wall of tracheids. Amer. Jour. Bot. 29:
231-241.
BAILEY, J. W. and KERR, T. 1935. The visible structure of the secondary wall and its
significance in physical and chemical investigations of tracheary cells and fibres.
Jour. Arnold Arb. 16: 273-300.
BAILEY, I. W. and KfRR, T. 1937. The structural variability of the secondary wall as
revealed by "lignin" residues. Jour. Arnold Arb. 18: 261-272.
BAILEY, 1. W. and VESTAL, M. R. 1937a. The orientation of cellulose in the secondary
wall of tracheary cells. Jour. Arnold Arb. 18: 185-195.
BAILEY, 1. W. and VESTAL, M. R. 1937b. The significance of certain wood-destroying
fungi in the study of the enzymatic hydrolysis of cellulose. Jour. Arnold Arb. 18:
196-205.
BANNAN, M. W. 1941. Variability in wood structure in roots of native Ontario conifers.
Bull. Torrey Bot. Club 68: 173-194.
BIERHORST, D. W. 1960. Observations on tracheary elements. Phytomorphology 10:
249-305.
42 Plant Anatomy
MERISTEMS
IN THE early stages of the development of the embryo, all the cells undergo "
division, but with further growth and development cell division and multi-
plication become restricted to special parts of the plant in which the
tissues remain embryonic in character and the cells retain the ability to di-
vide. These embryonic tissues in the mature plant body, are called meri-
stems. Cell division can also occur in tissues other than meristems, for in-
stance, in the cortex of the stem and in young, developing vascular tissues.
However, in these tissues the number of divisions is limited. On the other
hand, the cells of the meristems continue to divide indefinitely and as a
result new cells are continually added to the plant ·body. Meristems may
also be found in a temporary resting phase, for instance, in perennial .\
plants that are dormant in certain seasons and in axillary buds that may be
dormant even during the active phase of thee plant.
The process of the growth and morpho-physiological specialization of
the cells produced by the meristems'is called differentiation. Theoretically,
it was believed that the tissues that undergo differentiation ~radually lose'
the embryonic characteristics of the meristem and acquire the mature state.
Such tissues are called mature or permanent. Recently it has been shown
that the term permanent tissues.can only.be used in relation to certain cells
which have undergone' i~reversible differentia"tion, ~s, for instance, sieve
elements which have no nucleus and dead cells, such as tracheids, vessel
elements and cork cells. All ceUS' which contain nuclei possess, to 'a certain
degree, the ahility to grow and divide and redifferentiate if the appropriate
stimulus is present (Bloch, 1941; Buvat, 1944, 1945; Gautheret, ,1945,1957;
White, 1946; Wetmore, 1954, 1956).
I
Classification of meristems
The classification of meristems is made on the basis of various criteria
-their position in the plant body, their origin and the tissues which
they produce, their structure, their stage of development and their func-
tion.
According to the position of the meristems in the plant body they are
divided into the following types: (a) apical meristems, which are found in
44
Meristems 45
the apices of the main and secondary shoots and roots; (b) intercalary
meristems, which are found between mature tissues~ as, for example, in the
bases of the internodes of grasses; (c) iatera/.meristems, which are situated
parallel to the circumference of the organ in which they are found, as, for
instance, the vascular cambium and the phellogen.
It is customary to distinguish between primary and secondary meristems
-a classification based on the origin of the meristems. Accordingly, pri-
-mary meristems are those whose cells develop directly from the embryonic
cells and so constitute a direct continuation of the embryo, while secondary
meristems are those that develop from mature tissues which have already
undergone differentiation.
The above definitions of primary and secondary meristems, however, are
not always accurate. For example, the apical meristems of truly adventi-
tious organs develop secondarily within relatively mature tissues as well
as within secondary meristematic tissues, although according to their struc-
ture and function they are primary meristems. On the other hand, a large
!!i!part of, or sometimes even the entire, vascular cambium, which is generally
accepted to be a secondary meristem, develops. at a late stage, from the
"apical meristem, i.e. from a part of the procambium.
Examples of secondary meristems, which can be determined as such
·without doubt, according to origin, are the phellogen which develops from
parenchyma or collenchyma cells which have already undergone differen-
tiation and callous tissue ";hich develops in tissue cultures made from
mature tissues.
From the above it can be seen that it is more correct to use the terms pri-
mary and secondary meristems to refer. to. the stage of development at
which the meristems appear and to the types of tissue that develop from
them and not to their origin. From the primary meristems the fundamental
part~ of the plant, such as the epidermis, the cortical tissues of the stem
and root, the mesophyll of the leaf and the primary vascular tissues de-
velop, and from the secondary meristems the secondary vascular and pro-
tective tissues.
In certain monocotyledons, such as some palms, banana, Veratrum and
others, the thickening of the stem takes place near the apices and therefore
is regarded as being of primary nature. The meristem responsible for this
type of increase in thickness is termed primary thickening rneristem.
Api.cal mer;stems
In the nineteenth century research workers mainly dealt with the problem
of the number of the initials in the apices and the determination of the
tissues that were derived from them. Thus the histogen theory of Han stein
Meristems 47
(1868) and the apical cell theory of Nageli (1878) were developed. Modern
research on spermatophytes deals with histo-cytological arrangement of
the zones of cells and their activity in the apices. Recently experimental
research .on apices attached to entire plants or grown as tissue cultures has
contributed to the clarification of these problems (Ball, 1946, 1947, 1960;
Clowes, 1953, 1954; Wetmore, 1954, 1956; Wardlaw, 1957; Gifford and
Tepper, 1962a, b). Initials can be recognized by microscopical investigations
.and by the use of assumptions based o~ the orientation of cell divisions.
Experiments have been made to determine the location and number of the
initials by the application of colchicine. Using this substance it has been
possih\e to increase the number of chromosomes in a few cells, and as the
,derivatives of such cells possess the increased number of chromosomes it is
possible to identify all the cells that are derived from the colchicine-affected
cells. If the affected cell is an initial, entire regions of tissues, the cells
of which have the increased chromosome number, result; thus polyploid
chimeras are artificially formed. This phenomenon makes it possible to
identify the initials (Dermen, 1945, 1947, 1948, 1951). Although the initials
"are usually permanent, opinions exist that they may sometimes be replaced
by new initials. In addition to the above-mentioned studies of Dermen, the
"':.observations of other workers on polyploid chimeras (Satina et aI., 1940;
Satina and Blakeslee, 1941, 1943; Satina, 19'59) and on variegated chimeras
(ThieJke, 1948, 1954, 1955, 1957; Bartels, 1960; Moh, 1961) are of great
impodance in the 'study of apical meristems.
In this book the apical meristem will be divided, as already mentioned,
into two main regions-the promeristem which comprises the apical
initials and neighbouring cells, and the meristematic zone below it in which
the three basic meristems (the protoderm, procambium' and ground meris-
tern) of the tissue systems can be distinguished.
The following discussion deals mainly with the arrangement and function
of the cells in the promeristem.
Wolff, in 1759, discovered that the new leaves and tissues of the stem
arise in the very apex of the stem. He termed this region the "punctum
vegetationis". Today the term shoot apex is generally used (Fig. 19, no. I)
as it is the region of initiation of the primary organization of the shoot in
which the processes of growth take place and which cannot be limited to
a point. The shoot apex proper is considered as that terminal part of the
shoot immediately above the uppermost leaf primordium. There are great
differences in the shape and size of the shoot apices among the spermato-
phytes. In a median longitudinal section the apex generally appears more
or less convex. In Anacharis and Myriophyllum and some grasses the shane
48 Plant Anatomy
of the apex is.a narrow cone with a rounded tip (Fig. 20, nos. 1, 3; Fig.
21, nos. 1, 2), while in a rew plants, e.g. Drimys andl Hibiscus syriacus,
it is slightly concave (Gifford, 1950; Tolbert, 1961).
Before the initiation of each leaf the apical meristem widens considerably
and after the appearance of the leaf primordium ·it again· becomes narrow.
,
FlO. 19. Shoot apex of Vilis vim/era. 1, Vegetative shoot apex. 2, Vegetative shoot
apex in which the tip of the apex is seen to be asymmetrical; this is apparently
connected with the initiation of an axiUary bud. 3, Apex in which the main apex
and an axillary bud can be distinguished; the axillary bud_js~apparently repro-
ductive. 4, Primordial inflorescence (Drawings adapted from Z.' Bernstein.)
This phenomenon is rhythmic, i.e. it recurs with the initiation of each leaf
or pair of leaves. Schmidt (1924) introduced the terms minimal- and maxi-
mal-areas of the apex. For the period between the successive initiations of
two leaves or two pairs of leaves he suggested the use of the term plasto-
chron (Fig. 27, nos. 1-6) which had been used previously but with a much
wider meaning. The shoot apices of dicotyledons with opposite leaves
(such as Lonicera, Coleus, Vinca, Ligustrum, Syringa and others) are parti-
cularly suitable for the study of plastochronic changes.
Meristems 49
In the Angiospermae the shoot apices are usually small. The measure-
ment' of the diameter is taken as the 'width of the apex immediately above
FIG. 20. Photographs of shoot apices. 1, Vegetative shoot apex of Hordeum bu/-
bosum. 2, Early stage in development of inflorescence of Hordeum bulbosum. 3,
Shoot apex of Secale at the time of floral induction. 4, Early stage of floral devel-
opment in Secale. (Photographs courtesy of D . Koller.)
phaea, 500 p, and in Trichocereus itis between 700-800 p (Ball, 1941; Boke,
1941; Cutter, 1957; Fahnetal., 1963). The differences in the diameter of
the apices of gymnosperms are much greater (Kemp, 1943). The apices of
the conifers are cone-shaped and fairly narrow and the dimensions of
their diameter are similar to those typical for angiosperms. On the other
hand, the apices in Ginkgo and Cycas are three to eight times as wide as
they are high (Johnson, 1944). In Cycas ,evoluta the diameter of the maxi,
mal-area of the apex is 3·5 mm (Foster, 1940).
In the Pteridophyta there are one or more initials which can usually be
easily distinguished from the neighbouring cells. These initials give rise
to all the cells of the apex. If only one initial is present it is termed the
apical cell (Fig. 22, no. 2) and if more than one cell are ,present they are
termed apical initials (Fig. 22, no. 3). The single apical cell usually is tetra-
hedral in shape and its base is directed toward the surface of the apex.
A single apical cell is found in the Psilotales, in Eqllisetllm and in some
ferns. The single apical cell divides in such a manner that the new cells are'
~/
DoughIer cells
-~---
/ z
FIG. 23. Diagrams of apical cel1s to show the manner of division and addition of
cells to the plant body. 1, Tetrahedral apical cell with base directed towards the
surface of the apex and in {vhich the planes of division are parallel to the other
three faces. 2, Apical cell in which the planes of division are parallel to two faces
only. (Adapted from Schliepp, 1926.)
formed on all its sides with the exception of that on the surface of the apex.
The apical cells of pteridophytes are usually four-sided, but in SOme water
ferns, e.g. Salvinia and Azolla, and sometimes in Selaginella, they are only
three-sided. In the former, new cells are produced on three sides while in
the latter only on two sides (Fig. 23, nos. 1,2).
It is thought that the ferns (Filicinae) with a single apical cell are evolu-
tionarily more advanced than those with several apical initials.
Meristems 53
It was believed that the tissue of the shoot apex of spermatophytes was
a primordial meristem (promeristem) consisting of undifferentiated cells
which are morphologically equal. ~Recent cyto-histological research on the
shoot apices of spermatophytes has disproved this theory and has shown
that it is possible to distinguish, in these meristems, a complicated arrange-
ment of groups of cells which are characterized by the following features-
the size of the cell and the nucleus, differential staining, the relative thick-
ness of the cell walls and the frequency and orientation of cell division. The
plane of these divisions may be anticlinal, i.e. at right-angles to the surface
of'lthe apex, or periclinal, i.e. parallel to the surface of the apex, or dia-
gonal. ~
Since 1937'much research has been made on the structure of the shoot
apex of gymnosperms (Korody, 1937; Foster, 1938, 1939b, 1940, 1943;
Cross, 1939, 1942, 1943; lohnson, 1939, 1944; Gifford, 1943; Kemp, 1943;
Majumdar, 1945; Sterling, 1945, 1946; Allen, 1947; Gifford and Wetmore,
'1957; Guttenberg, 1961).
It is characteristic of all the gymnosperms that the direction of the cell
(li~visions in the surface of the apex is both anticlinal and periclinal and so
this layer represents the initiation zone of the entire apex and has been
tenned the surface meristem. The striking feature in the struct~re of most
gymnosperm' apices is the occurrence of a distinct zone of ceniral mother
cells. The cells of this zone are characterized by their size, the numerouS
large vacuoles and the presence, in many of them, of relatively thick walls.
Along the sides and the base orthe central mother cell zone the other apical
regions develop as a result orthe diagonal and horizontal divisions of the
central mother cells. In this way the peripheral zone or flank meristem is
developed laterally and the rib meristem zone (also known as central
meristem) from the base. The term rib meristem was introduced by Schuepp
(1926) to describe that type of meristematic tissue that consists of vertical
series of transversely dividing cells. According to Popham (1952), three
principal types of gymnosperms can be distinguished on the basis of the
structure of the shoot apex (Fig. 24, nos. 4-6; Fig. 25, nos. 1,2).
1. The Cycas type (Fig. 24, no. 4) in which three meristematic zones can
be distinguished. (a) The surface meristem in which the cells divide anti-
clinally, periclinally and diagonally. The cells of this zone are not uniform
in appearance and apical initials have been distinguished in the centre of
this zone in the seedlings of Cycas revoluta, but not in mature plants. The
cells of this zone give rise to the epidermis and the other apical meristema-
tic zones. (b) The rib meristem which is situated in the central region of the
apex below the surface layer. In the upper region of this zone vertical rows
of cells are obvious. The cells at the base of these rowS divide periclinally,
anticlinally and diagonally, and they are usually large and contain large
54 Plant Anatomy
vacuoles. In C. ,evoluta the pith develops from this tissue. (c) The flank
me,istem which enlarges by cell division within the zone itself and by the
addition of cells from the surface layer and from the periphery of the rib·
meristem. The cells of this zone are smaller than those of the rib meristem
and they are generally elongated. In C. ,evo/uta the cortex, the procambium
and the leaf primordia develop from this zone.
Cambium-like
transitional
7
/
Flank rneristem
meris~em
8
.,
FIG. 24. I, Schematic drawing of a gr;ss plant to show the regions of growth. In-
tercalary meristematic regions~ heavily shaded; regions that are still growing but
whose tissues have undergone< a certain degree of differentiation, lightly shaded;
mature regions. unshaded. 2-9, Diagrams of eyto-histological zonation in vege-
tative shoot apices. 2, Pteridophyte type with single apical cell. 3, Selaginella type
with 2-5 apical initials. 4, Cycas type. 5, Ginkgo type. 6, Crypto»}eria-Abies type.
7. Schematic representation of the histogen theory of angiosperms. 8, Opuntia
type. 9, Usual angiosperm type. (No.1, adapted from Esau. 1953; nos. 2-6. 8
and 9, adapted from Popham, 1952.)
Meristems 55
2. The Ginkgo type (Fig. 25, nb. ']) in which five meristematic zones can
be distinguished in ,t he apex. (a) The surface meristem, the cells of which
mostly divide anticlinalIy, but among which periclinal divisions also occur.
The peric1inal divisions are more frequent in the cells of- the summit.
rface meristem
Cambium-like
transitional zone
Flank meristem
Surface meristem
These cells are the apical initials. (b) The zone of central mother cells
which occurs in a median position below the surface layer. These cells are
large, polyhedral and they are irregularly arranged. They contain numer-
ous vacuoles and t he nuclei of many of them are large and the cell walls
are thick, particularly in the angles of the cells. The division of these cells
56 Plant Anatomy
which is cup-shaped, is found between the central mother cells and the rib
and flank meristems (Fig. 26, nos; 1, 2). The cambium-like transitional
zone differs from the other zones of the apical meristem in that its height
and diameter vary considerably during the plastochron, reaching a maxi-
mum development close to a developing primordium (Fahn et aL 1963).
FIG. 26. 1, Micrograph of a longitudinal section of the uPller portion of the vege-
tative shoot apex of the Dwarf Cavendish banana showing a two-layered tunica.
x 460. 2, Micrograph of a longitudinal section of the shoot apex of Coleus blumei
showing a four-layered tunica. :><: 450.
Mer/stems 59
From the point of view of activity (cell division) two zones, which are
parallel to the cyto-histological zones, are generally distinguished: (a) a
central apical zone which includes the initials of the tunica and of the cor-
pus and in which division is considered to occur rarely; and (b) a
peripheral zone to which much mitotic activity is ascribed.
Extreme views as to the activity of these two zones are held by
Plantefol (1947, 1948) and Buvat (1952) who, contrary to most investiga-
tors do not regard the central apical zone as having the role of cell-
producing cells during the vegetative development of the plant (Fig. 27,
no. 7), but as being active only when the vegetative apex becomes reproduc-
tive. They base their views mainly on the fact that in vegetative apices no
divisions are found in this region. Thus, according to the above authors
an inactive region (meristeme d' attente) is present in the shoot apex.
60 , Plant Anatomy
n leO!
primordium
\\1
7
FIG. 27. 1-6, Diagrams --of longitu-dinai sections 'of a-shoot '-apex with opposite
leaves showing the changes in shape and size of the apex during a plastochron.
In nos. 1 and 6 the area of the apex is minimal, while in no. 5 it is maximal. 7,
Diagram of the vegetative shoot apex after Buvat, showing the central inac-
tive region (meristeme d'attenle) and a peripheral zone of high mitotic 'activity
(anneau initial).
\
mother cells were demonstrated, With the use of microphotographs taken - .
with a cine camera of the surfaces of culture-grown shoot apices of several
plants, Ball (1960) came to the conclusion that all the cells in the surface
layer, including those at the summit of the apex, divide and do so quite fre-
quently. Also the application of radioactively-labelled precursors of DNA
(Partanen and Gifford, 1958; Gifford and Tepper, 1962a, b) did not reveal
the existence of an inactive zone in the shoot apex.
Branching of the shoot originates at the shoot apex. In many pterido-
phytes the branching is brought about by the equal division into two of
the single apical cell or group of apical cells, The resultant type of branch-
Meristems 61
REPRODUCT1VE APEX
The reproductive apex, which produces the flowers and bracts (Fig. 19,
no. 4; Fig. 20, nos. 1-4), usually develops from a vegetative apex, which
produces leaves and axillary buds. As stated by Philipson (1947, 1949), the
basic function of the vegetative apex is to promote longitudinal growth of
the axis and that of the reproductive apex is to produce a meristematic
envelope with large surface area from which the parts of a flower or
flowers develop .. This meristematic envelope is superimposed on a base
of parenchymatous tissue. Many investigators (Boke, 1947; Popham and
Chan, 1952; Wetmore et al., 1959; Fahn et al., 1963, and others) have
shown that the transition from vegetative to reproductive apex is ,gradual.
The first noticeable change' is the increase of mitotic activity on the
boundary between the central mother cell zone and the rib meristem zone.
Gradually this activity spreads into the central mother cell zone where the
Oellsthen become smaller and richer in protoplasm. In this way all the cells
above the rib meristemare added to the tunica, the cells of which are more
Dr iess isodiametric and are relatively small. Following these changes mi-
totic adivityand gro,!,th ceases, or almost so, in the cells of the rib meris-
tern and of the pith below it. Thus, in the apex a parenchymatous pith
mrrounded by meristematic cells develops (Fig. 28, no. I). Depending on
the species, the bracts, the axillary branches of the inflorescence and the
flowers themselves develop from these meristematic cells. An apex in this
sfage of development ceases to elongate in plants with capitula or single
flowers, and in other plants the rate of elongation is reduced. In certain
plants, such as banana, for example, an extensive and rapid elongation
takes place. In such apices a well-developed rib meristem can be observed,
and this meristem probably participates in the process of elongation.
These histological changes are no doubt accompanied by physiological
and biochemical changes. This may be demonstrated by the fact that
the dominance of the main apex, which suppresses the development of the
lateral buds, is lost with the production of the inflorescence.
In the embryo, within the seed, only the promeristem of the root or,
sometimes, an embryonic radicle may be seen at the base of the hypocotyl.
Only after the germination of the seed and with the development of the
62. Plant Anatomy
there is a single central initial or but a few initials in the root apex of the
Spermatopbyta (Fig. 30, no. 3; Fig. 31, no. I). Other investigators, how-
ever, such as'Clowes (1950; 1953, 1954, 1961), believe that there is a larger
group of initials in the median region of the root apex .
.Allen (1947), working on Pseudotsuga (Fig. 30, nos. 1,2), distinguished
a central group of permanent initials with three groups of temporary ini-
\
\
1 \ ,
\
\
I
a i .!
I i
'7
I
I"
\ \ ,., . I
,\ '~J"_}
\. I -' II
\.\'~.i},
"'l z ~\..-/
'1/
'K\rp-P'
s \ •
,, ,,,
J S
,
,.J, ,["
, , ,,,
,, ,,
,
,, ,
, ,,
,, ,
,
i i
\ '\ ,\
'lJ;~
J~ II
/
2
1./
tia[s (mother cells) on its periphery. He observed that the meristem of the
vascular cylinder developed from the first group of temporary initials, the
meristem of the cortey. from the second, and the columella from the third.
The columella is a group of cells that forms the longitudinal axis of the
root cap. In it the cells are arranged in longitudinal rows. Cells are added
to the root cap from the columella by pericJinal cell division on its peri-
phery. "Ihe protoderm was seen to develop from the young cortex. Accord-
ing to Wilcox's work on Abies procera(l954) there appear to be two groups
of temporary initials, one of which gives rise to the central cylinder and
the other to the columella, from which the root cap and cortex develop.
Although research on the development of the histogens in the shoot
apex has proved that they do not exist, many authors still use the terms
'dermatogen-meristem of the epidermis,periblem-meristem of the cortex,
and plerome-meristem of the central cylinder, in connection with footS.
However, the terms, as used today, have a somewhat different meaning to
those as ,,,od by Ha\\stdn. 1'he mothe! celh of the ';a!ious tissue systems
of the root are replaced, at relatively long intervals, by new cells which are
~derived from the common permanent initia1s. In many cases more than one
tissue develops from a group of mother cells (temporary initials) and so
it is desirable to use, wherever possible, instead of histogens, the terms
protoderm, meris/em of the cortex and meristem of the ,vascular cylinder
for th~ meristems that are derived from the promeristem, i.e. from the zone
of permanent and temporary initials of the root apex.
Adapting Guttenberg's view the meristems of the different tissue systems
can be traced, in the root apex, at various distances from the central cells
(i.e. the permanent initials). In some species the initials (temporary) of
the various tissue systems- are already discrete immediately adjace\\t to
the- central cells, i.e. closed type. These initials represent those of the
vascular cylinder, the cortex and the common initials of the protoderm
and root cap, e.g. as in Brassica, or the separate initials of the protoderm
and the root cap, e.g. as in ":ea and Triticum (Fig. 31, no. I). The special
initials of the root cap were termed calyptrogen by Ianczewski (1874). In
other species the meristems of the different tissue systems finally become
distinct only some distance away from the central cells, i.e. open type.
In this type. common initials for the cortex meristem, root cap and proto-
derm (e.g. Helianthus, Fig. 30, no. 3) or for the meristems of all the tissue
systems (e.g. Allium) appear on the periphery of the central cells. The
importance of the above types is queried as they have been observed to
occur in the roots of a single species.
Recent research (Jensen, 1957; Clowes, 1958 a, b, 1961; Jensen and
Kavaljian, 1958) on the promeristem of the root apex has shown that the
cells of the central part of the promeristem have very low mitotic activity.
This part is termed quiescent centre (Fig. 3 I. no. 2).
It is also necessary to mention here the Korper-Kappe theory (Fig. 31,
66 , Plant Anatomy
no. 3) which was put forward by Schiiepp (1917), This theory, similar to
the tunica-corpus theory of the shoot apex, is based on differences in the
planes of cell division. According to 'the Karper-Kappe theory the cells
FIG. 31. 1, Longitudinal section of the root tip of Triticum vulgare. 2, Diagram 'of
\
the foot tip of Allium cepa showing, by means of shading, the gradation of mitotic
activity in different zones; the most active zone is the most darkly shaded.- 3, Dia-
gram illustrating the Korper-Kappe pattern of the root apex of Zea mays. (No.1,
adapted from Schade and Guttenberg, 1951; no. 2, adapted from Jensen and Ka-
valjian, 1958; no. 3, adapted from Clowes, 1961.)
Intercalary rnerislems
Intercalary meristems are parts of the apical meristem that become
separated from the apex during the growth of the plant by regions of more
mature tissues (Fig. 24, no. I). In stems with intercalary meristems the
nodes mature earlier and the intercalary meristems are localized in the
:internodes. At first, the entire internode is meristemotic, but with further
development part of the internode matures faster and so various stages
'of-development can be found in the internode. In most plants with inter-
calary meristems the region with the cells showing the least degree of differ-
entiation is at.the base of the internode, but this region may sometimes
be found in the middle or at the ·top. of the internode (Eames and Mac-
Daniels, 1947). In more mature stages the intercalary meristems are sepa-
rated from each other by fully.matured tissues and they, themselves, are
penetrated 'by vascular bundles that consist of proto xylem and proto-
phloem. Finally, the intercalary meristems undergo complete differentiation
and so disappear.
The best-known examples of intercalary meristems are those in the
stems' of grasses, some other monocotyledons, some species of the Caryo-
phyllaceae, and articulated species of the Chenopodiaceae and Equisetum.
Intercalary meristems also occur in the peduncles of the inflorescences of
certain plants, in the leaves of many monocotyledons (e.g. in the Grami-
neae and Iris), Pinaceae and others. The gynophore of Arachis (ground nut)
also elongates as a result of the activity of an intercalary meristem (Jacobs,
1947).
Internodal elongation in many grasses is brought about by an intercalary
meristem, the cells of which divide to form parallel series of cells and which
is, therefore, termed rib meristem (not to be confused with the rib meristem
of apices). The enlargement of the derivatives of this meristem also con-
tributes to the elongation of the internode (Milt"nyi, 1931; Kaufman,
1959). Milt"nyi states that the intercalary meristems of grass internodes
have no fixed position, but that their position is altered as the internode
elongates. At first the intercalary meristematic activity occurs throughout
68. Plant Anatomy
the internode, as has already been mentioned above; but after the develop·
ment of the _lacunae, that are present in most grasses, this activity becomes
restricted to the peripheral ground tissue in the proximity of and above the
nodal plate, i.e. in the joint regions. The meristematic activity of the joints
.can be reactivated even in mature stems. It has been shown that~ in those
parts of plants that have already undergone a certain degree of differentia-
tion, such as in flowers, fruits, leaves and stems without special intercalary
meristems, the cells continue to divide for a long time after they have been
derived from the apical meristem. This type of growth can also be con-
sidered as intercalary, but the growth regions are less well defined.
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SATINA, S. and BLAKESLEE, A. F. 1943. Periclinal chimeras in Datura in relation to the
development of the carpe]. Amer. Jour. Bot. 30: 453--462.
72 Plant Anatomy
SATlNA, S., BLAKESLEE, A. F. and AVERY, A. G. 1940, Demonstration of the three germ
layers in the shoot apex of Datura by means of induced polyploidy in periciinal
chimeras',Amer. Jour. Bot. 27: 895-905. ~
SCHADE, C. and GUTTENBERG, H. v. 1951. Ober die Entwicklung des Wurzelvegetations-
punktes der Monokotyledonen. P/anta 40: 170-198.
SCHMIDT, A. 1924. Histologische Studien an phanerogamen Vegetationspunkten. Bot.
Arch. 8: 345-404.
SCHOEPP, O. 1917. Untersuchungen tiber Wachstum und Formwechsel von Vegetation-
spunkten. Jb. Wiss. Bot. 57: 17-79.
SCHOEPP,.a. 1926. Nferisteme. Gebr. Borntraeger,.Berlin.
STE'RLlNG, C. 1945: Growth and vascular development in the shoot apex of Sequoia
sempervirens (Lamb.) EndL I. Structure and growth of the shoot apex. Amer. Jour.
Bot. 32: 118~-126.
STERLING, C. 1946. Organization of the shoot of Pseudotsuga taxi/alia (Lamb.) Britt.
I. Structure of the shoot apex. Amer. JOur. Bot. 33: 742-760.
THIELKE, C. 1948. Beitraege zur Entwicklungsgeschichte und zur Physiologie pana~
schierter Blatter. Planta 36: 2-33.
THIELKE, C. 1954. Die histologische Struktur des Sprossvegetationskegels einiger
Commelinaceen unter Beriicksichtigung panaschierter Formen. Planta 44: 18-74.
THIELKE, C. 1955. Die Struktur des Vegetationskegels einer sektorial panaschierten
Hemerocallis /u/va. Ber. Dtsch. Bot. Ges. 68: 233-238.
THIELKE, C. 1957. Chimaren mit periklinal spaJtender Oberhaut am Scheitei. Acta
Soc. Bot. P%n. 26: 247-253.
TOLBERT, R, G. J961. A seasonal study of the vegetative shoot apex and the pattern of
pith development in Hibiscus syriacus. Amer. Jour. Bot. 48: 249-255, .
WARDLAW, C. W. 1957. On the organization and react(vity of the shoot apex in vascular
plants. Amer. Jour. Bot. 44: 176-185. ...
WETMORE, R. H~ ,1954: The"use of in vitro cultures in the investigation of growth and
differentiation in vascular plants. "Abnormal and Pathological Plant, Growth, Brook-
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WETMORE, R. H . .1956: Growth and development in the shoot system of plants. In:
Cellular Mechanism in Differentiation and Growth. Princeton Unlv. Press: 173-190.
WETMORE, R. R., GIFFORD, E. M., Jr. and GREEN, M. C. 1959. Development of vege-
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"Anima/s. Proc.Con/. Photoperiodism, Ocr. 29 - Nov. 2, 1957. A.A.A.S., Washington
255-273. I
WHITE, P. R, 1946. Plant tissue cultures. IJ. Bot, Rev. 12: 521-529.
WILCOX, H. 1954. Primary organization of active and 'dormant roots of ,noble fir,
Abies procera. Amer. Jour. Bot. 41: 812-821.
WOLFF, C. F. 1759. Theoria Generationis. W. Engelmann, Leipzig.
MATURE TISSUES
IT'IS the usual practice to divide the plant body into different tissues, but
with the accumulation of knowledge of tissue structure it becomes more
and more' difficult to give an exact definition of a tissue. The accepted
definition of a tissue is a group of cells with common origin, structure and
,function. However, this definition is not suitable for all cases. When dealing
with the tissues of higher plants a more flexible definition is necessary. If we
could base our descriptions on elements, i,e. the individual types of cells,
it would be easier to define these units. However, difficulties would also
arise from such a classification because of the transitional forms present.
, _For the sake of convenience, in this book the anatomical and histological
, structure of plants will be discussed on the basis of tissue classification,
Today a complex of cells of common origin is generally understood by the
~terin tissue. A tissue may consist of cells of different form and even differ-
-ent function, but in tissues consisting of different cell types the cell com-
posiiion is always ihe same. '
. The tissues in the plant body are classified on ihe [ollowing'bases: accord-
ing to their position in the plant; the cell types of which they consist; their
function; the manner and place of their origin; and their stage of develop-
ment. Tissues are also divided into simple and complex tissues according to
the number of cell types that they comprise, A simple tissue is homogeneoUs
and consists or'only one type of cell, while a complex tissue is heierogeneous
and comprises two or more cell types. -Parenchyma, collenchyma, scleren-
chyma and latieifers are examples of simple tissues. All other tissues are
complex. Complex tissues, however, may contain parenchyma, scleren-
chyma or' other elements. Examples of complex tissues are xylem and
'Iphloem,
CHAPTER 4
PARENCHYMA
THE parenchyma of the primary plant body develops from the ground
meristem, and that connected with the vascular elements, from the pro-
cambium or cambium. The phellogen in many plants also produces paren-
chyma (the phelloderm). Parenchyma consists of living cells of differing
shape and with differing physiological functions.
74 Plant Anatomy
Many parenchyma cells are polyhedral and their diameter.in the differ-
ent planes is more or less equal (Fig. 33, no. 1) but many other shapes are
common. Elongated parenchyma cells are found in the palisade tissue of
the leaf, in the medullary rays, etc.; lobed cells are found in spongy'
mesophyll and in the palisade parenchyma/of Lilium (Fig. 89, no. I);
and in the mesophyll of the Xanthorrhoeac;,ae the parenchyma cells have
folds or projections (Fahn, 1954). Stellate parenchyma cells ar"found in
the stems of plants with well developed air spaces, such is Scirpus and
Juncus, for example (Fig. 32, no. I). In Juncus, according'to Geesteranus
(1941), the stellate pith cells differentiate ontogenetically from a mass of
cubo-octahedral cells which are arranged in vertical rows on their hexagonal
faces. The mechanical stretching of. the pith, which is mainly in a radiiil
direction as a result of the growth of the surrounding tissues, as, well as
the special arrangement of the intercellular spaces, causes the development
of the characteristic arms, of the cells.
The medium-sized polyhedral parenchyma cells usually have fourteen
faces (Higinbotham, 1942; Lewis, 1944; Matzke, 1946; Hulbary, 1948); the
number of sides is less in the smaller cells and greater in'the larger cells \
(Marvin, 1944). The number and size of the in{ercellula(spaces also affects
the number of faces of the polyhedron as the presence of intercellular spaces
reduces the planes of contact between the cells. The polyhedral shape
of the parenchyma cells is the result of numerous factors among which are
pressure and surface tension (see Chapter 2).
Mature parenchymatous tissue may be tightly packed'and without inter-
cellular spaces or it may have a well developed system of intercellular
spaces. For example, the parenchyma of the endosperm of most seeds is
devoid, or almost so, of intercellular spaces, while in the stems and leaves
of hydrophytes the intercellular spaces reach maximal development.
Parenchyma 75
The development of intercellular spaces is either schizogenous or /ysi-
genous. The schizogenous development of .an intercellular space .t akes
place as follows: at the time when the primary wall is formed between two
. (Fig
new cells the middle lamella between the two new cell walls .
. 9, nos.
Extraxyla
fibres
7-10) comes into contact only with the.primary waiL of the mother cell and
not with the middle lamella between it. and the neighbouring cells. Thus,
a small space develops where the new middle lamella coloes into contact
with the mother cell wall. That portion of the mother cell wall opposite
this small space disintegrates and so forms the intercellular space which
can be enlarged by the formation of a similar space in the neighbouring
.'
cell. The intercellular space is lined with the substance of theimiddle la-
mella. These intercellular spaces may be further enlarged by divisions of
the surrounding cells in a plane perpendicular to the circumference of the
space. The resin ducts of the Coniferae, the secretory ducts of the Compo-
sitae, Umbelliferae, Hedera helix (Fig. 33, nos. 2-5) and other species,
are formed schizogenously. Lysigenous intercellular spaces are formed by
the disintegration of entire cells. Examples oflysigenous intercellular spaces
are the large spaces in water plants and in the roots of some monocotyle-
dons, and also the essential oil cavities in Eucalyptus, Citrus (Fig. 34, nos.
I -6) and Gossypium.
Parenchyma 77
Most parenchyma cells, e.g. those that contain chloroplasts and those
that act as storage cells, usuall)' have thin primary walls, but parenchyma
cells with thick primary walls also exist. Certain parenchyma storage cells,
as, for example, the endosperm of Phoenix(Fig. 212, no.,3), Diospyro"
Coffea and Asparagus, have very thick walls in which hemicellulose, which
Oil
FIG. 34. Lysigenolts development of an essential oil cavity in the peel of the
fruit of Citrus. 1-4, Sections at right-angles to the surface of the peel. 5 and 6,
Sections parallel to the surface of the peel. (Adapted from Martinet, 1871.)
serves as the reserve substance, accumulates. The walls of these cells gra-
dually become thinner during germination. Parenchyma cells with rela-
tively thick and lignifitd secondary walls are common, especially in the
secondary xylem.
The internal structure of the parenchyma cell varies according to its
function. Parenchyma cells which take part in photosynthesis contain
chloroplasts and then the tissue they form is termed chlorenchyma. In the
photosynthetic parenchyma there are usually many or large vacuoles.
Certain parenchyma cells contain leucoplasts. Parenchyma cells may serve
to store different res<rve materials which may be found in solution in the
vacuoles, or in the form of solid particles or liquid in the cytoplasm. Sugars
78 Plant Anatomy
References
EAMES, A. J. and MACDANIELS, L. H. 1947. ln~roduclion 10 Plant Anatomy. 2nd ed.
McGraw-Hill, New York-London.
ESAU, K. 1953. Plant Anatomy. John Wiley, New York.
FAHN, A. 1954. The anatom.ical structure of Xanthorrhoeaceae Dumort. Jour. Linn.
Soc. London, Bot. 55: 158-184.
Parenchyma 79
GEESTERANUS, R. A. M. 1941. On the development of the stellate form of the pith
cells of Juncus species. Proc. Nederl. Akad. Wetensch. 44: 489-501, 648-653.
HIGINBOTHAM, N. 1942. The three-dimensional shapeb'of undifferentiated cells in the
petiole of Angiopleris evecta. Amer. Jour. Bot. 29: 851-858.
HULBARY, R. L. 1948. Three-dimensional cell shape in the tuberous roots of Asparagus
and in the leaf of Rhoeo. Amer. Jour. Bot. 35: 558-566.
LEWIS, F. T. 1944. The geometry of growth and cell division in columnar parenchyma.
Amer. Jour. Bot. 31: 619-629.
MARTINET, J. B. 1871. Organes de secretion des vegetaux. Theses. Facult6 des Sciences
de Paris.
MARVIN, J. W. 1944. Cell shape and cell volume relations in the pith of Eupatorium
per/oUatum L. Amer. Jour. Bot. 31: 208-218.
MATZKE, E." B. 1946. The three-dimensional shape of bubbles of foam-an analysis
of the role of surface forces in three-dimensional cell shape determination. Amer.
\ Jour. Bot. 33: 58-80.
METCALFE, c._R. and CHALK, L. 1950. Anatomy o/the Dicotyledons. Clarendon Press,
, Oxford.
Sl'i.RUCH, A. 1939. Da~ trophische Parenchym. R. Exkretion~gewebe.ln: K. Lin-s.bauer.
Handbuch der Pjlanzenanatomie. Band 4, Lief. 38. Gebr. Borntraeger, Berliri.
CHAPTER 5
COLLENCHYMA
THE supporting tissues of the plant, i.e. the collenchyma and the scleren-
chyma, a;e designated from the functional point of view by the term
s(ereame (Haberlandt, 1918).
Ontogenetically collenchyma develops from elongated cells which re-
semble procambium and which appear in the very early stages of the
differentiation of the meristem. Collenchyma con,ists of living, slightly
elongated cells which, generally, have unevenly thickened wall,. Co\\en-
chyma functions as a supporting tissue in young growing organs and, in
herbaceous plants, even in mature organs (Ambronn, 1881; Miiller, 1890;
Anderson, 1927; Esau, 1936). Collenchyma is plastic and it stretches
irreversibly with the growth of the organ in which it occurs. Mature
collenchyma is less plastic, harder and more' brittle than young collen-'
chyma. There is a physiological and morphological relationship between
collenchyma and parenchyma and, in places where the two tissues occur
side by side, transitional forms can be found between typical collenchyma
and typical parenchyma. I
Collenchyma, like parenchyma, may contain chloroplasts. Chloroplasts
occur in larger numbers in less specialized collenchyma cells which re-
semble parenchyma and in smaller numbers, or not at all, in'the most
specialized collerichyma, which consists of elongated· narrow cells. Col'
lenchyma cells may also contain tannins. I
In a freshly made cross-section of collenchyma the cell w·alls appear
nacre-like. It has been seen in plants exposed to wind that the walls of
collenchyma cells become thicker. Collenchyma mdY become lignified and
the walls may thicken thus resulting in the fonriiition of sclerenchYma.
The thickened walls of the collenchyma may, secondarily, beCOMe thin
and then the cells may again become meristematic and start to divide as,
for instance, where the phellogen is formed in"collenchyma tissue. Primary
pit fields can be distinguished in the walls of collenchyma cells.
appears in those parts of the cell wall that face intercellular spaces. Such
collenchyma can be seen in the petioles of species of the Compositae,
Salvia, Malva, Althaea and Asclepias. However, as IDtercellular spaces
can be distinguished in other types of collenchyma, this does not seem
to be a valid criterion for the classification of a special type. Annular col-
lenchyma (Fig. 35, no. 3) is described as being collenchyma in which the
//
References
AMBRONN, H. 1881. (Ther die Entwickelungsgeschichte und die mechanischen Eigen-
schaften de's Collenchyms. Ein Beitrag zur Kenntnis des me~hanischen Gewebe-
systems. Iahrb. Wiss. Bot. 12: 473-":'541.
ANDERSON, D. 1927. Dber _die,Struktur der Kollenchyrnzellwand auf Grund mikro-
chemischer Untersuchungen. Sitzber. Akad. Wiss.' Wien, Math.-Naturw. Kl. 136:
429-440.
BEER, M. and SETTERFIELD, G. 1958. Fine structure in thickened primary walls of
collenchyma_cells of celery petioles. Amer. Jour.Bot. 45: 571-580.' "-...
DUCHAIGNE, A. 1955. Les divers types de collenchymes chez les Dicotyl6dones: leur
ontogenie et leur lignification. Ann. Sci. Nat., Bot. Ser. II, 16: 455-479. ..
ESAU, K. 1936. Ontogeny and structure of collenchyma and of vascular tissues in
celery petioles. Hilgardia 10: 431-476.
ESAU, K. 1953. Plant Anatomy. John Wiley, New York.
FOSTER, A. S. 1950. Practical Plant Anatomy. D. Van Nostrand, New York-London.
HABERLANDT, G. 1918. Physiologische Pjianzenanatomie. W. Engelmann, Leipzig.
MAJUMDAR, G. P. and PRESTON, R. D. 1941. The fine structure of coIlenchyma cells in
Heracleum sphondylium L. Proc. Roy'. Soc. (London) B 130: 201-217.
MULLER, C. 1890. Ein Beitrag zur Kenntnis der Formen des Collenchyms. Ber. DIsch.
Bot. Ges. 8: 150-166.
PRESTON, R. D. and DUCKWORTH, R. B. 1946. The fine structure of the walls of col·
lenchyma cells in Petasites vulgariS L. Proc. Leeds Phil. Lit. Soc. 4 (5) 343-351.
ROELOFSEN, P. A. 1959. The Plant Cell· Wall, Encyclopedia of Plant Anatomy, Gebr.
Borntraeger, Berlin. ~.
VAN FLEET, D. S. 1950. A comparison of histochemical and anatomical characteristics
of the"hypodermis with the endodermis in vascular plants. Amer., Jour. Bot. 37:
721-725.
Coilenchyma 83
SCLERENCHYMA
SCLERENCHYMA is a tissue composed of cells with thickened secondary
ceiL walls, lignified or not, whose principal function is support and some-
times protection. Sclerenchyma cells exhibit elastic properties unlike col-
lenchyma cells which exhibit plastic properties.
\ Sclerenchyma cells may differ in shape, structure, origin and develop-
ment. Many transitional f9rms exist between the various cell shapes and
thus it is difficult to classify the different types of sclerenchyma. Generally,
sclerenchyma is divided into fibres and sclereids. Fibres are usually defined
as long cells and sclereids as short cells. This definition is not sufficient,
as very long sclereids exist and relatively short fibres can be found. At-
tempts to describe the differences between fibres and sclereids were made
on the basis of the presence of pits which are more numerous in sclereids,
as well as on the origin of the elements. Sclereids develop from paren-
chyma cells whose walls becom~ secondarily thickened, whereas fibres de-
velop from meristematic cells and so they are determined from their
origin. Other research, 'however, has shown that these definitions are also
insufficient due to their inconstancies. ,Not only is it difficult to distinguish
between the different types of sclerenchyma cells because of the existing
transitional forms, but it is also somewhat difficult to distinguish between
sclerenchyma and parenchyma as there are parenchyma cells with thick
secondary walls, such as the xylem parenchyma.
Fibres
Fibres occur in different parts of the plant body. They may occur
singly as idioblasts (e.g. in the leaflets of Cyeas), but more usually they
form bands or a network or an uninterrupted hollow cylinder (Fig. 32,
no. 2). Fibres are most commonly found among the vascular tissues but
in many plants they are also well developed in the ground tissues. Accord-
ing to their position in the plant body, fibres are classified into two basic
types - xylary and exlraxylary fibres.
Xylary fibres constituto an integral part of the xylem and they develop
from the same meristematic tissues as do the other xylem elements. These
fibres are of varied shape in spite of their common origin. Two main types
of xylary fibres, i.e. libriform fibres and fibre-I,acheids, are distinguished
86 . Plant Anatomy
on the basis of wall thickness and type and amount of pits (Fig. 37, nos.
1-3). Libriform fibres resemble phloem fibres (liber=inner bark) and they
are usually longer than the tracheids of the plant in which they occur.
These fibres have extremely thick walls and simple pits. Fibre-tracheids
are forms intermediate between tracheids and libriform fibres. Their walls
are of medium thickness - not as thick as those of the libriform fibres but
thicker than those of the tracheids. The pits in fibre-tracheids are bordered
but their pit chambers are smaller than those oftracheids. In fibre-trac]l.eids
and sometimes also in libriform fibres the pit canal is elongated and·.the
inner pit aperture usually becomes slit-like (Fig. 18, no. I) as a result of
the thickening of the wall. In fibre-tracheids, therefore, the length of the
pit aperture usually exceeds the diameter of the pit chamber. In both libri-
form fibres and fibre-tracheids, the inner pit apertures of a pit-pair are
usually at right-angles to each other.
Another type of fibre present in the secondary xylem of dicotyledons
is the gelatinous or mucilaginous fibre (Fig. 138, no. I). In such fibres·the
innermost layer of the secondary wall contains much ",-cellulose and is
poor in lignin. This layer, termed the "G layer", absorbs much water and
may swell so as to fill the entire lumen of the fibre. On drying, these
layers shrink irreversibly. (Dadswdl and Wardrop 1955). The G-layers
wereJound to be relatively porous and. less/~ompact than the adjacent'
outer layers (Cote and Day, 1962). Gelatin6us fibres are characteristic. of
tension wood.
Extraxylary fibres occur elsewhere in the plant other than among the
xylem elements. They occur, for instance, in· the cortex or they may be
closely related to the phloem elements. In the stems of many monocoty-
ledons, the extraxylary fibres occur in an uninterrupted hollow cylinder
in the, ground tissue, and they may be situated at various·distances inside
the epidermis and may even -surround the outermost /
vascular bundles.
Commonly in the monocotyledons, the fibres form sheaths around the
vascular bundles. Such fibres develop partly from the procambium and
partly from the ground tissue (Esau, 1943).
In the stems of climbing and certain other dicotyledonous plants, such
as Aristolochia and Cucurbita, fibres are found on the inside of the inner- \.
most cortical layer and on the periphery of the central cylinder (Fig. 71).
It was thought that such fibres were not dejlelopmentally connected with
the phloem and so they were termed, by many workers, pericyclic fibres.
However, Esau (1938, 1943), Blyth (1958) and others, as a result of onto-
genetic studies on these fibres in many plants (Nicotiana, Linum, Corcho,rus,
Ricinus, Nerium, etc.), came to the conclusion that these fibres develop
from the procambium and so constitute part of the primary phloem.
The above classification into xylary and extraxylary fibres is not always
applicable as there are fibres~uch as the septate fibres (Fig. 37, no. 4),
which are found in the xylem and the phloem even of the same species,
Sclerenchyma 87
e.g. in Vitis where they are very common (Vestal and Vestal, 1940; Spack-
man and Swamy, 1949). These fibres are characterized by the presence of
internal septa and, usually, of a living protoplast. The internal septa are
formed by the inner lamellae of the secondary wali and middle lamellae.
The latter, however, do not connect with the middle laniella of the entire
fibre. Septate fibres contain starch, oils, resins and sometimes crystals of
calcium oxalate and, therefore, they are thought to have a storage function.
Mature fibres have well developed, usually lignified secondary walls
which are sometimes so thick as to obscure the lumen of the fibre. Lamellae
can be distinguished in these walls. In Linum, for example, each lamella is
0·1-0·2 /" thick as seen in a cross-section of the fibre.
\ Mention should be made here of those elongated cells that sometimes
occur in the secondary xylem and whose secondary walls are equal in
thickness to those of the xylem parenchyma. These cells contain living
protoplasts and, according to Haberlandt (1918), they were termed by
Sanio, substitute fibres (ErsatzJasern). It appears, however, that these cells
should be included among the xylem parenchyma and that they should
not be confused with the living libriform fibres and fibre-tracheids (Fahn
and Arnon, 1963; Fahn and Leshem, 1963) which are discussed later in
this chapter.
Fibres are usually very lo~g and narrow cells with tapered, and some-
times branched, ends. The length of fibres varies very greatly and generally
extraxylary fibres are longer than xylary fibres. In Cannabis sativa (hemp)
the fibres are 0·5-5·5 cm long, in Linum usitatissimum (flax), from 0·8 to
6·9 em, and in Boehmeria nivea (ramie) Aldaba (1927) showed, by means of
a special maceration, that the fibres may reach a length of 55 cm. These
ramie fibres are among the longest cells in the higher plants.
DEVELOPMENT OF FIBRES
j
FIG. 36. Stages in the ontogeny of the extraxylary fibres of Boehmeria nivea. I,
Elongation of the upper end, showing a series of young lamellae one within the
other; each lamella is open at its tip. 2 and 3. Development of chamb:::rs by inner
lamellae. 4-8, Diagrammatic representation of the differentiation of a phloem
fibre, in which the centripetal development of the lamellae of the secondary wall is
shown. 8, Widened chambers, formed by the relatively increased growth o~ inner
lamellae which are present only along part of the fibre. (Adapted from Aldaba,
1927.)
Sclerenchyma
2 3 4
FIG. 37. 1-3, Tips of elements from the secondary xylem of Quercus ithaburensis.
1, Tracheid. 2, Fibre-tracheid. 3, Libriform fibre. 4, Septate fibre of Vitis. 5,
Isolated sc1ereids from the leaf blade of Olea. (No.5, adapted from Arzee,
1953a.)
found that this process is gradual so that new lamellae of the secondary
wall are added centripetally in the form of cylinders which are open at
both ends. At the same time the first-formed lamellae continue to elongate
towards the fibre ends which they reach only when the fibre ceases to elon-
gate (Fig. 36, nos. 1-8). According to Kundu and Sen (1960) the upper
ends of ramie fibres continue to grow for a longer period than the basal
ends. Sometimes not all the lamellae reach the actual fibre end and in some
fibres chambers may be formed in the terminal portions by the ingrowth,
..
9Q Plant Anatomy
\
Sclerenchyma 91
toward the cell lumen, of these lamellae. The lamellae of the primary phloem
fibres, of .at least·of the immature fibres; are often not strongly attached
one. to another. This feature is easily demonstrated during the cross-sec-
tioning of such material when the different layers become torn one from
the other. In short fibres, such as those found in Agave, Sansevieria and
Musa textilis, whose total length is not more than a rew millimetres, all
portions of the cell wall grow at the same rate.
Differences existin the manner of growth of the fibres in the primary
body and of those in the secondary body. The initials of the primary fibres
appear early, before the organ in which they occur has elongated, and so
they may grow in length symplastically together with the neighbouring cells
, which continue to divide. The symplastic growth is augmented by intrusive
and gliding growth of the ends which thus penetrate between the surround-
ing cells. The initials of the secondary fibres develop in organs that have
ceased to elongate and therefore the growth of secondary fibres can be
intrusive only. This is apparently the reason why the primary fibres are
usually longer than the secondary fibres of the same plant. Thus it was
found in ramie that the average length of the primary phloem fibres is
164·5 mm while that of the secondary phloem fibres is 15·5 mm.
FIBRE PROTOPLASTS
As has been mentioned above, the xylary fibres differ in shape, size,
thickness of wall, type and amount of pits. It is assumed, from the evo-
lutionary point of view, that fibres have developed from tracheids. This
assumption is supported by the fact that many transitionai forms between
these two types of elements are found in some angiosperms, as, for example,
Quercus spp. From the many transitional forms that have been distinguished
it appears that the following changes have taken place during the
course of the evolution of fibres from tracheids. The wall has become
thickened, the number of pits and the size ofthepit chamber has been reduced
leading to the eventual disappearance of the bordered' pit, and the cells
have become shortened. Thisassumed shortening of the fib'~es refers'to the
shortening of the initials of the fibres in the cambium and not to the mature
fibres. In the mature tissues of one plant, the libriform fibres are usually
longer than the tracheids, and this increased length is secondary and is the
result of the additional growth of the ends of the fibres.
I
The'term fibre, as used in industry, does not generally have the same\
meaning as that defined by botanists. For instance, the- commercial fibres
of Linum, Boehmeria and Corchorus are, inreality, a bundle' of fibres and
those from monocotyledonous leaves, such as from Agave, Musa textilis,
and others, are usually the vascular bundles with the surrounding sheaths
of fibres. From some plants the commercial fibres comprise the vascular
system of the root, e.g. Muhlenbergia, Of of the entire plant, e.g. Tillandsia.
The commercial fibres of Gossypium (cotton) are the epidermal hairs of
the seeds. Kapok fibres are hairs produced on the inner surface of the
capsule of Ceiba penlandra.
Commercial fibres are divided into two types-hard fibres and soft
fibres. Hard fibres are those which have a high lignin content in the walls,
Sclerenchyma 93
and are of a stiff texture. Hard fibres are obtained from monocotyledons.
Soft fibres mayor may not contain lignin, they are flexible and elastic,
and are of dicotyledonous origin. The best-known plants from which hard
ONTOGENY OF SCLEREIDS
FIG. 41. Sc1ereids in the leaf blade of Olea e(Jyopaea. 1, Portion of a cross-section
of the blade in which parts of the sclereids (darkly stained) can be seen. X 160. 2.
Portion of a relatively thick, cleared cross-section of the leaf blade photographed
in polari:l:ed light in which the sclereids appear white. X 95. 3, Surface view of
portion of a cleared leaf, photographed in polarized light, showing the arrange-
ment Qfthe sclereids in the sPongy parenchyma. X 110. (From Arzee, 1953a.)
98, Plant Anatomy
the following histogenetic facts have been realized, In all the above exam-
ples the scleroids develop from small initials with thin walls which, already
in the early stages of development, begin to branch arid so acquire the
form of the mature sclereid. The branches or projections of the sclereid
penetrate into the intercellular spaces,but intrusive growth of these branches,
between the joined walls of neighbouring cells, is also common (Fig. 42,
nos. 1-3). The degree of pitting in these sclereids is not constant. .
Sclereids are usually described as non-living cells when 'mature, but it
has been seen that the protoplasts may remain viable throughout the life
of the organ in which the sclereids are found. In non-deciduous leaves and
in certain stems the life of the sclereids may sometimes be 4-5 years (Puch-
. inger, 1923). The protoplast in,the stone cells in the fruits of the pear and
quince also remains alive for relatively long periods. According to Alexan-
drovand Djaparidze (1927), during the ripening of the quince fruit, the
stone cells undergo a process of delignification, whiCh they believe is an
indication of the enzymatic activity of the protoplast of the stone cell
itself.
The structure of the fibre wall has been investigated comparatively tho- \
roughly and emphasis has.been laid on the, ~(ll structure offibresthat are
of economic value; Attention has also' been paid to the ontogenetic and
phylogenetic development of fibres, Cell growth, especially intrusive
growth, can be well studied in the course of nbre development.
As was mentioned in the chapter dealing with collenchyma, collenchyma
cells may often become sclerified during the maturation of the organs in
which they occur. This fact emphasizes the "iew of tire close reiatiorrs.lrip
between' these two tissues.
Because of the great variability in the form of nbres and because of the
existence of many transitional forms, fibres serve as favourable material
for the study of the evolution of an element or part of it, as, for instance,
the evolution of the pit.
,Substitute and septate fibres have, during the course of evolution, be-
come strikingly different from the typical fibre form and. should, actually,
be classified as parenchyma cells with secondarily thickened JaIls. A sub-
stitute fibre resembles an elongated parenchyma cell and a septate fibre a
longitudinal series of parenchyma cells derived from a single mother cell
FIG. 42. Stages of development of sclereids in the leaf blade of Olea europaea. 1,
Portion of a cross-section of a young leaf in which a sc1ereid initial can be dis-
tinguished in the as yet single row of palisade cells. x 370. 2, Portion of cross-sec-
tion of a leaf blade, showing the intrusive growth of an arm of a developing scle-
reid. x 940. 3, Portion of a section cut parallel to the blade surface in which a
scJereid initial can be seen. x 210. (From Arzee, 1953b.)
Sclerenchyma
99
100 Plant Anatomy
in which secondary wall lamellae develop before the cell divisions are com-
~~. . I
The libriform fibres and the fibre-tracheids have, till recently, generally
been described as non-living cells devoid of protoplasts and were regarded
as having only mechanical function or, at the most, as playing a small role
in water conduction in addition to the tracheary elements. However. it
now appears, in the light of recent research, that the libriform fibres and·
even fibre-tracheids of the sap wood of many woody plants contain living
protoplasts. Therefore we should begin to consider fibres not only as sup-
porting elements but also as elements that doubtless fulfill various other
important physiological functions. This aspect, the investigation of which
has been initiated in our laboratory, awaits still further research.
It is possible that the retention of living protoplasts in fibres is more
characteristic for certain life forms (e.g. shrubs and subshrubs) or for
woody plants of certain habitats, such asxeric ones. The evolutionary and
ecological investigation of these assumptions may possibly bring to light
some interesting results. \
It is also worth mentioning that the appearance' of the living protoplasts
in libriform fibres and fibre-tracheids represents a further example of the
indistinct.limits between the various elements·that form the highly differ'
entiated tissues of the higher plant. body:' This, together with similar
phenomena, are of grearimportance in oUf' understanding of the evolution
of the various elements.
The appearance ofidioblastic sclereids in the leaves of pla'nts that belong
to diversified taxonomic and ecological groups makes it difficult to under-
stand both their evolutionary and functional significance.
References /
Al.DABA, V. C. 1927. The structure 'arid development of the cell'wall in plants.'l. Bast
fibers of Boehmeria and Linum. Amer. Jou.r. Bot. 14: 16-24.
ALEXANDROV, W. G. and DJAPARIDZE, L. 1. 1927. Ubef das Entholzen und Verholzen
der Zetlhaut. Pianta 4: 467--475. .
ARZEE, T. 1953a. Morphology and ontogeny of foliar sc1ereids in Olea europaea. I.
Distribution and structure. Amer. Jour. Bot. 40: 680-687."'" I
ARZEE, T. 1953b. Morphology and ontogeny of foliar sclereids in Olea europaea. II.
Ontogeny. Amer. JOllY. Bot. 40: 745-752.
BAILEY, 1. W. 1953. Evolution of the tracheary tissue of land plants. Amer. JOllr. Bot,
40: 4-8.
BLOCH, R. 1944. Developmental potency, differentiation and pattern in meristems of
Monstera deliciosa. Amer. Jour. Bot. 31: 71-77. .
BLOCH, R. 1946. Differentiation and pattern in Monstera deliciosa. The idioblastic
development of the tr:..:hosclereids in the air root. Amer. Jour. Bot. 33: 544-551.
BLYTH, A. 1958. Origin of primary extraxylary stem fibers in Dicotyledons. Univ.
Calif. Publ. Bor. 30: 145-232. .
COTE, W. A. and DAY, A. C. 1962. The G layer in gelatinous fibers-electron micro-
scopic studies. Forest Prod. Jour. 12: 333-338.
Sclerenchyma 101
XYLEM
THE vascular system of the sporophytes of the higher plants consists of
xylem, the main function of which is the transport of water and solutes,
and phloem which mainly transports the products of photosynthesis.
On the basis of its physiological and phylogenetic importance, the vas-
cular system, and especially the xylem, has been used for the classification
of a large group of plants. The term vascular plants was first used in 19 I 7
by Jeffrey. Recently the term Tracheophyta has been introduced to cover
this group of plants which comprises the Pteridophyta and Spermato-
phyta. The term Tracheophyta has been derived from the xylem, and not
the phloem, because of the firm and enduring structure of the tracheary ,
elements. These elements have thick, hard walls. and so can be distinguished
more easily than the phloem elements. Also' the xylem is more readily
preserved in fossils and so can he identified more easily.
Xylem is a complex tissue as it consists of several types ,of cells. Tlk
most important cells are the tracheary elements which are,the non,living
cells that are principally concerned with the transport of water and which
-a\':)\), \~ 'a c't1ta)Tl oegtee, na~e a '5UPPOT\)tlg l\)D.c\)DTl. rlnTe'&o a1i~ 1>I't3ent
in the xylem where they are mainly concerned with the strengthening of
the plant body. Sclereids also may be sometimes present. Parenchyma
cells which have storage and other functions also occur in the xylem. The
xylem of some plants contains laticifers (see Chapter 9). .
The xylem and phloem elongates in developing prgans by the continual
differentiation of new elements produced by the procambium, which itself
is continuously produced by the apical promeristem. The xylem produced
by the procambium in the primary body is called·the primary/xylem. In
many plants, after the completion of the formation of the primary body,
secondary tissues are developed. The xylem that is produced as a result of
the activity of the vascular cambium is called the secondary xylem.
In the primary xylem the elements that are completed early, i.e. the
protoxylem, are distinguished from those completed later, i.e. the meia-
xylem.
Xylem 103
Tracheary elements
Two basic types oftracheary elements are distinguished-tracheids and
vessel members. The term tracheid was introduced in 1863 by'Sanio who
discussed the similarity and differences between this element and the
vessel member. Since then much work has been devoted to the investiga-
tion of the structure, shape, function, ontogeny and phylogeny of these
elements.
The main difference between tracheids and vessel members is that the
former are not perforated while the end walls of the latter are perforated
(Fig. 45). A vessel, which is also termed a trachea, is built of numerous
vessel members that are joined one to the other by their end walls. Vessels
are terminated by a vessel member of which the proximal end wall is
perforated, whereas the distal end wall is not, i.e. the distal parts of a vessel
is tracheid-like.
Narrow
Primary strip
wall
SeCondary
wall
8
(0
~
.
o Groove
Narrow
o strip
o
Primor~
wall
Secondary
wall
7 10 Primary II
wall
FIG. 43. 1, Tip of tracheid of Dryopferis .. with scalariform pitting. 2 and 3, Tips of
tracheids of Kingia. 4, Tracheid of Pinus. 5, Portion of a longitudinally sectioned
trachear), element showing the helical wall thickenings and the strips by which
they are joined to the primary wall. 6, As in no. 5, but in which the helical thicken-
ing is deeply grooved. 7-11, Different types of wall thickening in'tracheary ele-
ments. 7; Annular thickening. 8, Helical thickening. 9, Dense helical thickening.
10, Scalariform thickening. 11, Reticulate thickening.
elements and parenchyma cells the pit-pairs are mostly half-bordered,
i.e. bordered on the side of the tracheary element and simple on the side
of the parenchyma cell.
When the bordered pits are transversely elongated and are arranged in
longitudinal rows along the element, the pitting is termed scalariform
pitting (Fig. 18, no. 2). Circular and elliptical pits are arranged in horizontal
or diagonal rows. The former arrangement is called opposite pitting (Fig.
( 18, no. 4) and the latter alternate pitting (Fig. 18, no. 5). On the inside
surface of a pitted secondary wall a helical thickening may develop (Fig.
132). I
.In the Ophioglossales, the. Ginkgoales, the Coniferales and the Gnetales
\no"'scalariform pitteo .elements are found. Tn plants of these orders bor-
dered pits, which are similar to those found in the secondary tracheary
elements of the same plant, are found on the reticulate and helical thicken-
ings of the pri'm ary xylem (Fig. 46, no. 10). '
\ The following facts are known about the formation of the special wall
thickenings of the trachea~y element's.
F IG. 44. I and 2; Micrographs of longitudinal sections of the young stem of Clf<!U/,-
bila. >(' 150. 1, Protoxylem elements with annular and helical thickening. 2, Pitted
meta"xylem vessel. 3~ Micrograph of a radial longitudinal section in the secondary·
xylem of Viburnum linus showing a scalariform perforation plate. x 470. "
Criiger (1855) observed that in the positions where the thickenings of
the secondary wall will develop, strips of actively streaming cytoplasm
appear. Similar conclusions were reached by Barkley (1927) who believed
that the position of the cytoplasmic strips is determined by the position
of rows of vacuoles.
A similar phenomenon was also observed by Sinnott and Bloch
(1945) who studied the development of tracheary elements from paren-
chyma cells during the regeneration in the vascular bundles of Coleus.
Interesting observations were made by Majumdar (1940, 1941) who
worked on the development of vessels in the protoxylem of Heracleum.
Recently Wooding and Northcote (1964) suggested that the Golgi appara-
tuses are involved in the formation of the wall thickenings.
There is also evidence that the cytoplasm of the developing tracheary
element lines the cell wall with suberin (Scott ef al., 1960).
The functional significance of the different types of wall thickenings in
the tracheary elements is not clear. It is possible that the exclusive ap-
pearance of annular and spiral thickenings in elements in those organs that
are still elongating has some connection with the rapid increasdn length
of the organ. Investigations using X-rays together with the regulation of
light which altered the rate of stem elongation proved this assumption.
Goodwin (1942) and Smith and Kersten (1942) saw that if stem elongation
is inhibited the production of annular and spiral vessels is reduced !Jr. stop-
ped and pitted vessels develop.
g-Tail-
~- 0_
~ £:9
~;i :;
~~~~
o -Q ~
~ f. 5
4
/
/
FIG. 45. Dicotyledonous vessel members. 1 and 2. Vessel members in which the
perforation plates at Iboth ends are scalariform. 3, Vessel member with
one scalariform and one simple perforation plate. 4-6, Vessel members
with simple perforation plates. "Tails", the narrow elongated tips of the vessel
members, can be seen in nos. 2-5. (AdaI?ted from Failey.)
2 3
II 4
0
5
~
00
00
It,
0
00
0
o @
@ @\
(j) '@@
9 @8 @':-
6 7 8 10
"
FIG. 46. 1-5, Perforation plates of vessel members in the primary xylem of mono-
cotyledons. 1, Scalariform perforation plate from the stem of Phoenix dactylI/era.
X 70. 2, Reticulate perforation plate from the root of Hymenocallis caribaea.
x 200. 3-5, Vessel members from the stem of Rhoeo discolor. x 150.3, Scalari-
form perforation 'plate of a helically-thickened vessel member. 4, Reticulate
perforation plate of an annularly-thickened vessel member. 5, Simple perforation
plate. 6-9, Ends of vessel members with helical thickening from dicotyledonous
primary xylem. 6, Scalariform perforation plate. 7, Transitional form between a
scalariform and simple perforation plate. 8 and 9, Simple perforation plates. 10,
Tracheid of Gne/urn with helical thickening and circular bordered pits. 11, Vessel
member end of Ephedra with a foraminate perforation plate. (Nos. 1-5, adapted
from Cheadle, 1953; nos. 6-10, adapted from Bailey, 1944.)
not. This method also involves technical difficulties. It may be that som
of the gaps in the secondary wall are perforations and some are pit,
It has been suggested that such intermediate forms between typica
tracheids and typical vessels· should be termed vessel-traeheids (Fahn
1953) or vessel-·member-traeheids by analogy with fibre-tracheids, whicl
are intermediate between fibres and tracheids.
In many dicotyledonous species the middle portion of the vessel mem-
bers of the secondary xylem widens during ontogenetic development while
the tips remain narrow and elongated. These tips are not perforated and
they appear as projections that overlap the walls of the neighbouring vessel
members; these tips have been termed tails (Chalk and Chatlaway, 1934,
1935). The perforations are present at the end of the widened part of the
element, i.e. near the base of the tails (Fig. 45, nos. 2-5).
DEVELOPMENT OF VESSELS
I
/ -
Vessels develop from meristematic cells - procambial cells in the primary
xylem and cambial cells in the secondary xylem. The vessel members may
or may not elongate prior to the thickening of the wall but they usually
widen in, this stage of development. '
Much attention has been paid by workers studying the ontogeny of
vessels to the end walls in which the perforations develop. Different opin-
ions exist as to how the perforations develop. According to Esau and Hewitt
(1940), who worked on herbaceous plants in which the vessel members.
had simple perforation plates, layers of the secondary wall are deposited
on the primary wall in the pattern specific for each type of vessel after
the vessel ~~mbers have reached their maximum size., Those parts of the
primary wall in the position where the perforation will develop do not
become covered with secondary wall substance, but they become thicker'
relative to the other area of the primary wall of the element (Fig. 47, nos.
1-3). This thickening appa'rently is not the result of the addition of material
to the wall but the result of the swelling of intercellular substance. After
the secondary walls are completely developed and lignified the swollen
parts of the primary wall and middle lamella slowly disintegrate. This
process is apparently brought about by the protoplast which' itself later
dies and disintegrates (Fig. 47, no. 4). '
According to Priestley et al. (1935), who investigated the development
of the vessels in trees, the production of the perforation in the end wall is
a sudden process and no intermediate stages can be found. Apparently,
while the walls are still very thin, the end walls c'ontract suddenly, and so
the rim or rims around the perforations areformed (Fig. 47, no. 5). Often
a stretched pectic membrane remains in the position of the 'perforation.
By means of plasmolysis 'the ab'ove 'workers were able to show ·that each'
vessel member has a separale protoplast during all stages of the thickening
and lignification of the wall. ,/
In ring-porous and sometimes in diffuse-porous wood, in/which the
vessels are wide, as the vessei grows in width the cells neighbouring it may
become separated one from the other. In this way the vessei is brought into
•• ~ -, , . , , -';'<- ,_"
contact with new gel1s (Fig. 47, nos. 6, 7). In ma~y,cases it is possible to
110 Plant Anatomy
observe that where the position of the above separating cells is shifted rela-
tive to the widening vessel, the cells retain their original ~ttachments, or
at least partially so,-in those positions where there are pits".'\This is possii:>le
by the extension of the cell wall to forni bridge-like connections in the re-
gion of the pits (Fig. 47, no. 8). According to Priestley et af. (1935) this
'\. FlO. 47. 1--4, Development of a perforation plate in vessel members of Apiumgra~
veolens, after'Esau. 1936. The development of helical secondary thickening on
the ~~e walls and t~e presence ~f the primary end w~ll can ~e see~ ~n nos. 1~~.
The end wall has dIsappeared III no. '4.;5, End portIOns of two adjacent vessel
members of Fraxittus in which it is possible to distinguish the' perforation rim and
the two pro(?plasts that have separated from one another: x 125. 6 and,7. Dia~
grams of cross-~ections of a vessel and neighbouring cells showing how the v:essel,
during enlargement, comes into contact with new cells. Neighbouring cells indi ..
cated by numerals-:..,S, Drawing of portion of a vessel of Ulmus in surface view,
showing how the cells'around the vessel tear away from each other as a result of
the widening of the vess~b,The neighbouring cells retain their origimil attachments
to the vessels where there a'fe pits, resulting in the formation of bridge·like exten·
sions. x 95. (Nos. 5-8, adapted from Priestley et al., 1935.) T
""'"
Xylem 113
vessel members in these four groups are 3·96 mm, 2·58 mm, 1·47 mm and
.0·76 mm.·As the.vessel members are shorter than the·tracheids the shorter
the vessel member, the more advanced it is considered to be.
2. The diameter of the element. The diameter of the tracheid is smaller
than that of the vessel member.
J. Tile thickness of the wall. The wall of a typical tracheid 'is thin and is
of equal thickness over the entire circumference. This feature is also seen
in primitive vessel members.
4. The perforation plates. Those scalariform perforation plates that are
long, oblique and with numerous perforations are considered the most pri-
mitive and the simple, horizontal perforation plates the most advanced.
5. The shape of the element in cross-section. The shape of the tracheids
\ and theprimitive vessels in cross-section is angular, while that of advanced
vessel members is circular or nearly so.
6. The type of pitting. In the dicotyledons scalariform pitting in vessel
members is considered to be primitive. The structure and arrangement of
pits developed, from scalariform 'pitting, through intermediate forms in
which scalariform pits occur together with circular or elliptical pits (Fig.
18, no. 3), to forms with only circular or elliptical pits. Of this advanced
type of pitting, that in which the pits are arranged in parallel rows, i.e.
opposite pitting, is more primitive than alternate pitting, in which the cir-
cular or elliptical pits are arranged along more or less helical lines (Fig. 18,
nos. 4, 5). The appear,ance 1of the spiral thickenings on the inside of the
secondary wall of the tracheary elements is evidence of advanced develop-
ment.
The Qhyloil,enetic develoQment of the side walls of the trachear~ elements
was prior to that of the perforation of the.end_walls ..
From investigations based on the methods and facts that have been
mentioned above ano which have been made over the last 30 ye~is, the
present knowledge of the evolutionary development of the mono- and di-
cotyledonous vessel members can be summarized, after Cheadle(1953), as
follows:
Dicotyledons
I. Ten woody genera are known that completely lack vessels. These
genera belong to the following five families: Chloranthaceae, Winteraceae,
Tetracentraceae, Trochodendraceae and Monimiaceae.
2. There are 52 out of 147 families that consist of woody plants only and
that contain one or more species that have only scalariform-perforated
·
114' Plant Anatomy
MonocotyledonS'
The tracheary elements have developed during the evolution of the land
plants As has been pointed out by Bailey (1953), two main functional
trends have become evident during the course of the morphological evo-
lution of these elements, i.e. the development of those structures that en-
hance rapid conduction, on the one hand, and of those that strengthen the
elements, on the other hand. These two trends are antagonistic to a great
extent because certain structures that increase the efficiency of conduction
tend to weaken the cells and vice versa. However, during the course of
evolution, structures have been developed that have, to various extents,
resolved these two trends.
Pitted tracheary elements, in addition to those with annular and helical
wall thickenings, are found in most of the Tracheophyta, with the exception
of certain lower Devonian plants and some hydrophytes .. Elements with
such wall thickenings 'give support to the mature stem. The. absence of
living protoplasts in the 'tracheary elements, the development of elongated
tracheids and the occurrence of vessel members are all features that in-
crease the efficiency of water:'conduction. The bordered pit-pairs which are
characteristic of the tracheary elements, are, as has been shown by Bailey,
well adapted to their function and they combine the-two ab'ove-meritioned
trends. On the.one hand, the area of the pit membrane is comparatively
large and so the passage of water's fairly easy and, on the other hand, the
extent of the development of the secondary wall is maximal because the
secondary wall overafches the pit membrane in such a manner that the
pit membrane remains comparatively large whereas the pit aperture is very
small. This feature greatly strengthens the tracheary elements.
In tracheids more rapid conduction is obtained by the elongation of the
cells, the increase in diameter of the lumen and in the number of pits and
the reduction of wall thickness. Strengthening of the tissue is brought
about by the shortering of the cells, narrowing of the lumen, increase in
wall thickness and the reduction in the number of pits. In the secondary
xylem of conifers, for instance, the early wood is more adapted for efficient
water conduction and the tracheids of the late wood, for support.
Conduction is further facilitated by the complete disappearance of the
pit membranes in certain areas so resulting in the formation of vessel
members. In the secondary xylem of certain primitive dicotyledons primi-
116 Plant Anatomy
References
~ I
BAILEY, I. W. 1936. The problem of differentiating and classifying trad:eids, fiber~
'tracheids and libriform fibers. Trap. Woods. 45: 18-23.
BAILEY:-L~. 1944. The develo~ment of vessels in angiosperms and its significance in
morphological research. Amer. Jour. Bot. 31: 421-428.
BAILEY, I. W. 1953. Evolution of the tracheary tissue of land plants. Amer. Jour.~Bot.
40: 4-8. """
BAILEY, L W. 1957. Additional notes on the vesselless dicotyledon Amborella trichopoda
Baill. Jour. Arnold Arbor. 38: 374-378.
BARKLEY, G. 1927. Differentiation of vascular bundle of Trichosanthos anguina. Bal.
Gaz. 83: 173-184. ~.
BIERHORST, D. W. 1958. Vessels in Equisetum. Amer. Jour. Bot. 45: 534-537.
"
Xylem 117
PHLOEM
THE phloem together with the xylem constitute the conducting system
of vascular plants. The xylem functions principally in the conduction of
water, and the phloem of products of photosynthesis. Similarly to the
xylem the phloem also is a compound tissue. The important cells of the
phloem are the sieve elements which serve for the conduction of the photo-
synthetic products. Additional to these elements phloem contains typical
parenchyma cells in which reserve substances are stored, as well as spe-
cialized parenchyma cells, i.e. the companion cells and albuminous cells,
which are connected with the functioning of thelsieve elements. Fibres,
sclereids and sometimes laticifers may also be found in phloem tissue.
The primary phloem, similarly to the primary, xylem, develops from the
procambium. The primary phloem is divided)nto the protophloem, which
develops from the procambium during an early ontogenetic stage, "nd the
metaphloem which also develops from the procambium, but at a later stage
of development. J
The sieve elements were first discovered by Hartig in 1837 and the term
phloem was coined, from the Greek word for bark, by Nageli in 1858.
Tire plilaem iff tire stem, is asuilily external to the xylem Ollt, ill rome
ferns and in different species of numerous dicotyledonous families, e.g.
Asclepiadaceae, Cucurbitaceae, My~taceae, Apocynapeae, Convolvulaceae,
Compo sitae and Solanaceae, phloem is also present on the inside oftbe
xylem. Phloem.on the inside of the xylemds called internal or intraxylary
phloem (Fig. 70, no. 3) and it develop~ a little later than the external
phloem. In certain families, such as the Chenopodiaceae, Amarantha-
ceae, Nyctaginaceae, Salvadoraceae and others, phloem is also present
within the secondary xylem. This type of phloem is, called interxylary
phloem or included phloem (Fig. 158, nos. 1-3) .. '
Sieve elements
The mo;;Characteristic features of sieve elements are the sieve areas in
the walls and thedisappearance of the nucleus from the protol'last.
The sieve areas are'interpreted as being modified l'rimary l'it fields and
they appear as depressio~'in the wall in which groups of pores are 10-
~.
Phloem 119
'3
pit fields and then the connecting strands are derived from a single or
a group of plasmodesmata.- According to the second theory the pore sites
of the future sieve plates contain no plasmodesma'ta ana the'formation'of
the pores involves the dissolution of the wall at the pore site.
According to Esau et al. (1962), the sites of the future pores are first
• delimited by the appearance of small deposits of callose in the form of
~
~
Companion
cell
3 4 5
FIG. 49. 1-4, Sieve-tube members of Via's. 1 and 2, Longitudinal sections of com-
pound sieve plates between two elements. 1, Elements in dormant state in which
the plate is covered by a thick layer of callus. 2, Elements reactivated after the re-
moval of the callus. The slime which fills the sieve areas is indicated by heavy stip-
pling. 3, Surface view of two compound sieve plates. 4. Sieve-tube elements with
companion cells. 5, Portion of a sieve cell of Pinus. (Nos. 1-4, adapted from
Esau, 1948.)
12.2 Plant Anatomy
platelets increase in diameter, whereas the cellulosic bars between the pore
.sites..de,crease. in. ~idth. lhese cellulosic pars together form t~e basic
network of the sieve. plate. The platelets and' the bars incre_ase in thickness.
It appears that there is a single plasmodesma in each pore site. The per-
foration occurs in the centre of each platelet in that position where the
thin wall dissolves. ·The callose platelets of each pair fuse around the per-
foration and thus each pore is lined with callose from its inception. Later,
callose appears also on the surface of the sieve plate. The·perforation of
the sieve plate takes place after the disintegration of the nucleus.
Those sieve elements that have unspecialized sieve areas that are' similar
throughout the element are called sieve cells (Fig. 49, no. 5). Sieve cells,
thererore, do' not contain sieve plates. These 'cells are usually elongated
with tapering ends or their end walls are very oblique. In the positions
where sieve cells overlap one another the sieve areas are more numerous.
Elements in which sieve plates can be distinguished are called sieve-tube
members (Fig. 49, no. 4). Sieve plates are usually found on the end walls
which may be very oblique or horizontal or in intermediate planes. In
certain elements, e.g. those of Vilis and Pyrus malus, the sieve plate con-
tains several sieve areas; while in other elements, e.g. those of Cucurbita,
only one sieve area may be present. The former type of sieve plate is termed
a compound sieve plate (Fig. 49, nos. 1-4; Fig. 51 , nos. 2, 3) and the latter,
a simple sieve plate (Fig. 50, nos. 1-3). The sieve-tube members are con-
nected one to the other by the walls that contain the sieve plates and so
form sieve tubes. Sieve plates are found only very occasionally on the
longitudinal walls of the sieve-tube members. On these walls unspecialized
sieve areas develop.
124 Plant Anatomy
The walls of siev" elements, are usually only primary and consisLmainly
of cellulose. Only in a single family of the Coniferale;, the Pinaceae, has
a secondary, 'non,lignified cell wall been found in the sieve cells (Abbe
and Crafts, 1939). The thickness of the wall of the sieve elements varies in
different cpecies; in some species the cell wall is I p. thick while in other
species the wall nearly fills the cell lumen. Esau and Cheadle (1958) also
found differences in the structure of the wall. In certain species they found
that the wall is homogeneous, while in others the wall is composed of
two layers-a thin layer close to the middle lamella and a thicker layer
next to the cytoplasm. The inner layer as seen in cross-sections of fresh
material has a sheen similar to mother-of-pearl, and therefore has been
termed the,nacreous layer. The thickness of the wall of the sieve elements
usually decreases with the aging of the element. Thick nacreous walls can
be seen, for example, in Magnolia, Laurus, Rhamnus and Persea but they
are not present in Casuarina, Crataegus, Fraxinus" Morus, Populus, Salix
and Passiflora, among others.
\
THE PROTOPLAST
, '
The most characteristic feature of the protoplast of the sieve elemenhs
the absence ofa nucleus in the mature,'active cell., The structure of im-
mature sieve elements resembles that of the procambial and cambial cells
'from which they develop. In this stage the protoplast contains vacuoles
and a large nucleus. With the specialization of the element the nucleus
disintegrates and disappears. In certain plants the nucleolus or nucleoli are
extruded from the nucleus prior to its disintegration and they remain
within Jhe sieve,element.
A more or less viscous substance, which stains readily with cytoplasmic~
stains, is present in the sieve-tube members of dicotyledons. This substance
has been termed slime. It is thought that slime is of a proteinaceous nature.
It is located in the vacoule and in the preparation of sections it accumu-
lates at the ends of the cells near the sieve plates. These slime accumulations
are termed slime plugs (Fig. 50, no. I; Fig. 51, no. I). The slime is produce~,
in the cytoplasm in the form of small, variously shapO'd slime bodies that,
with the specialization of the sieve element, become' more iiquid and pass
into the vacuole where they become amorphous. This process takes place at
the same time as the disintegration of the nucleus. In monocotyledons,
gymnosperms and pteridophytes slime bodies have not been observed,
and in these plants the vacuole of the sieve elements is aqueous with only
small quantities of slime.
In the sieve elements of many species small plastids that take part in
the synthesis of carbohydrate granules are present. These granules are
similar to starch but stain red with iodine, and apparently contain a high
Phloem 125
osmometers. In.the above case, molecules of solute flow passively with the
solvent. Therefore, according
. to the theory of mass flow,
, the·.molecules
of sugar and other dIssolved substances are conducted through the phloem
as a result of the flow of the aqueous solution. Thus the force, which
enables the flow, is a result of the differences in the osmotic pressure be-
tween the supplying organs (the leaves) and the receiving organs (the roots,
tubers, etc.). Supporters of this theory regard the phloem as the .tube
that connects the above-mentioned osmometers, as the cytoplasm in these'
elements, contrary to that of other cells, is permeable to the products of
photosynthesis (Huber, 1941). According to these investigators the cyto-
plasm of the sieve elements lacks a tonoplast, is incapable of accumulat-
ing "neutral red" and does not plasmolyse when the cells are placed in a
hypertonic solution.
Another theory is that of the active transport of the solutes. The sup-
porters of this theory are of the opinion that the cytoplasm of the sieve
elements takes an active part in the transfer of the substances. They sug-
gest that the protoplasts of the sieve elements have the property of selec-
tive permeability, and they explain the difficulty ~f demonstrating plasmo-
lysis of these cells by the particular sensitivity of the nucleus-free proto-
plast which therefore necessarily demands, extremely careful treatment
(Rouschal, 1941). /
It is accepted 'by" all workers thai the passage of photosynthates in the
sieve elements is much faster than that in ordinary parenchyma cells.
I
Phylogeny of sieve elements
In the most priinitive form the sieve elements are parenchyma cells that
i
have undergone modifications in connection with their function. This was t
tube members. Esau (1947, 1948) found slime bodies in the companio]l
cells of Vitis and observed that, with the dispersal of this slime, the pro-
toplast stained more' intensely. 'i
Starch has not been found in companion cells.
In pteridophytes and gymnosperms companion cells, as described above,
do not occur, but cells which stain intensely with cytoplasmic stain? a~e
present. These cells are apparently connected physiologically and mor-
phologically to.the.sieve cellsand have ceen termed alhuminous cells. Onto,
genetically these cells develop from the phloem parenchyma or from cells
of the phloem .rays. Albuminous cells do not contain starch during the
period that the phloem is active, but they may store it· during the rest
period.
References
ASBE, L. B. and CRAFTS, A. S. 1939. Phloem of white pine and other coniferous species.
Bot. Goz. 100: 695-722.
ASPINALL, G:O. and KESSLER, G. 1957. The structure of callose from the grape vine.
Chern. Ind. 1957: 1296., '
BERNSTEIN, Z. and FAHN, A. 1960. The effect of annual and bi-annual pruning on the
seasonal changes in xylem formation in the grapevine. Ann. Bot., N. 5.;24: 159-171.
CHEADLE, V. I. 1948. Observations on the phloem in the Monocotyledoneae. II. Addi-
tional data on the occurrence and phylogenetic specialization in structure of the
sieve tubes in the metaphloem. Amer. Jour. Bot. 35: 129-131.
CHEADLE, V', 1. and UHL, N. W. 1948. The relation of metaphloem to the types of
vascular bundles in the Monocotyledoneae. Amer. Jour. Bot. 35: 578-583.
CHEADLE, V. I. and WHITFORD, N. B. 1941.-f>bservations on the phloem in the Mono-
cotyledoneae. r. The occurrence and phylogenetic specialization in structure of the
sieve tubes in the "ltaphloern. Amer. Jour. Bot. 28: 623-627.
CURRIER, H. B. and STRUGGER, S. 1956. Aniline blue and fluorescence microscopy of
callose in bulb scales of Allium cepa,L. Protoplasma 45: 552-559.
ESAu, K. 1947 .. A study of some sieve-tube inclusions. ,Amer. Jour. Bot. 34: 224-233.
ESAU, K. 1948. Phloem structure in the grapevine and its seasonal changes. Hilgardia
18: 217-296.
:ESAU, K. 1950. Development and structure of the phloem tissue. 1L Bot. Rev. 16: 67-114.
-ESAU, K. 1961. Plants, Viruses and Insects. Harvard Univ. Press, Cambridge, Mass.
ESAU, K. and CHEADLE, V. I. 1958. Wall thickening in sieve elements. Proc. Nation.
Acad. Sci. 44: 546-553.
ESAu, X. and CHEADLE, V. I. 1959. Size of pores and their contents -in sieve elements of
dicotyledons. Proc. Nation. Acad. Sci. 45: 156-162.
ESAU, K. and CHEADLE, ".1.1962. An ~~aluation of studies on ultrastructure oftono-
plast in sieve elements. Proc! Nat. Acad. Sci. 48: 1-8.
ESAU, K., CHEADLE, V. i. and RISLEY, E. B. 1962. Development of sieve-plate pores.
Bot. Gaz. 123: 233-243. .
ESAU, K., CURRIER, H. B, and CHEADLE, V. L 1957. Physiology of phloem. Annual
Rev. Pl. Physiol. 8: 349-374.
FREY-WYSSlING, A., EpPRECHT, :W. and KESSLER, G. 1957. Zur Charakterisierung der
Siebroehren-Kallose. Experientia 13: 22-23.
HUBER, B. 1941. Gesichertes und Problematisches. in der Wanderung der Assimilate.
Ber. DIsch. Bot. Ges. 59: 181-194.
MUNCH, E. 1930. Die Stojfbewegungen in der Pfianze. G. Fischer, lena.
ROUSCHAL. E. 1941. Untersuchungen tiber die Protoplasmatik und Funktion der
Siebrohren. Flora 35: 135-200.
ZAHUR, M. S. 1959. Comparative study of secondary phloem 01' 423 species of woody
dicotyledons belonging to 85 families. Cornell Unlv. Agric. Exp. Sta. Memoir 358.
CHAPTER 9
LATICIFERS
Types of laticifers
According to De Bary (1877), the laticifers are divided into two main
types: non-articulated and articulated. This classification has no relation-
ship to taxonomic groups and thus different types of laticifers may be
found in different species of one family.
The non-articulated laticifers develop from a single cell which greatly
'elongates with the growth of the plant and which is sometimes branched.
Such laticifers are also termed laticiferous cells. Articulated laticifers con-
sist of sim!,le or branched series of cells which are usually elongated. The
end walls of such cells remain entire or become porous or disappear com-
pletely. Such laticifers are also termed laticiferous vessels.
NON·ARTICULATED LATICIFERS
ARTICULATED LATICIFERS
FIG . 52. I, Micrograph of a stem of Sonchus oleraceus, cleared with lactic aci d,
to show the branched, articulated laticifers. X 140. 2, Tangential section at a depth
of two cells below the abaxial epidermis of a bulb scale of Allium cepa in which
simple articulated laticifers can be distinguished. x 115.
RUBBER PLANTS
There are many plant species from which rubber can be obtained, but
of them, Herea brasiliensis in the Euphorbiaceae, which is known as the
Para rubber tree, is the most important in world economics. This plant is
today grown in central America, the West Jndies, Brazil, Liberia, Ceylon,
Malayan Archipelago, Sumatra, Java and eastern India. The latex of
H: brasiliensis contains about 30 % rubber.
Other rubber plants of secondary economic importance (Schery, 1954)
are CastUra (Panama rubber) in the Moraceae, Manihot (Ceara rubber) in
the Euphorbiaceae, Parthenium argentatum (guayule) and Taraxacum kok-
saghyz in the Compositae, Hancornia and Lando/phia in the Apocynaceae,
and Cryptostegia in the Asclepiadaceae. Parthenium argenta tum was in-
vestigated thoroughly in the U.S.A., during World War II, when rubber
was unobtainable from the Heyea plantations in Asia.
Rejel'ences
ARTSCHWAGER, E. and MCGUIRE, R. C. 1943. ContritlItion to the morphology and
anatomy of the Russian dandelion (Taraxacum Kok-saghyz). U.S.D.A. Tech.
Bull. 843.
BLASER, H. W. 1945. Anatomy of Cryptostegia grandiflora with special reference to
the latex. Amer. Jour. Bot. 32; 135-141. ..
BONNER, J. and GALSTON, A. W. 1947. The physiology and biochemistry of rubber
formation in plants. Bot. Rev. 13: 543-596.
DE BARY, A. 1877. Vergleichende Anatomie dey Vegetationsorgane dey Phanerogamen
und Fame. W. Engelmann, Leipzig.
ESAU, K. 1953. Plant Anatomy. John Wiley, New York.
FosTER, A. S. 1950. Pract{cal Plant Anatomy. D. Van Nostrand, New York-London.
FREY~WYSSLING, A. 1952. Latex flow. In: Deformation and Flow in. Biological Systems.
Ed. A. Frey-Wyssling, pp. 322-343. North Holland Publ. Co., Amsterdam.
136 Plant Anatomy
EPIDERMIS
THE epidermis constitutes the outermost layer of cells of the leaves, floral
parts, f~uits and seeds, and of stems and roots before they undergo con-
siderable secondary thickening. Functionally and morphologically the
epidermal cells are not uniform and among them, apart from the ordinary
cells, many types of hairs, stomatal guard cells, and other specialized cells
are found. Topographically, however, and to a certain extent also onto-
genetically, the epidermis constitutes 'a uniform tissue.
The earliest stages of the ontogenetic development of the epidermis
differ in the rool and shoot (see Chapter 3). This fact caused certain in-
vestigators to coin special terms, epiblem and rhizodermis, for the outer-
most layer of the root (Linsbauer, 1930; Guttenberg, 1940, and others).
However, if the development of the epidermis from the, protoderm is
traced ignoring the problem of the origin of this meristematic tissue, it
is possible to apply the term epidermis to all the organs of the various
groups of vascular plants.
The epidermis usually exists throughout the entire life of those organs
that have no secondary thickening. In a few plants, s,-\ch as long-lived
moho-cotyledons with no secondary thickening, the epidermis is replaced
by a cork tissues as the organs age. The duration of the epidermis in organs
with secondary growth differs; usually in stems and r~ots the epidermis
is replaced by the periderm during the plan!'s first year, but there are
certain trees, e.g. Acer striatum, in which the periderm develops only after
several years of secondary growth of the organ (De Bary, 1877). In such
cases the epidermal cells continue to divide anticlil.ally and to enlarge
tangentially (Fig. 148, nos. 3-6).
4:ti~~9W~ ) Ep:d"m;,
/'
FIG. 55. Portions of cross-sections of the leaf blade of- Ficus elastica. approx.
X 500. 1. Adaxial side of the blade in which the multiseriate epidermis the
cells of which include a lithocyst (a cystolith-containing cell), can be distin-
guished. 2 and 3, Two early stages in the development of the adaxial multiseriate
epidermis and a lithocyst. 4-,6, Stages in the development of the multiseriate
abaxial epidermis. (Adapted from De Bary, 1877.)
when the leaf begins to expand in the bud and the stipules are shed (Fig.
55, nos. 1-6). Multiseriate epidermis occurs in the Moraceae, certain
species of the Begoniaceae and Piperaceae, and some articulated Cheno-
podiaceae. In Anabasis articulata the multiseriate epidermis develops in
the lower region of each internode (Fip"_ flO:; n()~ l_h.1· -riifT ';:Ll .,...n<o 1 "'t\
Epidermis 139
Epidermal cells
Various types of epidermal cells can be distinguished in different plants:
the ordinary cells of the epidermis; single cells or groups of cells with
special structure, form or content; cells connected with stomata; and epi-
dermal appendages termed trichomes.
C The ordinary cells of the epidermis vary in shape, size and arrangement,
but they are always closely attached to form a compact layer devoid of
"intercellular spaces. In the epidermis of petals air spaces may sometimes
occur but they are always covered by the cuticle (Eames and MacDaniels,
'l947)., Many epidermal cells are tabular, and in the leaf blade of dicoty-
ledons the anticlinal walls ar~ mostly sinuous. In the stems, and especially
in the leaves'of many monocotyledons, the epidermal cells are elongated.
In-the epidermis of certain seeds (in species of the Leguminosae and in
Punica) the cells are relatively very elongated in a radial direction and ,are
rod-shaped. In certain plants; for instance Aloi' aristata, the epidermal' cells.
appear to be hexagonal in surface view but actually they are polyhedral,
and according to Matzke (1947), the average number of faces is 10·885.
The external wall of the epidermal cells of certain leaves and petals ,is
raised in the form of papillae. In certain pteridophytes such papillae are
found on the epidermal cell wall facing the mesophyll.
Walt structure
The above structure of the cuticle suggests that the passage of the cutin
is from the inside outwards and that it accumulates on the surface~ where
it forms the cuticle. .." ~ I ~
Various theories have been proposed to explain this outward movement
of the cutin. According to someinvestigators special channels are present
in the outer epidermal walls through which the fatty substances, which
form the cutin, pass (Scott et al., 1957); otlier investigators believe that
the .porous nature of the wall suffices. The latter~ view is more accept·
able. "-
Deposits of wax in the form of granules, as-in Brassica, Dianthus, or of
rods, as in Saccharum (Fig. 56, no. I) or as continuous layers, as in Thuja
orientalis, are. often found on the surface of the cuticle. In the leaves of
Agave a continuous layer of wax is also found below the cuticle (Schiefer·
stein and Loomis, 1959). In certain plants platelets of wax have been found
within the ~utin of outer epidermal cell.walls (Roelofsen, 1952). Deposits
of salts in the form of crystals, e.g. in Tamarix and Plumbago capensis, of
caoutchouc, e.g.,Eucalyptus, or of oils and re~ins sometimes occur on the
surface of cuticle or within it. Deposits of silico'n salts are found in the
epidermal cell walls of many plants, as, for example, Equisetum, the Gra·
mineae, many species of the Cyperaceae, the,Palmae and certain species
of the Moraceae, the Aristolochiaceae and the'Magnoliaceae (Metcalfe and
Chalk, 1950). '
Lignin is rarely found in the epidermal cell walls. When it is present it
may be found in all the walls or only in the ouier wall. Lignified epidermal
walls are found in the leaves oflhe Cycadaceae, in the needles of conifers,
the rhizomes of the Gramineae, in the strips of epidermis above the bundles
of sclerenchyma in the leaves of the Gramineae, Juncaceae and Cyperaceae,
in the leaves of certain species of Eucalyptus and Quercus and in Laurus
nobilis and Nerium oleander. /
Parts of the epidermal cell walls of groups of cells or of single cells may
become mucilaginous in certain dicotyledonous families, such as the Mora-
ceae, Malvaceae, Rhamnaceae, Thymelaeaceae and Euphorbiaceae. In
certain seeds, such as those of Linum usiiatissimum (Fig. 216, no. 2) and
species of Alyssum, the outer walls of the epidermal cells'become mucila· \.
ginous. In the nectaries of certain plants the epidermal cells! also become
mucilaginous at the time of nectar secretion (Fahn, 1952).
Protoplast
Epiderinis 143
flowers, in the leaves of Zebrinapendula and red cabbage, in the stems and
petioles of Ricinus and indifferent organs o(many other plants. Tannins,.
mucilage and crystals lI1ay be present in epidermal cells .
\.
FIG. 57. 1, Surface view of the epidermis of a grass leaf ( Crypsis schoenoides).
X 600. 2, Portion of a cross-section of a grass leaf ( Dactylocteflium robecchi) in
which bulliform cells can be distinguished in the epidermis. x 110. 3, Surface view
of a stoma of Crypsis schoefloides. x 1000.
144 . Plant Anatomy
and cork-cells. The latter two types of cells often occur successively in pairs,
throughout the length of the leaf. The silica-cells contain silica-bodies which
are isotropic masses of silica in the centre of which are usually minute gran-
ules. In surface view the silica-bodies may be circular, elliptic, dumb-
bell or saddle-shaped (Fig. 56, no. 5; Fig. 57, no. I). The walls of the cork-
cells are impregnated with suberin and many of them contain solid organic
substances. The above short-cells sometimes bear papillae, setae, spines or
hairs. Metcalfe (1960) draws attention to the fact that the cork-cells in
many plant,s contain silica-bodies, and·that in certain grasses silica-bodies
also occur in some elongated cells. Silica-bodies also occur in specialized
epidermal cells of the Cyperaceae and some other monocotyledons (Met-
calfe, 1963).
In the Gramineae and many other monocotyledons, with the exception
of the Helobiae, bulliform cells are found in the epidermis. These cells are
larger than the typical epidermal cells and they are thin-walled and have
a large vacuole. The bulliform cells may constitute the entire adaxial epi-
.dermis of the leaf or they form isolated parallel strips.in the.area.between
the veins. In a cross-section of the leaf"these cells appear in·a fan-like
arrangement in which the central cell is the tallest. In certain plants bulli-
form cells are also found on the abaxial surface of the leaf. These cells are
sometimes accompanied bysimilar mesophyll Cells. Bulliform cells contain
much water and are devoid, or nearly seC of chloroplasts. Their wall
cons.ists of cellulose and pectic substances, and the outermost ~all contains
cutin and is covered by cuticle (Fig. 57, no. 2). I
Different opinions exist as to the function of the bulliform cells. Accord-
ing to one opinion they function in the opening of the rolled leaf as present
in the bud. According to a second view they bring about the rolling or
unrolling of mature leaves as a result of their loss or uptake of water. Re-
cent research carried out by Shields (1951) on tweIve xerophytic grass
species has questioned the importance of the bulliform cells in both the open'
ing of the young leaves from the bud and in the hygrochastic movements
(opening due to water absorption) of the matru, leaves. According to
Metcalfe (1959), the bulliform cells often become filled with large masses of
silica and their outer walls often become thick and cutinized.
Other specialized epidermal cells are the cystoliths which are found in
the Acanthaceae, Moraceae, Urticaceae and Eucurbitaceae. In the Cruci-
ferae myrosin cells are sometimes found in the epidermis. These cells are
sac-like secretory cells which contain the enzyme myrosin, and they stain
red in the Millon test, or violet with orcein solution and concentrated
hydrochloric acid.
Epidermis 145
STOMATA
FIG. 58. I, Micrograph of the cuticle o f the fo ssil Cupre!o'Sinoc ladus from which
the shape of the epidermal cells can be d etermined . x 700. 2, Micrograph ofa s ur-
f ace view of \he abaxial epid erma l ce\l~ in a regi011 above a "Vein uf a pe\a \ o f P eiUT-
gonium zonale .. it is possible to d istinguish cuticula r striations particula rly in' the
centra l porti on of the cells which is papill ate. x 780. 3, Microgra ph of a surface
view of the a baxial epidermis of Zebrina pel1dllla in which g uard cells and subsidi-
Epidemiis 147
Outer
ledge" Front cavity
Back cavity
FIG. 59. Various types of stomata. _1 and 2. Abies pinsapo. ,3 and 4. Juniperus
chinensis. 5, SY1{echanthus fibrosus. X 280. 6, Corypha talieri: x'175. 7, Chamaerops
humilis. x280. Nos. 2 and 4, longitudinal sections; the rest cross-sections.
Cuticle represented by solid black, lignified areas by dark shading, (Nos. 1-4,
adapted from Florin, 1931; nos, 5-7, adapted from Tomlinson, 1961.)
the middle portions are elongated and thick walled and the cell lumen is
narrow (Fig. 56, no. 5). As a result of .turgor increase in this type of guard
cell, the expanded tips swell and so push apart the middle blongated por-
tions of the cells. Because of the above structure the nuclei in the guard
cells of grasses appear as two ellipses connected by a. narrow thread.
According to Flint and Moreland (1946) the two parts of the nucleus may
become completely separated.
The chemical composition of the guard-cell wall is the same as that of .
the ordinary epidermal cells of the same plant. They are usually covered,
by cuticle which generally continues on that wall that faces the aperture
"'---j
"""- » Cross - section
ot end of
guard cells
,
FIG. 60. Structure of stomata of Haloxylon articulatum. 1, Surface view showing
thin-walled areas at the ends' of the guard cells. 2, Portion lor a cross-section of
the siem~ showing a guard cell in longitudinal section. The cell-lumen is dumb-
ben shaped; nucleus heavily stippled. 3, Portion of longitudinal section of stem
showing the biseriate epidermis and sunken stomata, sectioned transversely at
various levels. x 400.
,
and it also reaches the cells abutting on the substomatal chamber. In
Citrus, cuticle is absent from the cell wall facing' the stomatal aperture
(Turrell, 1947).
Apart from the types of guard-cell structure described above many other
structural variations exist in the mono- and dicotyledons, e.g. in species
of Haloxylon (Fig. 60, nos. 1-3) and Anabasis and in the Palmae (Fig.
59, nos. 5-7). These variations probably result in different methods of
functioning (Tomlinson, 1961; Fahn and Dembo, 1964).
The various types of stomata in the Coniferales (Fig. 59, nos. ,1-4)
have been described by Florin (1931). He distinguishes several types accord-
Epidermis 149
iug to the movements of the guard cells during the opening orthe stomata.
These.variations in movement are due to the different positions of the thin
parts of the wall as well as to the presence and position of the lignified and
non-lignified wall areas.
FIG. 61. Different types of arrangement, as seen in surface view of the leaf, of the
subsidiary cells relative to the stoma. 1, Acacia; rubiaceous or paracytic type. 2,
Brassica; cruciferous or anisocytic type. 3, Dianthus; caryophyllaceous or diacy~
tic type. 4, Pelargonium; ranunculaceous or anomocytic type.
3 4 5
8
f
FIG. 62.1~5, Ontogeny of stomata of Allium cepa; 1 a'od 2, Elongated epidermal
cells before unequal division; 3, Showing that the smaller cell resulting from
this division is richer in protoplasm. It is this cell that gives rise, by a longitudinal
division, to the two guard cells which in no. 4 are not yet separated by the stoma-
tal aperture. In no. 5 the stomatal aperture has developed. 6 and 7, Epidermis of'
Sedum pubescens. 6, Early stages in the development of stomata up to the stage
where three subsidiary cells and two guard cel1s, but no aperture, can be distin-
guished. Numerals indicate ontogenetic stages. 7, Portion of epidermis with ma-
ture stoma. 8-10, Types of monocotyledonous stomata. 8, Strelitzia nicolei. 9,
CommeUna communis. 10, Pandanus haerhachii. (Nos. 1-5, adapted from Blinning
and Biegert, 1953; nos. 8-10, adapted from Stebbins and Khush, 1961.)
Epidermis 151
Ontogeny of stomata
The stomata develop from the protoderm (Fig. 62, nos. 1-7; Fig. 63,
nos. 1-6; Fig. 64, nos. 1.2). The mother cell of the guard cells (Fig. 62,
, no. 3) is usually tbe smaller of the two cells that result from an unequal
division of a protodermal cell (Bunning and Biegert, 1953; Bonnett, 1961,
and others). Tbe motber cell divides to form two cells which differentiate
into the guard cells. At first these cells are small and have no special shape
but, as tbey develop, they enlarge and become characteristically sbaped.
During their development the middle lamella between the two guard cells
swells and becomes lens-shaped shortly before the time when it disintegrates
to form the stomatal aperture (Ziegenspeck, 1944). Even in those cases
in which the mature guard cells are sunken or raised relative to the ordinary
cells of the uniseriate epidermis, the guard-cell mother cells and the guard
cells are level with the other epidermal cells immediately after their for-
mation. The sinking and the raising is brought about during the maturation
152 Plant Anatomy
Epidermis 153
of the guard cells. The development of stomata in the leaf continues for
a relatively long period during·the.growth of the leaf.
On the basis of the order of the appearance of the stomata OIl the photo-
synthesizing organ, two main types of development can be distinguished:
(I) that in which the stomata appear gradually in a basipetal sequence, i.e.
from the tip of the organ to its base, as is the case in leaves with parallel
venation and in the internodes of the articulated species of the Cheno-
'podiaceae; (2) that in which there is no regularity in the appearance of the
stomata in the various regions of the growing organ, as is the case in
leaves with reticulate venation.
EPIDERMAL APPENDAGES
(d) Shaggy hairs which consist, at least at the base, of two or mare con-
tiguous rows of cells. Such hairs may be seen on the petiole base of Por-
tulaca oleracea (Fig. 65, no. 4), in Schizanthus, and cert'ain species of the
Compositaec
;nl-,~
I)
':, "
'\'/ i;
In some species the hairs may show movements. This may be brought
about in two ways: either by hygroscopic mechanisms, that is, by the
differential swelling and shrinking of the cell walls (e.g. as on the seed of
~2
I Gland
4 5 7
FIG. 66. Trichomes and epidermal glands. 1 and 2, Peltate hair of Olea europaea.
1, Surface view showing in the centre the stalk cell around which the "shield" de·
velops. 2, Lateral view. 3, Portion of cross-section of the leaf of Tamarix showing
a multicellular salt gland. 4-7, Stinging hair of Urtiea dioica. 4, As seen under a
mkroscope with the focus on the surface of the trichome. S, As in no. 4, but with
focus on centre of the trichome. 6, Intact tip. 7, Trichome with broken tip. 8 and
9, Calcium-secreting gland in epidermis of Plumbago capensis. 8, In surface view
of the epidermis, 9, In cross-section of the leaf. 10, Portion of a cross-section of
the leaf of Thymus capitatus showing a secretory gland. (Nos. 4-6. adapted
from Troll, 1948.)
Tarnarix); or by the action of living cells which may comprise the hair
itself or which may be present only at the base of the hair or close to it
(Uphof, 1962).
156 . Plant Anatomy
2. Glandular trichomes
These may be unicellular; multicellular or scale-like.IThe simple multi-
cellular glandular trichomes consist of a stalk and a uni- or multicellular
head. Such trichomes occur, for instance, on the leaves of Nicotiana,
Primula and many species of the Labiatae (Fig. 66, no. 10). Certain gland-
ular trichomes consist of a multicellular mass surrounded by palisade-
like secretory cells.
Glandular trichomes, which secrete a sticky substance and which usually
consis't of a multicellular stalk and head, have been termed colleters. Such
trichomes are mainly found on bud scale; and stipules, e.g. of Rosa,
Syringa, Aesculus and on the stems and leaves of Ononis, and on the calyx
of Plumbago capensis.
Another type of glandular trichome is the digestive gland which is
found on insectivorous plants, e.g. in the Nepenthaceae, Droseraceae and
Sarraceniaceae.
Highly specialized glandular trichomes are the stinging hairs of Urtica.
These trichomes consist of a single, long cell which has a broad, bladder-
like base and narrow, needle-like upper part (Fig: 66, nos. 4-7). The broad
base is surrounded by epidermal celIs which are raised above the level of
the other epidermal cells. The wall of the/distal needle-like part of the
secreting cell is impregnated with silica adhe tip and with calcium some-
what lower. The very tip is spherical and breaks off, along a predetermined
line, ,when the hair is touched. The broken tip resembles} the tip of a sy-
ringe and so easily penetrates the skin into which the poisonous, irritating
cell contents (hystamine and acetylcholine) are injected.
Uni- and multicellular appendages exist from which nectar is secreted.
Some of these "re devoid of cuticle and the. nectar is secreted by diffusion,
while oth'erg have a cuticle and then the secreting mechanism is more
I \
complex. In the latter the outermost layer of the cell ,wall of the head of .
the secreting trichome gradually s,,:ells and expands so thit, a crescent-
shaped mucilaginous layer is formed below the cuticle. This layer con-
tinues to enlarge and so depresses the innermost layer of the cell wall to-
ward the cell lumen which becomes almost obliterated, Eventually the
cuticle bursts and the mucilaginous' mass, in which. the nectariferous
I
sub- \
stances have accumulated, is brought to the surface (e.g, Hibiscus, Abuti/on
and Tropaeolum). Active secretory cells have a dense protoplast. External
glandular layers may develop on epidermal outgrowths and emergences,
or independently of them. Thus, for instance, nectariferous tissues may
occur on the teeth of leaf margins (Prunus amygdalus, Ailanthus altissima) or
on different parts of the floral organs. The manner of secretion from these
glands differs in the various plant species, Here also the secretion may be
accomplished by simple diffusion, by the swelling of the outermost layers
of the epidermal cell wall and the rupture of the cuticle, or special aper-
Epidermis 157
tures, which are modified stomata, may be present on the epidermis (Fahn,
1952). (See Chapter 19 for further details.)
Multicellular epidermal glands of special interest are the salt. and chalk
glands. These glands are somewhat sunken or are level in relation to the
epidermis. Salt-secreting glands (Fig. 66, no. 3; Fig. 67) nos~ 1, 2) are
/ 2
FIG. 68. 1, Longitudinal section through a gland present on the petiole of Prunus "
amygda/us showing a palisade-like_.secretory epidermis. 2 and 3, Development of
root hairs. 2, Epidermal cells. showing the begin'ning· of-a protubera'nce at the
apical end of the cell. 3, Maturing root hairs that develop from the above pro-
tuberances as the ceJls become further -distant from 'the root ape,Y. (Nos. 2
a~d 3, adapted from Troll, 1948.)
Root hairs are tubular elongations of epidermal cells. They are branched
in only very few plants. Root hairs are 80--1500J' long and 5-17 I" in dia~.
meter (Dittmer, 1949). The root hairs have large vacuoles and they are
usually thin-walled. On the aerial, adventitious roots of Kallmchoii fed-
tschenkoi multicellular root hairs have been found (Popham and Henry,
1955).
Root hairs begin to form beyond the meristematic zone of the ,young'
roots in regions where the epidermal cells can still elongate. The root hairs
usually first appear as small protuberances near the apical end of the
epidermal cell. If the epidermal cell continues to elongate after the appear-
ance of the protuberance the root hair is found somewhat distant from
the apical end of the mature epidermal cell (Fig. 68, nos. 2, 3). Root hairs
Epidermis 159
elongate at their tips where the wall is thinner, softer and more delicate.
The nucleus is usually located close to the growing tip of. the root. hair.
The epidermal cells which give rise to Toot hairs elongate less than the
other epidermal cells. (See Cormack, 1949, for further anatomical and
physiological details.)
In some plants only certain of the root epidermal cells, termed tricho-
blasts or pi/iferous cells, can produce root hairs. These are small cells
which result from unequal divisions of epidermal cells.
Root hairs are usually viable for only a short period, generally only
a few days. With the death of the root hairs and if the cells are not slough-
ed, the walls of the epidermal cells become suberized and lignified. In
some plants root hairs hav, been found that remain permanently on the
plant. The walls of such root hairs become thick and then apparently
lose their ability to take up water from the soil (Artschwager, 1925;
Cormack, 1949).
Refel'clIces
CHAPTER II
THE STEM_
The axis of the embryo in the seed consists of a hypocotyl and radicle.
At the tip of the hypocotyl one or more cotyledons and the bud of the
shoot, i.e. the plumule, are found. At the tip of the radicle is the root cap.
The bud of the shoot usually consists of an axis containing a few
internodes, which have not elongated, and some lear primordia. With the
The Stem 163
germination of the seed the embryo enlarges and starts to grow, the apical
meristem of the young shoot adds further leaf primordia and the inter-
nodes between the lower primordia, which in the meantime .have become
distant from the apex, elongate. In many plants buds develop in the axils
of the developing leaves giving rise to a branched shoot.
In mature plants the development of leaf primordia at the shoot apex
and the elongation of the nodes below it are the same as that in the growing
embryo of the germinating seed. The oider of appearance and the arrange-
ment of leaves on the stem is more or'less characteristic 'of each species.
FIG. 69. Portion of a branch of Prunus. The broken line which passes from leaf
to leaf indicates the phyllotaxis which, in this case,- is i.
That part of the stem from which a leaf or leaves develop is called the
node and that portion of the stem between two such nodes, the internode.
The length of the internodes varies in the different species. In certain
plants, such as CichoriunJ, Thrincia and others, where the leaves are ar-
ranged in a basal rosette, the internodes hardly elongate at all, but in most
spermatophytes the internodes elongate to differing extents. At each node
one, two or more leaves may be found. The arrangement of the leaves
on the stem is termed phyllotaxy. When there are more than two leaves at
one node the arrangement is termed whorled. When there are two leaves at
each node the leaves are said to be opposite; in this type of arrangement the
leaves of the successive nodes may be at right-angles to each other and then
the arrangement is termed decussate; or the leaves may form two parallel
ranks along the stem, i.e. distichous. When there is a single leaf at each node
and the leaves are arranged spirally on the stem the phyllotaxy is ~aid to be
164· Plant Anatamy
alternate. The space, on the circumference of the stem, between two succes-
sive leaves, whether they arise at a single node or whether t,hey are arranged
spirally on the stem, is constant, i.e. two successive leaves are separated by
a constant portion of the perimeter of the stem (Fig. 69).
The position of the leaf primordia on the stem apex is determined before
it is possible to distinguish any feature that indicates that such develop-
ment has begun. Therefore i1 appears that. the factors determining the posi-
tion of the primordia on the stem apex' are internal and, in general, they
are identical with those factors that control the distribution of the growth
potential in the apical meristem. There is reason to assume that each leaf,
together with that portion orthe axis around it and below its junction to
the axis, forms a single physiological unit. Such leaf fields or primordial
fields are already present in the shoot apex. This association between stem
and leaf is evident, among others, in the relationship between the phylo-.
taxy and the vascularization and general structure of the stem.
I
Arrangement of primary tissues in the stem
The primary body deveJops from the protoderm, procambium and
ground meristem. The arrangement and stru~ture of the primary tissues
is as follows. ,/ .
THE EPIDERMIS
The stem cortex is that cylindrical region between the epidermis and the
vascular cylinder (Fig. 70, nos. 1-4; Fig. 82, ;0'. 2). It may comprise various
cell types. In the simplest case, the cortex consists entirely of' thin-walled
parenchyma tissue. In many stems, as for instance Pelargonium, Retarna
and Salicornia, this parenchyma may have a photosynthetic function in
addition to that of the temporary storage of starch and other metabolites.
In other cases the outer region of the cortex, which borders on the epider-
mis, may include collenchyma or fibres, and the inner region of paren-
chyma. The collenchyma or fibres may form a continuous cylinder or they
may be present in the form of separated strips. The stem cortex may con-
tain sclereids, secretory cells and laticifers.
The Stem 165
THE ENDODERMIS
Internal to the cortex is the vascular system of the stem. In the gymno-
sperms and most of the dicotyledons the vascular system consists of a continu-
ous or a split cylinder which encloses the pith, i.e. the central portion of
the stem (Fig. 70, no. I; Fig. 77, nos. 1-4). In this cylinder two types of
vascular tissues can be distinguished-the phloem which is usually exter-
nal, and the xylem which is usually internal. In the case of the split cylinder
each strand is termed a vascular bundle. A vascular bundle in which the
phloem is only external to the xylem is said to be a collaterat' bundle (Fig.
70, no. 2; Fig. 72, nos. 3, 4; Fig. 83, no. 1). In some dicotyledonous fami-
lies, e.g. the Solanaceae, Cucurbitaceae, Asclepiadaceae, Apocynaceae,
Convolvulaceae and Compositae, internal phloem is also ptesent. The in-
ternal phloem may be present as separate strands on the border of the pith,
168 . Plant Anatomy
as in Lycopersicon (Fig. 70, no. 3), or it may be in close contact with the
inner side of the xylem, as in the stems of the Cucurbitaceae and Myrta-
ceae. The latter type of bundle is termed a bicollateral bundle (Fig. 82, no .
TYPES OF STELE
According to Smith (1955), Esau (1953) and other authors, the stele
of the sporophyte of vascular plants may be divided into two basic types:
(I) protostele which consists of a solid central cylinder of xylem surrounded
17(} Plant Anatomy
/
4
7
/
-"
FIG. 73. 1-3, Three-dimensional diagrams of different types of protostele. The
diagrams represent the stele alone without the cortex and epidermis. The micro-
ph~llls appear in those positions where there are protuberances on the surface of
the stele. 1, Hapiosteie. 2, Actinostele. 3, Plectostele. 4-8, Diagrams of cross-sec-
tions of stems with siphonosteles showing different stages in evolutionary develop-
ment. 4, Amphiphloic siphonosteie (solenostele), 5, Dictyosteie. 6, Ectophloic
siphonostele. 7, Eustele. 8, Atactostele. Xylem-hatched; phloem-stippled.
Three types of proto stele can be distinguished: (I) haplostele (Fig. 73,
no. 1) which is the simplest type, in which the xylem appears more or less
circular in cross-section, e.g. Rhynia and Selaginella .. (2) actiflostele (Fig.
The Stem 171
73, no. 2) in which the xylem is stellate in cross-section, e.g. Psilotum; and
(3) plectostele (F)g. 73, no. 3) in which.the.xylem is split into longitudinal
plates of which some are joined and others separate, e.g. Lycopodium.
The various types of proto stele are characteristic of the Lycopsida (the
lower Pteridophyta) and the siphonostele of the Pteropsida (the phylo-
genetically more advanced Pteri<iophyta and the Spermatophyta.).
Two types of siphonostele are distinguished according to the positions
dfthe phloem and xylem: (1) ectophloic siphonostele in which phloem only
surrounds the xylem externally (Fig. 73, no. 6); and (2) amphiphloic
"=
siphonostele (Fig. 73, no. 4) in which the phloem surrounds the xylem both
externally and internally and where the endodermis appears both outside
and inside the vascular tissue on the borders of the cortex and pith, res-
pectively (e.g. Adiantum and Marsilea).
The siphonostele may consist of a continuous cylinder of vascular tissue
or of a network of bundles (Fig. 74, no. 2; Fig. 75, nos. 1-5). The latter type
is the more advanced and the vascular tissues of this type appear in cross-
FIG. 75. Diagrams ofsiphonosteles with different types of arrangement of the leaf·
and branch-traces and gaps. 1, Unilacunar node with one leaf-trace. 2, Unilacu-
nar node with associated branch-traces and gap. 3, Overlapping gaps so that the
stele forms a network of bundles. (, Trilacunar node with three leaf-traces.
5, Unilacunar node with three traces.
section as a ring of separate bundles. The regions between the bundles are
parenchymatous. Those parenchymatous regions that occur in the stele,
above the positions where the leaf-traces pass out from the stele to the
leaves, are termed leafgaps (Fig. 75, no. I). An amphiphloic siphonostele
in which the successive leaf-gaps are considerably distant, one from the
other, is termed a solenostele. An amphiphloic siphonostele with overlap-
The Stem 173
ping gaps, i.e. that in which the lower part of one gap is parallel with the
upper part of another gap, is termed a dictyostele (Fig. 73, no. 5). In this
case the bundles are interconnected to form a cylindrical network (Fig. 74,
no. 2) and each bundle is of concentric structure consisting of a central
strand of xylem surrounded by phloem. Individually such buudles are
termed meristeles. From the anatomicaJ"point of view these are amphicri-
bral.bundles.
During the course of evolutionary development the eustele (Fig. 73, no.
7), with collateral bundles, was formed by,the splitting of the ectophloic
siphonostele. Bicollateral vascular bundles'in which the xylem strands are
accompanied externally and internally by phloem strands and which are
'found in advanced dicotyledonous families appear to be the result of a
secondary specialization and not a relic of the primitive structure charac-
teristic of the Filicinae.
In some plants, e.g. Marattia,Pteridium and Maton/a, two or more con-
centric cylinders of vascular tissue are present. Such a stele is termed a
polycyclic stele. The individual cylinders in this case are interconnected.
)n rare cases stems and roots contain more than one stele; such a condition
_is termed polystelic (see Chapter 13).
A different interpretation of the above nomenclature exists and has been
summarized by Sporne '(1962). According to this interpretation the·ecto-
phloic .siphonostele without leaf-gaps, such as is found in the Pteridophyta,
is considered as a protosteleand is termed a medullated protostele. Sporne
does not use the term siphonostele for the Pteridophyta.
As in the dictyostele, the bundles of the eustele are usually intercon-
nected. That type of stele in which the bundles are scattered (Fig. 73, no. 8),
such as is characteristic of the monocotyledons, 'is called an aiactostele
(Nast, 1944; Esau, 1953).
In the siphonostele not all the interruptions in the vascular tissue are
leaf-gaps as described above. Some interruptions result from the secondary
reduction of vascular tissue and the f-ormation of interfascicular paren-
chyma. Such interruptions are termed perforations. When such perforations
occur in a solenostele it may be confused with a dictyostele. The paren-
chymatous connections between the pith and cortex are termed medullary
rays. •
There are certain plants, such as Populus, for example (Fig. 74, no. I),
in which the primary vascular cylinder consists of a thin layer of vascular
tissue which is interrupted only by leaf- and small branch-gaps. Rib-like
projections composed entirely of protoxylem are found on the inner sur-
face of this cylinder (Eames and MacDaniels, 1947).
Differing from the Pteropsida whose steles form leaf-gaps, the steles in
microphyllous plants, i.e. the Psilopsida, Lycopsida and Sphenopsida, are
devoid of such parenchymatous regions (Fig. 73, nos. 1-3). In microphyl-
lOllS plants with a siphonostele the only gaDS oresent are hran('h-fTnn,,, whi~h
174 Plant Allatomy
.are tho~e gaps associated with bundles that depart from the central cylinder
to the lateral branches. This type of siphonostele has been termed by some
authors cladosiphonic, and that found in the. Pteropsida phyllosiphonic
(ie·ffrey,.1910).
FIG. 76. 1 and 2, Micrographs of the central portion of cross-sections'of the erect
rhizome of Ophioglossum /usitanicltm showing the transition from protostele to
siphonostele. I, Protostele. 2, Siphonostele. 3, A branch tip of Chimonanthus (a
woody genus of the Ranales) cleared by treatment with lactic acid to show the pat-
tern of vascularization in which the nodes are unllacunar with two traces; the
traces fuse on entry into the leaf. Nos. 1 and 2, x 200. '
The development of siphonostele f~om the proto stele has been a point
of discussion (Bower, 1911 a, b). According to one view the pith, i.e. the
parenchymatous core, in the siphonostele originated from the cortex.
The supporters of this vie\. use the presence of the endodermis, in certain
The Stem 175
ferns, between the pith and :vascular tissue as proof of this view. According
to them.the endodermis penetrated inwards together with the'parenchyma
of the cortex, and they explain the absence of such an endodermis in
other plants as being the result of further development. According to
another theory the siphonostele developed from the protostele by the
alteration of the inner vascular initials to parenchyma initials; This
theory is supported by the fact,that tracheary elements may be found in
the centre of the stele scattered among the parenchyma cells. This feature
is common in relatively primitive plants, both living and fossil. Such steles
may be considered as intermediate stages between protosteles and siphono-
steles. In the light of our present knowledge of morphogenesis it is difficult
to accept the former theory. Research on Ophioglossum lusitanicum
(Gewirtz and Fahn, 1960) showed that the stele.of the rhizome of the
sporophyte that developed from the gametophyte (not as a result of
vegetative reproduction) was protostelic at the base and siphonostelic
(dictyostelic) in its upper portion. Furthermore, it was clearly seen in
the transition zone, below the level of the first leaf-gap, that the pith
undoubtedly originated from the xylem. Here parenchyma cells were
seen mingled with tracheids and the number of parenchyma cells was
'seen to increase in an upwards direction (Fig. 76, nOs. 1, 2).
/ 4
FIG. 77. Diagrams showing the relationship between the vascular systems of the
leaf and the stem. 1-4, Cross-sections of the nodes of young stems. 1, Eucalyptus
camaldulensis in which the node is unilacunar. 2, Laurus nobilis, also with un i-
lacunar node. 3, Chrysanthemum ane/hi/olium in which the node is trilacunar with
three traces. 4, Dianthus caryophyllus which has opposite leaves and unilacunar
nodes. 5, Diagrams showing the possible ways of development of the nodal
vascularization in dicotyledons from the unilacunar node with two traces. (No.
5, adapted from Marsden and Bailey, 1955.)
The Stem 177
Bailey, 1957), By these workers and others it has been shown that in many
Pteridophyta, in the Cordaitales,. Bennettitales, in ,Ginkgo and Ephedra,
a single gap is found in that position where the leaf-traces depart from
the stele. Similarly, in many dicotyledons unilacnnar nodes with two leaf-
traces have also been found. Many of the dicotyledonous genera with
unilacunar nodes with a ,double leaf-trace belong to the primitive groups
of the Ranales and the Chenopodiaceae. Bailey (1956) found in many
dicotyledons that the vascular supply to the cotyledons consists of a
double leaf-trace which arises from a unilacunar node (Fig. 78).
\fl.. W
'
()# ••~
()
~ .
'~ '~
() ()
t"~\
~rJ
i/o,
. I 'cto .,
Bailey is of the opinion that the leaves of angiosperms are able to un-
dergo reversible changes in shape and vascularization.
In the light of the above facts it can be assumed: (I) that the unilacunar
node of certain genera of the Ranales is primitive and has not changed
during its evolution, from that of the lower Pteropsida; (2) in certain
other dicotyledonous genera, e.g. genera of the Leguminosae, Ana-
cardiaceae and others, the unilacunar node has apparently been derived,
178 . Plant Anatomy
by reduction, from a trilacunar node; (3) there are positive indications in-
certain dicotyledonous. groups, such as the Epacridaceaej and Chloran-
thaceae, that in the course of evolution the tri- and multilacunar .nodes
have arisen from the unilacunar node (Bailey, 1956).
The evolutionary development from.a unilacunar node· with two leaf-
traces to other types of unilacunar nodes with one, th,ree, .or more traces
may occur in a single family as can be demonstrated in the Chenopodiace!,e
(Bisalputra, 1962; Fahn and Broido, 1963). -
Co" """1-
FIG. 79. Diagrams of the primary vascular system of the stem of Anabasis articu-
lata. 1. Vascular system spread out on one plane. 2, Three-dimensional diagram
showing the bundles on one side of the stem. The number of bundles and their
arrangement as seen in cross-section can be seen at the level of the cut. \
In order further to clarify the nature of the leaf-traces and the nodal
type, it is necessary to study more accurately-the nature of the traces and
to follow their passage downwards in the stem. Often the picture obtained
at the node is not a true reflection of the situation but lower in the stem
the stele is more conservative (Fig. 76, no. 3) and a more correct picture
of the arrangement of the leaf-traces is obtained (Fahn and Bailey, 1957).
The suggested evolutionary trends of the basic types of nodal structure
are given in Fig. 77, no. 5.
In the literature the bundles of the stem have been variously classified,
i.e. as leaf-trace bundles, cauline bundles and common bundles. Leaf-trace
The Stem 179
bundle was used to designate those bundles that directly connect the leaf
and stele. Cauline bundle refers to those bundles that form the· major
vascular system of the stem and which may anastomose and give rise to
leaf-traces. The term common bundle has been used for those bundles
that run unbranched for a relatively long distance in the stem and which
eventually terminate in a leaf-trace. However, it appears that. in most
plants the vascular system of the stem can be interpreted as being a system
of leaf-traces that continue downwards in the stem for one or more inter-
/~'---'---'-'''''''
l
~ J
,
I
m 2
FIG. 80. Diagrams of the primary vascular system of the stem of Chenopodium
glaucum. 1, Vascular system spread out on one plane. 2, Three-dimensional dia-
gram showing the bundles on one side of the stem. The number of bundles and
their arrangement as seen in a cross-section can be seen at the level of the cut.
nodes where they fuse with the leaf-traces of Jower nodes (Figs. 79 and
80). This view is strengthened by the fact that the stem mainly functions
as a connecting organ between the leaves and the root. The number of
bundles in the stem is thus determined by the phyllotaxis and by the degree
to which the traces continue down in the stem. Inasmuch as the phyllo-
t~xis is more dense and the leaf-traces continue down along a larger Dum-
180 Plant Anatomy
BRANCH-TRACES
lne s\em, D~ \ne vaDDu, Q1CD\),leoom Ql'lieT ~Tom one ano\neT 1n \ne
pattern of the primary vascularization (Balfour and Philipson, 1962, and'
others). These differences are' apparently connected ~ith evolutionary
development.
The amount of primary .vascular tissue, as has been described above,
varies from a solid through a hollow unintO'rrupted cylinder to a small
number of narrow separate bundles. It is assumed that during the course
of evolution the primary vascular cylinder became thinner, i.e. it under-
went reduction in a radial direction and because of the appearance of
leaf-gaps, branch-gaps and perforations, and because of further reduction
of the vascular tissue in a tangential direction, the cylinder became split
into the longitudinal strands such as are seen in most dicotyledons.
The arrangement of secondary vascular tissue of the gymnosperms and
dicotyledons bears no relation to the arrangement of the primary vascular
tissue, and may be in the form of an entire cylinder. However. the amount
and arrangement of the secondary vascular tissues and especially that
of the xylem may also vary from an entire cylinder of various widths, as
in trees, to separate strands. as in the herbaceous stems of certain annuaJ
·-i- 1
The Stem 181
FIG. 82. 1, Micrograph of the outer portion of a cross-section of the stern of Zea
mays showing the vascula r bundles scatiered in the ground parenchyma. x 50.
184 Plant Anatomy
of the plants die during the dry season (Orshan, 1953) so reducing the plant
body. and its requirements to a minimum. However; although' most desert
perennials appear dead at the end of the dry season they are capable of
developing new shoots with the onset of the rainy season.
As no or little foliage'actually remains on desert plants during the dry
season, the main problem of adaptation is not by what means transpiration
is reduced in the leaves, but how the plant remains viable until the onsel
of the rainy season. The answer to this question must therefore be sought
in the axis of the plant.
In some desert plants the function of photosynthesis is taken over by
the cortex of the stem. In others, in addition to the presence of photo-
synthesizing tissues, water-storing parenchyma, is developed.
Another commonly observed character of desert shrubs is the occurrence
of a split axis. The axes of Artemisia herba,alba, Peganum harmala, Zygo-
phyllum dumosum and Zilla spinosa, for instance, become split by various
anatomical mechanisms into separate parts or "splits" (Ginzburg, 1963}
which may compete with one another; the split in the most favourable
'microhabitat around the mother plant will probably be the one to sur-
vive.
- 'In primary stems the cortex is generally narrow and the vascular tissues
are situated' on the periphery of a wide pith; However, it has been observed
that the cortex of the primary stem of plants growing in deserts or salt
marshes is recognizably thicker than in mesophytes. This feature, which is
accompanied by the Hcontr~ction" of the vascular strands around a narrow
pith, may be an adaptive one by which the vascular tissues are protected
from drought or other damage in the early stage,s of development before
the periderm is developed.\
It is known that in the articulated Chenopodiaceae, such as Anabasis
spp., Haloxylon spp. and Arthrocnemum glaucum, the fleshy photosynthe-
sizing cortex is'shed from the mature stems in the summer as the result of
the formation of a periderm which develops in the phloem parenchyma
(Fahn, 1963). It is also of interest that in some desert plants, such as Atri-
plex halimus, Zygophyllum dumosum and Fagonia cretica, for example,
which do not have a fleshy cortex, the first-formed periderm also develops
deep within the stem in the pericycle or phloem parenchyma. This is an
adaptive feature.
In some desert shrubs, such as Artemisia spp. and Achillea jragrantissima,
interxylary cork rings are produced at the end of each annual wood incre-
ment. Moss (1940) has already pointed out the important adaptive value
of these interxylary cork tissues which reduce water loss and restrict the
upward passage of water to a narrow zone of functioning secondary xylem.
The anatomical structure of the stems of some xerophytes (plants grow-
ing in arid habitats) and one halophyte (plant growing in saline soil) will
be described here,
IB8 Plant Anatomy
Retama raetam. (Fig. 84, no. 3) may be cited as an example of plants with
xeromorphic.stem.(Evenari, 1938)._Along;the.green~ brapches.o( this plants
there are ribs and furrows. The stomata are situated in the furrows in
which there are also nllmerous hairs ..:rhe central portion ofthe .ribs .con~
sists of a sclerenchymatous strand which is accompanied laterally, on the
sides facing the furrows, by one .or two rows of large colourless parenchyma
cells which serve as water~storjng cells and usually contain crystals. Be~
use
tween these cells and the epidermis is the photosynthesizing tissue, which
,consists of small and dense parenchyma cells containing chloroplasts and
crystals. The epidermal cells have very thick outer walls and cu!icle.
The succulent stems of desert plants are characterized by a well developed
water-storing tissue in the cortex and pith. As a result of this the ratio be-
tween the cortex and vascular cylinder in these plants is considerably larger
than that of other dicotyledons (Fig. 84, nos, 1,3), Anabatiis articulata (Fig.
84, no: '2; -Fig, 85) may be cited as an example of desert plants with a
Multiseriate
epidermis
Branches of
. vQscular bundles
Central vascular
cylinder
---,c"','jI
Water-storing
, parenchyma
succulent stem (Volkens, 1887; Evenari, 1938; Fahn and Arzee, '1959), In
the young green internodes the epidermis consists of three to four layers
of thick-walled cells and it is covered by a very thick cuticle, Below the
epidermis is a hypodermis of thinner-walled cells which similarly to the
epidermis contain crystals, On the inside of this layer there is a layer
of palisade cells with chloroplasts, Immediately inside the palisade tissue
is a layer of more or less cubical cells which also contain chlorophyll. Still
further inwards is the water-storing parenchyma which also contains, here
and ther", large druses. With the maturation of the stem, cork tissue devel-
ops in the outer phloem parenchyma. Branches of the vascular bundles of
the internodes pass through this cork tissue into the cortex, In still older
branches the connection between the bundles and their branches is disrupt-
ed by the completion of theglrk cylinder and, as a result, the outer layers
dry out and are shed,
In Salicornia fruticosa (Fig. 86, nos. 1-3) which grows in salines, the
structure of the cortex is simpler (Fahn and Arzee, 1959). The epidermis
190 Plant Anatomy
References
ARBER, A. 1950. The Natural Philosophy of Plant Form. Cambridge Univ. Press, Cam~
bridge.
BAILEY, 1. W. 1956. Nodal anatomy in retrospect. Jour. Arnold Arb. 37: 269-287.
BALFOUR, E. E. and PHlLIPSON, W. R. 1962.' The development of the primary vascular
system of certain dicotyledons. Phylomorpho!ogy 12:.110-143.
BISALPU"fRA, T. 1962. Anatomical and morphological studies in the Chenopodiaceae.
III. The primary vascular system and nodal anatomy. Aust. Jour. Bot. 10: 13-24.
BLYTH', A. 1958, Origin of primary extraxylary stem fibers in dicotyledons. Univ. Calif.
Publ. Bat. 30: 145-232.
BOWER, F. O. 1911a. On the primary xylem and the origin of medullation in the Ophio~
glossum. Ann. Bot. 25: 537-555.
BOWER, F. O. 1911b. On the medullation in the Pteridophyta. Ann. Bot. 25: 555-575.
CRAFTS. A. S. 1943. Vascular differentiation in the shoot apex of Sequoia sempervirens.
Amer. Jour. Bot. '3~: 110-121.
CUTTER, E. G. 1959. Formation of lateral members. Proc. IX. Intern. Bot. Congr.
Montreal: 84-85.
DE BARY, A. 1877. Vergleichende Anatolnie der Vegetationsorgane der Phanerogamen
und Farne. W. Engelman, Leipzig.
EAMES. A.]; and MACDANIELS, L. H. 1947. An Itllroduction to Plant Anatomy, 2nd ed.
. McGraw-Hill, New York-London.
ESAU, K. 1950. Development and structure of the phloem tissue. If. Bot. Rev. 16:
.",_. 67~1I4.
ESAu,_K.J953._Plant Anatomy. John Wiley, New York.
EVENARI, M. 1938. The physiological anatomy of the transpiratory organs- and the
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Linn. Soc. London, Bot. 51: 389-407.
FAHN, A. 1963. The fleshy cortex of. articulated Chenopodit1ceae. Maheshwari Camm.
Vol, Jour. Indian Bot. Soc. 42A: 39-45. '
FAHN, A. and ARZEE; T. 1959. Vascularization of articulated Chenopodiaceae and the
nature of their fleshy cortex. Amer. Jour. Bot. 46: 33()..-338.
FAlIN, A. and BAILEY, 1. W. 1957. The nodal anatomy and the primary vascular cylinder
of the Calycanthaceae. Jour. Arnold Arh. 38: 107-117.
FAHN, A. and BROIDO, S. 1963. The primary vasculariiation of the stems and leaves
of the genera Salsola and Suaeda (Chenopodiaceae). Phytomorphology 13: 156-165.
GEWIRTZ, M. and FAHN, A. 1960. The anatomy of sporophyte, and gametophyte of
Ophioglossum lusitanicum L. ssp. lusitanicum. Phytomorphology 10: 342-351.
GINZBURG, C. 1963. Some anatomic features of splitting of desert shrubs. Phyto-
morphology 13: 92-97.
GUNCKEL, J. E. and WETMORE, R. H. 1946a. Studies of development in long shoots of
Ginkgo hi/oba L. I. The origin and pattern of development of the cortex, pith and
procambium. Amer. Jout. Bot. 33: 285-295.
GUNCKEL, J. E. and WETMORE, R. H. 1946b. Studies on the development of long and
short shoots of Ginkgo hiloba L. II. PhyIlotaxis and organization of the primary
vascular system, primary phloem and primary xylem. Amer. Jour. Bot. 33: 532-543.
GWYNNE-VAUGHAN, D. T. 1901. Observations on the anatomy of solenostelic ferns.
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JEFFREY, E. C. 1910. The Pteropsida. Bot. Gaz. 50: 401--414.
KUMAZAWA, M, 1961. Studies on the vascular course in maize plant. Phytomorpho{ogy
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MARSDEN. M. P. F. and BAILEY, 1. W. 1955. A fourth type of nodal anatomy in dieD--
tyledons illustrated by Clerodendrolt trichotomum Thunb. Jour. Arnold Arb. 36:
I-51.
CHAPTER 12
THE LEAF
MARSDEN, M. P. F; and STEEVES, T. A. 1955. On the primary vascular system and nodal
anatomy of Ephedra. Jour. Arnold Arb. 36: 241-258.
METCA~FE, C. R. and CHALK, L. 1950. Anatomy of the DicotyleJons. Clarendon Press;
Oxford.
Moss, E. H. 1940. Interxylary cork in Artemisia with a refere-nee to its taxonomic signi-
ficance. Amer. Jour. Bot. 27: 762-768.
NAST, C. G. 1944. The comparative morphology of the Wintera(:eae. VI. Vascular
anatomy of the .flowerjng shoot. JOJlr. Arnold Arb.-25: 454-466.
ORSHAN, G. 1~53. Note on the application of Raunkiaers system of life forms in arid
regions. Palest, iour. Bot. Jerusalem 6: 120-122.
PALLADlN, W.l. 1914. Pjlanzenanatomie. B. G. Teubner, l:eipzig and Berlin. "'"
PmuPsoN, W. R. 1949. The ontogeny of the shoot apex in dicotyledons. Bioi. Rev.
24: 21-50.
PRIESTLEY, J. H. 1926. Light and growth. II. On the anatomy of etiolated plants. New
Phytol. 25: 145-170. .
SHARMAN, B. C. 1942. Development anatomy of the shoot of Zea mays L. Ann. Bot.
6: 245-282.
SINNOTT, E. W. 1914. Investigations on the phylogeny of angiosperms. r. The anatomy
of the node as an aid in the classification of angiosperms. Amer. Jour. Bot. 1:
303-322.
SMITH, G. M. 1955. Cryptogamic Botany. Vol. II. Bryophytes and Pteridophytes. 2nd
ed. McGraw-Hill, New York.
SNOW, M. and SNOW, R. 1947. On the determination of leaves. Nelv Phytol. 46: 5-19.
SPORNE, K. R. 1962. The Morphology of Pteridophytes. Hutchinson Univ. Library,
London. ~
TROLL, W.-1948.' Allgemeine Botanik. F. Enke,"".Stuttgart.·
VAN FLEET, D. S. [941a. The development and distribution oftfte endodermis'and an
associated oxidase system in monocotyledonous'plants. Amer. Jour. Bot: 29:,)-15.
VAN FLEET, D. S. 1942b. The significance of oxidation in the endo,dermis. Amer.'Jour.
Bot. 29: 747-755. /
V AN FLEET, D. S. 1950a. The cell forms and their common substance reactions in the
parenchyma-vascular boundary. Bull. Torrey Bot. Club 77: 340-353. ,
VAN FLEET, D. S. 1950b . A comparison ofhis~oc~e~i~a_! apd anatomicaI"characteristics
of the hypodermis with the- endodermis in vascular plants. Amer. 'lour. Bot. 37:
721-725. I
VAN TIEGHEM, P. and DOULIOT, H: 1886: Sur la polystelie. Ann. Sci. Nat. Bot., Ser. 7,3:
275-322. .
VOLKENS, G. 1887. Die Flora der aegyptisch-arabischen Wilste auf Grundla'ge anatomisch
physiologischer Forschungen. Gebr. Borntraeger, Berlin.
WARDLAW, C. w. 1948. Experimental and analytical studies ,of Pteridophytes. XI.
Preliminary observations on tensile stress as a factor in_fern phyllotaxis. Am};.
Bot. N.S. 12: 97-109. .. I.
WARDLAW, C. W. 1950. Experimental and analytical studIes of pterIdophytes. XVI.
The induction of leaves and buds in Dryop/eris aristata Druce. Ann. Bot. N.S. 14:
435-455.
WARDLAW, C. W. 1955. Evidence relating to the diffusion-reaction theory of morpho-
genesis. New Phytol. 54: 39-48.
WARDLAW, C. W, 1960. The inception of shoot organization. Phytomorphology 10:
107-110.
WETMORE, R. H. 1943. Leaf-stem relationships in the vascular plants. Torreya 43: 16-28.
ZOHARY, M. 1961. On hydro-ecological relations of the Near East desert vegetation.
Plant water relationships in arid and semi-arid conditions. Proc. Madrid Symp.
Unesco, Arid Zone Res. 16: 199-212.
Plant Anatomy
194
Morphology of the leaf
In this chapter we shall deal with foliage leaves which/among the Angio-
spermae in particular, themselves exhibit great variation in anatomical and
morphological structure. The foliage leaves in certain· angiosperm genera
are sessile and consist almost entirely of lamina, but in most genera several
distinct parts can be distinguished in. the foliage leaf, i.e. the leaf base,
petiole and lamina. The shape, structure and relative size of these parts
differ and are used to classify the foliage leaves into different types. In
many dicotyledonous genera two appendages, i.e. the stipules, develop" at
the leaf base. The stipules may be attached to the leaf base or they may
be free appendages. Even when they are free appendages they develop as
outgrowths of the leaf primordia. The vascular supply of the stipules is
derived from the leaf traces (Fig. 97, no. 2). In some plants the stipules
are green and resemble leaflets and have a photosynthetic function, but
their main function is -the protection of young developing leaves. In some
I~~
(~J
\
4 primordial 5
lamina
FIG. 87. Bud scales of Vilis vinijera. Numerals indicate the centripetal order of
appearance of the scales and young foliage leaves. 1, Cataphyll which envelops
the bud. 2-5~ Young foliage leaves in order of their appearance in the bud. The
stipules of the outer leaves are relatively large and they serve to protect the bud.
(Adapted from drawings made by Z. Bernstein.)
The Leaf 1<:)5
woody dicotyledons (e.g. Ficus and Vitis) the outermost scales protecting
the buds are stipules (Fig. 87, nos. 1-5). In most monocotyledons and some
dicotyledons (the Umbelliferae and Polygonaceae) the leaf base is widened
so as to form a sheath which surrounds the node. Usually a relationship
exists between the anatomical structure of the node and the appearance of
stipules and a sh~athin the dicotyledons (Sinnott and Bailey, 1914). Most
plants with trilacunar nodes (i.e. those in which the.re are three leaf-gaps
at 'each node) have stipules and those with multilacunar nodes, have
sheath-like leaf bases.
Leaves are divided into simple and compound depending on whether the
stalk bears one or more leaflets. In the case of compound leaves the com-
mon stalk is termed the rachis. If the leaflets arise from the sides of the rachis,
the leaf is said to be pinnate, and if all from one point, as rays, palmate.
The margin of the leaf, and similarly the leaflets, may be entire or variously
notched.
In some plants, such as many species of Acacia of Australian origin, the
lamina of. the leaves is reduced, and from the remaining parts a flat, leaf-
.:like photosynthetic organ is developed; this type of organ is termed a
~phyllode. In certain plants (Opuntia spp., Muehlenbeckia platyclados) the
stems are photosynthetic and have become flat; such organs are termed
platyclades, When the platyclade appears very leaf-like, as in Ruscus, it is
billed a phylloclade.
THE EPIDERMIS
THE MESOPHYLL
Spongy
paren chyma
Sclerenchymo
Palisa de
parenchyma
FIG. 89. 1, Portion of across-section of the Jamina of L ifium candidum showing lob-
ed palisade parenchyma cells immediately below the epidermis. x 430. 2, Cross-
section of the leaf of Thymelaea hirsuta in which the palisade parenchyma is pre-
sent on the abaxial side of the leaf and the spongy parenchyma on the adaxial
side. X 55. 3, As in no. 2, but portion around mid-rib en larged. x 200.
FIG. 90. 1, Portion of a cross~section of the leaf of Atriplex portulacoides showing
the vesiculate salt trichomes, thick~wal1ed epidermis and isobilateral arrange-
ment of the mesophyll tissues. The inner mesophyll celIs are large, contain few
chloroplasts and store water; occasional druses occur in these cells. 2, As above,
but of Atriplex halimus in which the epidermis is relatively thin-walled, the uni~
seriate hypodermis is devoid of chloroplasts, stores water and contains occa-
sional druses. The chlorenchyma, which consists of uniform elongated cells, is pre-
sent between the abaxial and adllxial hypodermal layers. The bundle sheaths are
open on the abaxial sicif' nf th", " .. ",.,,~ ~J..~ ~ ... 11~ -~ ~,_- -'_
i,."",
200 Plant Anatomy
FlO. 91. Micrographs of sectiOr1s cut parallel to the leaf su rface of Rosa. 1, Section
through the palisade parenchyma. x 520. 2, Sectio n through spongy parenchyma.
x 420.
The Leaf 201
external surface area of the leaf. The ratio of the volume of the intercellular
~paces, to the,total volume of the leaf varies between 77 ;.1 pOD and 713: 1000
(Sift on, 1945). The ratio of the internalsurface area to the external surface
area is of ecological importance (Turrell, 1936, 1939, 1942, 1944). The inter-
nal surface area of Styrax officinalis is eight times larger than the external
surface area, and in Olea it is eighteen ,times larger.
The specialization of the palisade tissue that results in more efficient
photosynthesis is brought about not only by the increased number of
chloroplasts in the cells but also by the dimensions of its free surface area,
Although the volume of the intercellular spaces in the spongy tissue is
much larger than that,in the palisade tissue, the free surface area is greater
in the palisade tissue. This feature becomes obvious when sections parallel
to the leaf surface are examined (Fig. 91, nos, 1,2). In such sections it is
seen that the palisade cells are round in cross-section and that the areas of
contact between the cells are restricted to very narrow strips, while the
areas of contact in the spongy tissue are fiatter and wider. Thus, for exam-
ple, in Styrax officinalis the free surface area of th.palisade tissue is about
twice as large as that of the spongy tissue. \
The intercellular spaces of the mesophyU usually develop schizogenously,
but in certain plants the development may also be lysigenous by the dis-
integration of gr()UPS of cells, Examples oUhe latter type o(development
are seen in water and marshy plants, and also in the banana leaf (Skutch,
1927) (Fig. 92, no. I).
/
STRUCTURAL CHANGES OF EPIDERMIS AND MESOPHYLL IN LEAVES OF
XEROPHYTES
(1907) these cells transport water to the peripheral layers. Other in-
vestigators (Heinricher, 1885; Volkens, 1887; Solereder, 1908; De Fraine,
1912) believe that these tracheid-like cells have a water-storing function
(Fig. 94). Tracheoid idioblasts may be scattered throughout the mesophyll
as, for instance, in Pogonophora schomburgkiana of the Euphorbiaceae
(Foster, 1956)'.
Involution of the leaves, which is especjally typical of grasses, is a cha-
racter" of xerophytes. This feature is brought about by the action of bulli- •
form cells and lor other epidermal and mesophyll elements which may be
parenchymatous or sclerenchymatous (Shields, 1951).
There are plants, such as Nerium oleander, which, although growing in
favourable wet conditions, have xeromorphic leaves, as these are defined
today. On the other hand, plants, such as Prunus amygdalus and Anagyris,
which grow in dry hahitats, have leaves of mesomorphic charader. How-
ever, in the majority' of cases there is a correlation between all the above-
mentioned xeromorphic features, or some of them, and the dry conditions
ortlle 'habitat. It is necessary to continue the investigation of additional
-anatomical and physiological features in order to understand better how
.desert plants withstand conditions of extreme drought.
spaces which are usually regular in shape and which pass through the
entire leaf. In many plants the air chambersomay penetrate deep into the
tissues of the stem. This type of structure can be seen in the leaves of
Potamogeton and Eichhornia. The air chambers are usually separated from
one another by thin partitions of one or two layers of chloroplast-contain-
ing cells. Cross partitions also occur in the air passages-elongated air
cavities arranged parallel to the longitudinal axis of the organs-where
they are termed diaphragms (Fig. 96, nos. I, 2). The diaphragms consist
of a single layer of cells with small intercellular spaces. which appear as
small pores and which apparently allow the passage of gases but not of
water.
The most specialized tissue found in the stem and respiratory roots of
many water plants is the aerenchyma (Fig. 95, no. 3). Aerenchyma is,
strictly speaking, a phellem tissue derived from a typical phellogen of
either cortical or epidermal origin (Haberlandt, 1918). During the devel-
opment of this phellem single cells, at constant intervals from each cell
.layer, elongate in a radial direction (in relation to the organ) while the
~cells between them remain short. This lengthening of the cells pushes the
previously formed layer of cells outwards so that numerous elongated air
~chambers, parallel to the axis of the pbnt, are formed. The elongated
cells form the radial walls of the air' chambers, while the non-elongated
'cells form the tangential' walls. From the physiological point of view,
however, any tissue that,
contains
I
large intercellular spaces is termed
aerenchyma.
Immersed hydrophytes contain very little sclerenchyma and may even
be devoid of this tissue. Strips of sclerenchyma are, however, sometimes
found along the leaf margins.
The root system ofhydrophytes °is usually very reduced and it principally
provides an anchorage in the soil as the uptake of water and salts is car-
ried out by the leaves and stems, For the same reason the vascular system
is also much reduced. The reduction is especially obvious in the xylem
tissue. Tn many species the xylem of the large veins is represented by only
a few elements and in many of the small veins tracheary elements are
completely absent. In such plants a duct similar to an air passage is pre-
sent in the position of the xylem. The phloem is also reduced in comparison
with that of land plants, but not to the same extent as is the xylem.
A similarity :exists between the tissues of the petiole; arid those of the
stem. The epidermis of the petiole is continuous with that of the stem.
The parenchyma cells of the petiole" like those of the, cortex, contain,only
a few chloroplasts especially as compared with the cells of tqe lamina.
The supporting tissues of the petiole are collenchyma and jor sc\erenchyma.
The vascular.bundles of thepetiol<'".l]lay be collateral"e.g. Ligustrum (Fig.
98, no. 1), bicollateral, c;.g. _Ner_ium, or concentric, as in certain pterido-
phytes and many dicotyledons. The phloem is accompanied, in many
species, by groups of fibres. The arrangement" of the vascular tissues in
the petiole differs in different plants. It may appear in a cross-section of
the petiole as a continuous or an
int~rrupted cre~cent, e.g. Olea, Nicotiana,
Nerium, Of as an entire or interrupted'ring, e.g. -Ricinus, Quercus calliprinos,
\'] I
Aerenchyll"lO
FIG. 95. 1 and 2, Succulent leaf of Sa/sola kali. 1, Outline of entire cross-section
of lamina. 2, Detailed diagram of one half of the lamina showing palisade paren-
chyma on both sides of the leaf and the central portion of large, water-storing
parenchyma cells. 3, Portion of a cross-section of an aerial pneumatophore of
Jussiaea peruviana in which secondary vascular tissues, phellogen and aerenchy-
rna can be distinguished. (Nos. 1 and 2, adapted from Shields, 1951; no. 3,
aClantprl from Palbrlin lQ,A ...
210 plant Anatomy
Q. boissieri (Fig. 98, no. 2), Citrus (Fig. 102, no. 1), or as a ring with
additional internal and external bundles, e.g. Vilis, Platanus, Robinia,
Pelargonium. An arrangement of· scattered bundles is ~een in many mono-
In certain plants the base of the petiole appears swollen and contains
a larger amount of parenchyma than·the rest· of the petiole. Such a petiole
base is termed a pulvinus. In such cases where this area is capable of bring-
ing about leaf movement, as in Mimosa pudica, for example, the paren-
cbyma cells on the active side of the pulvinus have thinner walls. Inter:
cellular spaces are present and there appears to be a close association
between the cytoplasm and the cell wall.
As has already been noted in the chapter on the stem, one, two. three
or many leaf traces enter the leaf. The leaf traces may continue in the
same number throughout the entire length of the leaf or they may divide.
fuse and branch again later. Single or several closely associated vascular
bundles form the veins. The term vein is sometimes used to include the
vascular tissue together with the non-vascular tissue that surrounds it.
212 Plant Anatomy
There are plants, such as certain species of the Coniferales and Equ;setum,
in which theJeaf has but a single ve.in .. However, in most of the higher
pteridophytes and the majority of the angiosperms the' leaf contains nu-
merous veins. The arrangement of the veins in-the leaf is termed venation.
, .,'
-_ ._
• >
," _',
of their length but approach one another and fuse at the leaf tip or both
at the leaf tip and base. These "parallel""veins are interconnected by very
thin bundles which are scattered throughout the lamina (Fig. 99, no. 2).
In certain monocotyledons, e.g. Zantedeschia, a special type of venation
is found. Here the veins are parallel for a certain distance, after which
they spread out in a feather-like pattern. In these leaves there are also
small veins that connect the main veins. Parallel venation can also be
found in certain dicotyledons, e.g. Plantago, Geropogon and Tragopogon,
and reticulate venation also occurs in certain monocotyledons, e.g. genera
of the Orchidaceae, in Smilax and Arum.
When the venation is reticulate the largest vein passes through the
median part of the leaf andit forms the main or central vein from which
smaller veins branch. In certain leaves numerous 'large veins can be seen
spreading out, as rays, from the base of the leaf lamina toward its margins.
Those parts of the lamina through which the larger veins, both main and
secondary, pass are usually thicker and project as ribs on the abaxial side
()f the leaf. These ribs are formed of parenchymatous tissue which is poor
'iii: chloroplasts, and of supporting tissue which is usually collenchyma.
'.Therefore the larger veins have no direct contact with the mesophyll, in
.the narrow se::!se of the word. The small veins that form a network between
the'larger veins, and which occur in the mesophyll proper, are usually
situated in the outermost layer of the spongy mesophyll which borders
the palisade cells (Fig.,,88, l10s. 2, 3; Fig. 92, no. I; Fig. 93, nos. I, 3).
The small veins usually form net,,:orks. These networks vary in size
aL<d shape, and they accordingly subdivide the area of the mesophyll.
The smallest areas, which are bounded by the thinnest branches of the
b4ndles, are called areoli, and they usually contain terminal vein-endings
which end blindly in the mesophyll (Fig. 99, no. 3). The degree of branch-
ing of these vein-endings differs in the leaves of different plants. Thus, for
instance, in the leaves of Euphorbia (Fig. 99, no. 3) or Ricinus, very many
such blind ends may be found in a single areole, in Morus there are some-
what fewer, in Quercus boissieri very few, and in the leaves of Q. calliprinos
(Fig. 97, no. 3) blind vein-endings are absent or almost so.
In monocotyledons with parallel venation, the veins that pass along
the entire leaf may be almost of the same thickness or they may be of
different thicknesses. In the latter case the thick and thinner veins are
arranged alternately. The median vein is usually the thickest.
In Ginkgo and many pteridophytes the veins do not form a closed system
since the adjacent branches do not anastomose (Arnott, 1959). In such
leaves all the terminal branches terminate freely within the lamina or along
its margins. In many leaves of this type the branching of the veins is
dichotomous.
In most cases the arrangement of the vascular tissue in the main vein
resembles that in the petiole.
214 Plant Anatomy
Bundle sheaths
bundle sheath consists of parenchyma cells, there are some families, such
as the Winteraceae (Bailey and Nast, 1944),.in which ~he sheath is scleren-
chymatous.
(
FIG. 100. 1. Micrograph of portion of a cross-section of the leaf blade of SIYIUJ(
officinalis in which stellate hairs on the abaxial surface and bundle-sheath exten-
sions can be distinguished. x 150. 2. Surface view of cleared leaf of Atrip/ex hal!- /
mUll in which the bundle sheaths surrounding the veins can be seen.
I
SECRETORY STRUCTURES
~.. ~.
ceae and Leguminosae. Cells with tannin compounds are also fo und in
parenchyma of fruits, e.g. of Ceratonia. Cells with mucilaginous contents .
a re found in the Buxaceae, many species of the Malvaceae, Chenopodia-
The Leaf 221
Most gymnosperms are evergreen and their leaves are usually xero-
morphic. Two types of gymnosperm leaf will be described here - that of
Cycas and that of conifers, such as Pinus and Cedrus.
The leaf of Cycas (Fig. 103, no. I) is leathery and stiff, the epidermal
cells are thick walled and have a thick cuticle, and the stomata are sunken
and occur on the abaxial surface of the leaf. The mesophyll consists of
palisade and spongy parenchyma as in angiosperms. A un i- or biseriate
hypodermis is present between the adaxial epidermis and the pali..de
parenchyma. The xylem of the median vein is of a special primitive type.
The protoxylem is present on the abaxial side and the metaxylem on the
adaxial side. The protoxylem is accompanied by a small amount of paren--
chyma. Secondary xylem develops near the phloem from a cambium situated
between the two types of vascular tissue. The vein is surrounded by an
endodennis. The transfusion tissue, which consists of tracheids and elon-
gated parenchyma cells, occurs on both sides of the vein. This tissue is cha-
222 Plant Anatomy
The epidermis of the needle~like leaves · of Pinus (Fig. 103, no.' 2) and·
Cedrus (Fig. 104, nos. 1,2) consists of extremely thickcwalled cells and is cov-
ered with a thick cuticle. The stomata are scattered on all sides of the leaf;
they are sunken and are overarched by the subsidiary cells (Fig. 103,
no . 2). A hypodermis of fibre-like cells is present except in the areas below the
stomata. The mesophyll is of a parenchymatous nature. The walls of the
mesophyll cells h ave characteristic ridge-like invaginations into the cells.
The Leaf 223
These cells contain chloroplasts. Resin ducts are also present in the meso-
phyll .. In the centre of the leaLthere is.a single vascular. bundle, or .two,
which are then close to one another. The arrangement bf the proto- and
metaxylem is as in angiosperms, j.e. the protoxylem is on the adaxial side
and the metaxylein.on the abaxial side close to' the phloem. The bundle is
surrounded by transfusion tissue consisting oftracheids and of living paren-
chyma cells. The parenchyma cells contain tannins"resins, and also starch
in certain seasons'ofihe year. The tracheids closest to the'bundles are long
while those further away are more parenchyma-like and have relatively
thin, slightly lignified walls and bordered pits. Because of their thinner
walls these'tracheids are not able te withstand the pressure of the living
cells around them in which the turgor is higher, so they become somewhat
crushed. In the transfusion tissue close to the phloem there are certain cells
that have dense cytoplasm and which are similar to albuminous cells. The
vascular bundles together with the transfusion tissue are surrounded by
a sheath of relatively thick-walled,cells-the.,endodermis (Fig. 103, no. 2;
Fig. 104, no. I).
INITIATION
The initiation of the leaf commences' with periclinill divisions in 'a small
group of cells on the sides of the apex.The number of cell layers, however,
th'at begin to divide thus and their position on the apex varies'considerably
in different plants. For example, in many grasses, it was found that lear'
initiation starts with periclinal division in the cells of the surface layer of
the apex (i.e. in the outermost layer of the tunica) and 'in~ cells of the layer,
immediately below it (Sharman, 1942, 1945; Thielke, 1951). In this case the'
main portion of the leaf primordium originates·from the butermost cell
layer of the shoot apex. .
Contrary to the situation in the grasses, in other monocotyledons, e.g.
Tulipa (Sass, 1944), and apparently in all the dicotyledons thus far exam-
ined, the first periclinal divisions do not take place in the cells of the surface
Jayer, but in the cells of one or more Jayers below it. In the apices of such
plants, therefore, the surface layer does not take part in the initiation of
the inner tissues of the leaf, This layer enlarges by numerous anticlinal
divisions of its cells and so becomes adapted to the growth of the primor-
dium. The surface layer gives rise to the protoderm of the young leaf.
The Leaf 225
EARLY DIFFERENTIATION
-,
-------0
------0
-----w ------6
n---o -----u ----cC1
L~--o --u 2
-cC:\ 3
: ;' ii>
I",
------~
--------0
/
------D,
:;,?,] W3"
---A
--0 \\~:;! (/ ~ \
~4 )__L._ ___ ~ __ -- _. 5
Sfster cell of
Submarginal submarginal.
""1
The marginal growth apparently continues longer than does the apical
growth but it, too, ceases relatively early. In Nicotiana tabacum, for ex-
ample, Avery (1933) observed that marginal growth continues, at least
The Leaf 229
in the lower part of the lamina, until .lhat stage where the leaf is several
centimetres long. In. Cercis.siliquastrum.(Slade,.1957).marginal growth of
the lamina is completed by the time the leaf measures 2-2·5 mm. In the
leaf of the Dwarf Cavendish banana, in which. a marginal .vein is differ-
entiated in the early stages of development of the leaf primordium, mar-
Middle spongy
parenchyma
Aooxiol - - - - - - - - _ A b o K i a l epidermis
protoderm
Adaxial _ _ _ _ _ _ _ __
protoderm Adaxial epidermis
Ad . If______'"Middle spongy
, . / OXIO ler parenchyma
/
Marginal Submorgin~Middle __...Procombium_....Vo.sculor ,tissue of
initials ' initials layers' ~ _ vems
are mostly anticlinal and thus a plate meristem (Fig. 106, nos. 3, 4) is
formed._A. plate meristem. is· one in which the planes orcell divisions"in
each layer are perpendicular to the surface of the organ in which the
meristem occurs. The activity of such a meristem results "in increase in sur-
face area but not in thickness of the organ. In the lamiea the cells of this
meristem ,have a stratified arrangement and therefore it is possible to
Adaxial _ _ _ _ _ _ _ _ _ _ Adoxiol
protoderm ep'laermis
Adaxial Palisade
Ioyer - - - - - - - -....parenchymo
_ _ _~V,osculor bundles
/ ProCombium OM sheath
/
Marginal Submorginal~Middle loyers _ _ _ _ _ _~Middle spongy
Inifiois ~irli1iOiS
~Abo,(,.'a)1 parer\chymo.
~ Abaxial spongy
loyer ---------+-:porenChyma
Abaxial Abmdal
. protoderm epidermis
Adaxial
//
----,-C---......-Adoxial epidermis
protoderm
~
do)',\al mesofiliyll loyer
~~~:fal _ _ _ _ _ _ _ _".~Midri~thjCkening
Marginal
initials
/
:~~margiM\,
IMlals ~
\
',',f
Central,'O, and
procamblol strands
Bundle sheo1h
Central mesophyll
Vascular bundles
.
. Bundle sheath
trace, with relative ease, the origin of the epidermis, palisade and spongy
tissues, and the vascular bundles. In Fig. 107 and 108, it can be seen how,
in different plants, these tissues develop from the cell layers of the y~ung
1amina.
The regular arrangement of the cell layers is interrupted to differing
extents by the development of the vascular bundles, their sheaths and
supporting tissues. As a result of this, in the final stages of the expansion
of the leaf surface the regular arrangement of the cell layers becomes
The Leaf 231
restricted to those areas of the lamina between the large lateral veins. The
palisade parenchyma'is'one'of the last·tissueS"to cease growing and divid·
ing. This tissue -may continue to Junction as a meristem for- some time
after the cells of the spongy parenchyma and of the epidermis have ceased
to divide.
The different parts of the leaf expand at different rates and in different
directions (Avery, ·1933). This type of growth has been termed anisotropic
growth (Ashby, 1948a). The type of growth of a leaf is controlled by
genetic factors, but it is also influenced by internal and external environ·
mental conditions (McCallum, 1902; Ashby, 1948b; Allsopp, 1955; Jones,
1956). Thus, the shape of leaves on different parts ofthe stem of the same
plant is apparently' influenced by internal factors. Among the external
factors that influence the sbape of leaves are water supply, nutrients, day
length, amount of light, etc.
The development of tbe vascular system in tbe leaf has, as yet, been
'studied only in a small number of plants. From what is known of the
development of dicotyledonous leaves, it appeal's that development of
the procambial strand of the mid·rib precedes that of the lamina and
it pro,eeds in the acropetal direCtiojj (Fig. 106; no. 5). With the com·
mencement of the development of the lamina the procambi,il strands
of the large lateral veins and, later, of the smaller veins begin to form
gradually. As was seen in j-V;cotiana tabacum (Avery, 1933), the procambial
strands of the small vein;, which form in a basipetal direction, develop
mainly during the intercalary growth of the lamina (Fig. 105, nos. 4, 5).
However, deviations from the above- pattern of 'oiiferehtiation- are· also
known to occur (Slade, 1957).
In the leaves of Zea (Sharman, 1942) the procambial strands of the
median vein and of the principal lateral veins develop acropetally while
those of the smaller lateral veins, which are arranged alternately with tbe
larger ('nes, develop basipetally, i.e. from tbe tip to tbe base of the leaf.
The latter development takes place only after the appearance of proto·
phloem in the larger veins. The cross veins which connect the parallel
veins are tbe last to form and they develop basipetally. The procambial
strands of the marginal veins of the banana leaf also develop in a basipetal
direction. The differentiation of the conducting elements begins before
the completion of the procambial system. The protophloem and proto·
xylem both differentiate acropetally and the differentiation of the proto·
xylem follows that of the protophloem. After the final elongation of the
veins, the development of the meta phloem and metaxylem commences in
a more or less definite basipetal direction, first in the large strands in
232 Plant Anatomy
~
\
MONOCOTYLEDONOUS LEAVES
.... \ ,
Grass leaves have linear laminae and sheathing bases·~urrounding the
stem. The development of the leaf of Oryza sativa as described by Kauf-
man (1959) will be used here as an example-of the development of this
type of leaf.
The leaf primordia are initiated in the tunica from which the ground
rneristem and protoderm of the leaf develop. At the shoot apex, in'the
early stage of initiation, a localized protuberance appears which later
becomes crescent-shaped and then as a result of further marginal and
apical growth, eventually surrounds the apex. As the young primordium
grows upwards it becomes hood-shaped (Fig. 109, no. I). Apical growth
of the leaf ceases during the third plastochron when the primordium is
the Leaf 233
labout 0·9 mm long, but the margins continue to grow and the primordium
elongates further. The continued marginal growth is brought about by
the activity ofthe marginal meristem andlhe elongation of the primordium
by a rib-meristem form of growth. A meristem of this kind is characterized
by parallel series of cells in which transverse divisions take place. The rib
. meristem and adaxial meristem (Fig. 104, no. 3) become distinguishable
during the second and third plastochrons.
Cone shaped
II
leaf primordium '
13)~
;
1. 2
'\
Hood- shaped
'\ ShOot
leof primordium. ... /' (lpel(
12)~-
,
.
.
1
Crescent-shaped
leof primordium
III
I
FIG. 109. 1, Three-dimensional drawing of the shoot apex of Oryza, showing the
apex and first three leaf primordia.' NUOlerals indicate the relative plastochrons.
2-4, Median longitudinal sections of Iigules at different stages of development
(Adaptedfrom Kaufman, 1959.)
The development of the sheath differs from that of the lamina in that
no distinct plate meristem is. seen in the wings of the, sheath and the ex-
tension of them is accomplished, primarily, by the activity of the marginal
meristem and by the enlargement of the cells derived from it.
As the differentiation in the lamina precedes that in the sheath, the
meristematic activity becomes more and more restricted to the base of the
sheath where the region of actively dividing and enlarging cells should,
therefore, be regarded as an intercalary ineristem. ~
The direction of the cellular differentiation and maturation in the leaf
of Oryza and Musa is basipetal. The development of the laminar mesophyll
in the former is depicted in Fig, 108, no, 2,
In certain monocotyledons the apical meristem of the primordium
ceases its activity very early and then a new centre of growth arises on the
abaxial side of the original leaf apex, That part of the leaf that develops
from the new apex is unifacial as it consists of tissues from the abaxial
side of the leaf only (Knoll, 1948; Thielke, 1948). The unifacial part of
the leaf may be cylindrical, as for instance in Allium cepa and }IincUs
maritimus, or it may be fiat, as in Iris (Fig, 92, no\ 2),
Leaj . a b SClSSlon
.. /
FIG. 110. Leaf abscission. 1, Longitudinal section of the leaf base of ,Prunus
showing the cells that divide to form the separation layer. 2, Coleus, longitudinal
section of portion of the stem together with the leaf base after the abscission of
the leaf. (Adapted from Gibbs, 1950.)
from the stem. After the leaf dies its base breaks away from the stem
through the weakened region of the abscission zone (Addicott, 1945;
Facey, 1950; Addicott and Lynch, 1955).
All the parenchyma cells including those of the vascular tissues of the
abscission zone take part in the process of abscission so that the leaf re·
mains attached to the stem only by the vascular elements. According to
many investigators these elements eventually break as a result of the weight
of the leaf or the action of the wind and so the leaf is shed. According to
236' Plant Anatomy
Facey (1950) the middle lamellae between the cells of the vascular tissues
also disintegrate. .\
The tissues below the separation layer, which become exposed to the air
wjth the shedding of the leaf, are protected from desiccation and the entry
of agents of disease by the formation of the protective layer. This layer
may be of two types - primary' protective layer or secondary protective
layer, Le. periderm. The primary protective layer is formed as a res.ult
of the lignification and suberization of the parenchyma cells in ·this region
or of the cells arismg from them by irregular cell divisions (Pfeiffer, 1928).
An opinion exists that the substance appearing in the cell walls of the pro-
tective tissue, and which has been defined as lignin, is really wound gum
which gives reactions similar to lignin (Hewitt, 1938).
The time at which the primary protective layer and periderm appear
differs in different plants. This fact has resulted in the development of a
complicated classification of abscission types (Pfeiffer, 1928).
Many factors exist that apparently influence the abscission ofleaves, and
growth regulators are the most important among them (Addicott and
Lynch, .1955). It is known, for example, that auxin inhibits abscission
(Gawadi and Avery, 1950).
References
/'
ADDICOTT, F. T. 1945. The anatomy of the leaf abscission and experimental defoliation
in guayule. Amer. Jour. Bot. 32: 250--256. I
ADDICOTT, F. T. and LYNCH, R. S. 1955. Physiology of abscission. Annual Rev. PI.
Physiol. 6: 211-238.
ALLSOPP, A. 1955. Experimental and analytical studies of pteridophytes. XXVII.
Investigations on-Marsilea, 5. Cultural conditions and morphogenesis with special
reference to the origin of land and water forms. Ann. Bot. N. S., 19: 247-264.
ARBER, A. 1950. The Natural Philosophy of Plant Form. Cambridge University ,Press,
Cambridge. - - -
ARNOTT; H. J. 1959. Anastomoses in the venation of Ginkgo biloba. Amer> Jour. Bal.
46: 405-411.
ASHBY, E. 194%a. S.tudies in the morphogenesis of leaves. I.-'An essay on the 1eaf shape.
New Phytol. 47: 153-176. , \
ASHBY, E. 1948b. Studies in the morphogenesis of leaves. II. The area, cell size and cell
number of leaves of Ipomoea in relation to their position on~ the shodt. New Phytol.
47: 177-195.
AVERY, G. S., Jr. 1933. Structure and development of tobacco leaf. Amer. Jour. Bot.
20: 565-592.
BAILEY, I. W. and NAST, C. G. 1944. The comparative morphology of the Winteraceae.
V. Foliar epidermis and sclerenchyma. Jour. Arnold Arb. 25:'342-348.
COULTER, J. M., BARNES, C. R. and CLOWES, H. C. 1931. A Textbook of Botany for
Colleges and Universities. Vol. 3. Ecology, Amer. Book Company, New York.
CROSS, G. L. 1940. Development of the foliage leaves of Taxodium distichum. Amer.
Jour. Bot. 27: 471-482.
DJ! FRAINE, E. 1912. The anatomy of the genus Salicornia. Jour. Linn. Soc. London, Bot.
41: 317-348.
The Leaf 237
THE ROOT
THE root constitutes the lower portion of the piant axis and it usually
develops below the soil surface, although there are roots that grow in the
air as there are stems that develop below soil surface. However, basic
differences in the development and arrangement of the primary tissues in
these two organs are always distinguishable. The histogenesis of the epi-
dermis of the root differs from that of the stem (see Chapter 3). In sperma-
tophytes the primary xylem in the roo~is .exarch .and that in the ~tem is
endarch. The xylem and phloem strands in the root do not form common
bundles but are arranged alternately, while in the stem the vascular bundles
are collateral, bicollateral or amphivasal. Roots bear no appendages that
are comparable to the leaves of the stem; roo.ts~re devoid of stOlnata lInd
their branches originate in the relatively, mature tissue of the pericycle
in contrast to the stem where the branches originate from the apical meris-
tern. Roots also possess a root cap which has"no parallel in. stems.
Much variability exists in the shape and structure of roofs. This varia-
bility, in many cases, is related to the function of the roots, i.e, whether
they are storage roots, succulent roots, aerial roots, pneumatophores, climb-
ing roots, prop roots, or whether they contain symbiotic fungi (mycorrhi-
za). Environmental conditions often influence the root system. Plants grow-
ing in dry soils usually have better developed root systems. Many plants
growing in sandy soils. develop shallow, horizontal, lateral roots which
spread out, close below the soil surface, over a distance of tens of metres
(e.g. Tamarix and Retama).
On the basis of origin two types of roots - primary roots and adventi-
tious roots-are distinguished. Primary roots develop'f(om the apex of
the embryo that is destined, from its origin,_to give rise to I a root, and
from the pericycle of relatively mature parts of roots, while adventitious
roots develop from other tissues of mature roots or from other parts of the
plant body, such as stems and leaves. Special importance has been given
to those adventitious roots that develop from the callus of cuttings.
In most dicotyledons and gymnosperms the root system consists of a
tap root from which side branches arise. The order of appearance of the
lateral roots is from the root neck (that part where the root joins the stem)
towards the root tip, but in some cases the primordia of some of the lateral
roots remain dormant. The mature portions of the root, which usually
The Root 241
The root cap is situated at the tip of roots (Fig. 29, no, 2), it protects
the root promeristem and aids the penetration of the growing foot into
the soil. The root penetrates into the soil more easily because of the mucila-
242. Plant Anatomy
gino us nature of the walls of the outermost cells of the root cap. Theroot
cap cpnsists ofli~ing.parenchyma. cells"which. often .contain starch. These
cells may have no special arrangement or they may be arfanged in radiat-
ing rows which originate from the initials. In many plants the central
cells of the root cap form a more distinct and constant structure which is
termed the columella (see Chapter 3).
The root-cap develops continuously. The outermost cells die, becor_ne
separated from one another and disintegrate, and· they are replaced by new
cells which, are produced by the initials. Root caps are apparently found
on roots of all plants except for the roots of some parasites and some my-
corrhizal roots. External factors ·influence the structure of the root cap.
Root caps develop in true water plants but they degenerate early.
THE EPIDERMIS
The epidermal cells of roots are thin walled and are usually devoid of
cuticle, although sometimes the outermost cell walls, including those of
the root hairs, undergo cutinization (Guttenberg, 1940; Scott et al.,
1963). On those parts of roots that are exposed to the air and on those
parts in the soil on which the epidermis persis(s for a long time, the outer-
most cell walls become thick, and may sometimes contain lignin or dark-
coloured substances which have not been fully,identified. The epidermis
of roots is usually uniseriate but exceptions do exist. On th~ aerial roots
of plants belonging to the Orchidaceae and in the epiphytic, tropical genera
of the Araceae the epidermis is multiseriate and it is specialized to form
a velamen (Fig. 67, nos. 3, 4). The velamen is a sheath of compactly arran-
ged dead cells, the walls· of which are strengthened by band'like or reti-
culate thickenings and which contain many primary pit-fields. When the
air is dry these cells are filled with air, but when rain falls they become filled
with water. Special structures, termed pneumcitodes, are present in the
velamen. The function of these structures is~t_9~nable gas exchange during
these periods when the root is saturated with' moisture. The pneumatodes
consist of groups of cells with very dense spiral wall thickenirigs. These
groups extend, in a ray-like fashion, from the periphery of the epidermis to
the endodermis. Oil droplets can be discerned in these cells. The endo-
dermal cells that are continuous with the pneumatodes are filled with air
(Gessner, 1956).
The most characteristic feature of the root epidermis)s the production
of root hairs which are organs well adapte.d to the efficient uptake of water
and salts. The region of root hairs is usually restricted to one or a few centi-
metres from the root apex. Root hairs are absent close to the apical meris-
tern and they usually die and dry out on the more mature portions of the
root. Certain herbaceous plants, and especially water-plants, lack root
The Roof 243
hairs. Plants that usually grow in soil and which produce root hairs fail
to do so when they ·are grown in water. Calcium is one of the factors'
controlling the normal development of root hairs (Cormack et al., 1963).
In some plants the root hairs remain on the root for a long time. In Gle-
ditschia triacanthos, for example, the root hairs remain viable for some
months and their walls become thickened. Long-lived root hairs have
been found in certain species .of the Compositae and in some plants of
other families (Cormack, 1949; Scott" et al., 1963) but it is doubtful
whether these root hairs take part in the uptake of water from the soil. In
many cases the presence of sue'> long-lived root hairs is connected with a
small amount of secondary thickening and absence of periderm.
I In certain plants all the epidermal cells may give rise to root hairs while
in others only certain cells, frichoblasts, may do so. Some workers have
found that root hairs develop from a subepidermal layer in the Commeli-
naceae and related families, and also in Citrus (Hayward and Long, 1942).
(For more details see Chapter 10).
In most of the dicotyledons and gymnosperms the cortex of the root con-
sists mainly of parenchyma cells. Tn many monocotyledons in which the
ioot cortex is not shed while the root remains viable, much sclerenchyma
develops in addition to' the parenchyma. The root cortex is usually wider
than the stem cortex (Fig. 112, no. I) and therefore it plays a larger role in
storage. The innermost layer of the cortex constitutes the endodermis
(Fig. II I, nos. 1-3; Fig. 1I2, no. 2). Incerlain plants such as Smilax, Iris
(Fig. 112, no. I), Citrus (Coss"llan, 1940) and Phoenix, for example, there
is a special layer below the epidermis; this layer is termed the exodermis.
The arrangement of the cells of the cortex, as seen in cross-section of
the root, may be in radial rows, at least in the inner layers, or the cells of
two adjacent concentric layers may be arranged alternately. The radial
arrangement is the result of the way in which the cells divide during the
formation of the cortex (Guttenberg, 1940; Heimsch, 1960). Repeated
peridinal divisions increase the number of cell layers in a radial direction,
while anticlinal divisions add to the periphery and length of the cortex.
The cells that undergo periclinal divisions are the inner cortical cells and,
after the periclinal divisions are completed the innermost layer of the
cortex differentiates to form the endodermis.
SChizogenous intercellular spaces, which appear in the early ontogenetic
stages, are very common in the Toot cortex . .In certain plants, such as the
Gramineae and Cyperaceae, large lysigenous intercellular spaces often
develop in addition to the schizogenous ones. Large air canals are com~
mon in the root cortex of the Palmae (Tomlinson, 1961).
244 Plant Anatomy
Radial
wall
2 3
FIG. 111.1, Cross-section of a root of a seedling of Triticum.2, Three-dimensional
diagram of a single endodermal cell with Casparian strip. 3, Portion of a cross-
section of a root showing part of the endodermis and a row of cortical parenchy-
maceU!; in a 'btateQ,f }'.\a~mo\y'bi,:>.l\ can be s.een that the protoplast of the endoder-
mal cell remains attached to the Casparian strips. (No. t, adapted from Avery,
1930; no. 3, adapted from Esau, 1953.)
The Root
Vascular
cylinder
Exodermis
Endodermis
THE EXODERMIS
In many plants the waJls of the cells of the outer subepidermal layers of
the cortex become suberized. In this way the exodermis, a protective
tissue, is formed (Guttenberg, 1943). The exodermis is similar in structure
and cytocbemical characteristics to the endodermis (Van Fleet, 1950). An
almost continuous ,suberin lamella lines the primary cell wall internally,
and it in turn is usu'ally lined with layers of cellulose which develop centri-
petally. Also lignin is often deposited in the walls of these cells and, in
246 Plant Anatomy
certain cases, Casparian strips have also been distinguished (Van Fleet,
1950). The cells of the exodermis contain viable protoplasts even when
mature; In,"the 'pteridophytes no exoderm"is is developed: "but sometimes,
as, for instance, in Ophioglossum. certain fatty substances are deposited
in the walls of the subepidermal cells but no special suberin lamella is
formed.
The thickness of the exodermis varies from a single cell layer to many
layers. The exodermis" may s.ometimes .be accompanied, on its inner side,
by sclerenchyma as, for example, in the root of Ananas (Krauss, 1949).
In Phoenix the exodermis is fibrous (Tomlinson, 1961).
THE ENDODERMIS
the radial walls, the walls parallel to a cross-section of the root, ana on
the inner tangential walls of the endodermal cells (Fig. 112, cno. 2).
This lype of endodermal cell is common in the roots of most monocotyled-
ons (Guttenberg, 1943). These thickened endodermal cell walls may be-
come lignified. The endodermis 'of the 'conifers is characterized by 'the
second stage of wall development only, i.e. by the development ofa suberin
lamella on the inner side of the walls (Guttenberg, 1941; Wilcox, 1962a).
The additional wall layers of the endodermal cells do not develop simult-
aneously in all the endodermal cells as seen in a single cross-section.
Casparian strips and the sucess;ve stages of the development of the typical
wall first appear opposite the phloem strands from where the development
~preads towards those endodermal cells that are opposite the xylem strands
(Van Fleet, 1942a, b; Guttenberg, 1943; Clowes, 1951). Because of the
delay in wall differentiation of the endodermal cells opposite xylem these
cells often have Casparian strips only, These cells are termed passage cells
as they are thought to provide passage for substances between the cortex
,and vascular cylinder, The passage cells may remain unaltered throughout
the entire life of the root or they, too, may develop thick secondary walls
as do the other endodermal cells,
The production of the suberin lamellae on the endodermal cell walls
results from the polymerization of unsaturated fatty compounds which is
brought about .by oxidases and peroxidases. The peroxidases are brought
to the"endodermal cells via the sieve elements. This has led Van Fleet
(1942b) ,to suggest that thid is the reason why the greatest amount of
suberin is .laid down on the inner walls of the endodermal cells and why
the passage cells, which lack .suberin, appear mostly opposite the xylem,
and not the phloem, strand's.
The vascular cylinder occupies the central portion of the root. In roots
it is more clearly delimited from the cortex than in the stem, because of
the presence of the endodermis which is characteristically better developed
in roots.
The primary vascular tissue is surrounded by a region of cells which is
termed the pericycle (Fig. 112, no. 2), The pericycle generally consists of
one or more layers of thin-walled parenchyma cells, It is in direct contact
with the protophloem and protoxylem and can already be distinguished
prior to the lignification of the protoxylem elements, The pericycle retains
its meristematic characteristics, The primordia of the lateral roots in all
spetmat()l'h~tes and the l'hellogen and portions of the vascular cambium
in the dicotyledons develop from the l'eric~cle, In monocotyledons the
phellogen usually develops in the outer parts of the cortex, In the roots
248 Plant Anatomy
\;
adventitious roots are polyarch and the number of strands in the Palmae
and the Pandanaceae may be 100 or more, In the roots of the Filicinae
different numbers of;xylem strands may be found~froii1 one, 'as in Ophio-
glossum /usitanicum, to many .a s in Marattia fraX_inea.
In certain monocotyledons, such as Triticum (Fig, 1 I 1, no, 1), one large
vessel is found in the centre of the vascular cylinder. Between this .meta~
-;
/.
E"dOr
m
".
-;
-;
,-; Immature
f
protox lem
II I
II 1000).1
Immature II
protophlaern_ I
II
II
II
Differentiatin·i:~~~~~~RJ~~
tracheary
elements
I
Tissue, differentiation in"the root
Some distance from the apical promeristem of the root the epidermis,
cortex, and vascular cylinder can be distmguished.
~
The,pericycle
- can also ,
be identified close to the apical meristem. As it is not. possible to distin-
guish clearly between the meristems of the vascular and' non-vascular
tissues in the vascular cylinder, it is not yet clear whether the peri cycle devel-
ops from the pro cambium or from the ground meristem. The cells of the
procambium that differentiate into the tracheary elements soon become
distinguishable from those cells from which th'e phloem elements will devel-
op. The former cells enlarge and they havi'ilirge vacuoles, while the latter
undergo numerous divisions without enlarging so that they become very
small. :
The order of appearance of the different tracheary elements, in com-
parison to the order in which they undergo maturation, is of interest. The
The Root 253
cells that develop into metaxylem elements enlarge; together with the
vacuoles in them, prior to those cells that differentiate into the protoxylem
elements while the order of maturation is, of course, the contrary. There-
fore the fil\al dimensions of the metaxylem elements are rar larger than are
those of the protoxylem. This is especially obvious in the monocotyledons
(Heimsch, 1951).
The ontogenetic development of the primary vascular system of the
'rooris simpler than that of the stem because the differentiation of the
vascular system of the latter is connected with the development of the
leaves. The vascular system of the root develops independently of, the late-
ral organs and the procambium develops acropetally as an uninterrupted
continuation of the vascular tissues in the more mature parts of the root.
Th-e differentiation and maturation of the xylem and phloem is also acro.
petal (Popham, 1955) and follows that of the procambium. From the accu-
rate investigations that have been carried out till now it appears that the
pro,t"phloem elements mature closer to the apical meristem than do the
.earliest tracheary elements (Fig. 115, no. 2). From this it is seen that the
~process of maturation of the protoxylem and proto phloem elements is
also simpler in the root than in the stem where the early differentiation of
-tlie-xylem close to a leaf primordium is in two directions.
"Generally the differentiation of the root tIssues behind the apical pro-
meristem can be summarized as follows: periclinal diVIsions in the cortex
cease near the level where the sieve' elements mature; beyond this region
the root"undergoes rapid elotigation, and the maturation of the protoxylem
usually takes place only when the process of elongation is almost complet-
ed; Casparian strips develop in the endodermal cells before the matura-
tion of the:protoxylemelements and generally also before the appearance
of root hairs.
The pr,oximity of the mature conducting elements-to the root apex is
dependent on the rate of growth and both these processes are dependent
on the external conditions, the type of root, and the stage of its develop-
ment (Wilcox, I 962a). H"imsch (1951) found the following distances be-
tween the root apex and the first mature vascular elements in different roots
of Hordeum: protophloemelements, 0·25-0·75 mm; protoxylem elements,
0·40-8·5 mm; elements of the early metaxylem, 0·55-21·6 mm or more,
While the large central vessels mature at even greater distances (Fig. I I 5,
no. 2). The earliest appearance of Casparian strips is at a distance of about
O· 75 mm from the apex.
CAMBIUM IN ROOTS
the base of earlier-formed lateral roots that have dried out (Esau, '1940,
1953).
In angiosperms the primordia of the laterarroots are formed by the peri-
dinal and anticlinal divisions of a group of pericycle cells, The initiating
divisions are periclinal. As a result of further growth tlie primordium
penetrates through the cortex ofthe parent root. It is possible to distinguish
the zones of primary ·tissues, apical meristem and root-cap of the lateral
Endodermis
4 5
FIG. 116. I, Basal portion of a plant of Zea mays in which the remains of the
seedling roots, and the adventitious roots, arising at the base of the internodes,
can be seen. 2-4, Portions of radial longitudinal sections of foots of Hypericum
showing eady stages in the development of lateral roots. 5, A seedling of Sinapis
alba in which the region of root hairs can be seen. (Adapted from Troll, 1948.)
root primordium even before it appears on the surface of the parent root
(Fig. 116, nos. 2-4). Different opinions exist as to how the passage of the
growing lateral root is effected through the cortex of the original root.
According to one view the lateral roots partially digest the cortical tissue
during penetration, while according to another view the process of pene-
tration is purely a mechanical one. However, it is generally agreed that the
developing lateral roots do not form any connection with the tissues through
which they penetrate.
256 Plnnt Anntomy
In many plants, as, for instancc::, Dqucus carota, the endodermis of the
parent root takes. part in the formation, of. the,primordi~m of the .lateral
roots (Esau, 19~0). Insuch cases the endoderm is may divide only anticli-
nally, but sometimes it may divide periclinally as well and thus forms more
than one layer. With the eruption of the lateral root on the surface of the
parent root, or even prior to it, the tissue that developed from the, ensJo-
dermis dies and it is eventually shed. Incertain -.yater-plants. and in species
of the Papilionaceae'; Cucurbitaceae and some others families, the inner-
most layers of the cortex also take part in the development of the lateral
roots (Esau, 1953).
The connection between the vascular systems of the lateral and parent
roots is brought about by intervening cells. As the lateral roots originate in
the pericycle the distance between the two vascular systems is small. Of
the intervening cells, which also develop from the pericycle, some differ-
entiate into sieve elements and some into tracheary elements.
The xylem of the lateral roots of many monocotyledons is connected
with two or more xylem strands of the original root. This can be seen in
Monstera, for example, where the connections ar~ not only with the peri-
pheral xylem strands but also with the innermost large vessels of the meta-
xylem. This is brought about by the modification, into tracheary elements;
of parenchyma cells between the xylem and phloem strands (Rywosch,
1909). /
Adventitious roots /
Adventitious roots, as defined above, may develop from large roo is,
from the hypocotyl of young plants, from the primary and secondary body
of stems, and from leaves. In the roots and stems of most plants adventi-
tious roots develop endogenously, but there are/examples in which the
development is exogenous. Primordia of adventitious roots may be formed
by the following tissues;·the epidermis, together with corticaHissue, of
buds and hypocotyls (e.g. Cardamine prJtensis, Rorippa austriaca); stem
peri cycle (e.g~ Coleus, Zea mays); ray parenchyma between pericycle and \
cambium (e.g. Tropaeolum majus, Lonicerajaponica, Tami'rix); non-differ-
entiated secondary phloem and cambium between the vasc'ular. bundles
(e.g. Rosa); interfascicular cambium and 'pericycle (e.g. Portulaca olera-
cea); interfascicular cambium, pericycle and phloem (e.g. Begonia); the
pith of the stem (e.g. Portulaca oleracea); parenchymatous interruptions
in the secondary xylem which are formed due to the presence of leaf-gaps
(e.g. Ribes nigrum) or buds (e.g. Cotoneaster dammeri) (Hayward, 1938;
Boureau, 1954); tissues of leaf margins ,and. petioles (e.g. Begonia, Kalan-
choe.)
The development of adventitious roots has been described in some' species
of Salix (Carlson, 1938, 1950). In these species the adventitious roots
The Root 257
develop from primordia which appear in the stem prior to its removal as a
cutting. These primordia are· formed from secondary parenchymatous
tissue in the leaf- or branch-gaps. Several layers of cells external to the
cambium take part in the formation of a primordium, to whose.inner side
cells are: also added by the cambium. The primordium becomes dome-
shaped as a result of the intensified growth of the secondary xylem imme-
diately inwards of it. These primordia remain dormant withinthe inner
bark as long as the branch is not removed from the tree. The differentiation
of these primordia is extremely slow so that even in 9-year-old branches,
the typical root'tip structure is not discernible. After the first year of
growth, additional primordia may develop vertically above and below the
.first-formed primordia on both branches left on the tree and on cuttings.
On cuttings most of the primordia develop rapidly into roots. Similar
adventitious root primordia have been observed on woody roots of
Zygophyl/um dumosum.
Plant species differ from one another in their ability to produce roots on
cuttings. Cuttings of planfs with dormant adventitious root primordia
(e.g. Salix) root easily as do many plants with broad vascular rays but
without such primordia (e.g. Vitis vinifera, Tamal'ix spp.). Cuttings of
'Ceratonia, Pyrus and Carya, for example, which have no dormant pr~
mordia and in which the rays are narrow, root with difficulty.
The ability to produce adventitious roots varies with age-generally,
they develop more easily on younger plants and plant organs.
I
In all primary roots reserve substances (mostly starch) are 'stored in the
cortex, which in most plants is relatively 'thick. In ordinary roots with
secondary thickening reserve substances are stored similarly as in stems,
i.e. in the parenchyma and sclerenchyma tissues of the secondary xylem
and phloem. Usually roots contain more parenchyma than do stems.'
There are plants in which certain parts of the root system develop into
thick, fleshy organs which function especially as storage organs. In many
plants the tap-root and hypocotyl undergo such modification.
The origin of the storage tissue may differ. In the carrot, for example
(Esau, 1940), the hypocotyl and tap-root become thickened and, with the
The Root 259
FIG. 117. Micrograph of the outer portion of a cross-section of the root tuber of
Ipomoea balalas. x 20.
Plant Anatomy
260
development of the periderm, the narrow cortex is shed. The organ be~
comes fleshy as a result of the excessive development of parenchyma in the
secondary xylem and especially in the secondary phloem. '
!n the sugar beet, according to Artschwager (1926), the hypocotyl and
root become fleshy as a result of an anomalous secondary thickening which
is characteristic of the Chenopodiaceae and which is discussed in more
detail in a later chapter. Here it will only be mentioned that, as a result of
the activityoofnumerous cambia, layer~ of secondary" tissue consisting of
parenchyma in which groups of conducting elements are scattered, ari,
formed. The sugar is found as a reserve substance in the cells of this secon-
dary parenchyma.
In Ipomoea batatas (Fig. 117) the fleshiness of the root is due to the
following development (Hayward, 1938). Both the primary and secondary
xylem develop normally and contain a large amount of parenchyma. How-
ever, with further development many anomalous secondary cambia are for-
med around single vessels or groups o( them. These cambia, which are
annular' in 'cross'sectiori "Fthe root, produce some phfoem but'"mainly
parenchyma. Some distance from the vessels, lati~ifers are also formed.
Tertiary tracheary elements develop close to and around the vessels
that are encircled by these special secondary cambia. Still later, secondary
cambia may be formed in the parenchym!,'~ot associated with vascular
elements.
In the radish the fleshiness of the root and hypocotyl 'is due to the' eX"
cessive development of parenchyma in the secondary xylem which is prod uc-
ed by the normal cambium, as well as secondary parenchyma produced
by additional cambia which also produce tertiary conducting elements
(Hayward, 1938).
/
ROOTS AS ANCHORAGE ORGANS
r ,
The anchorage function 'of the robt in the soil is aided by
, ),;
the following
structural features: the branching of many lateral roots from a tap-root
and the development of many adventitious roots in fibrous root systems; \
the growth of root hairs which are of great importance in'young roots; the
development of sclerified tissues (principally_xylem) in the cenire of young
roots and the development of sclerenchyma in old roots.
CONTRACTILE ROOTS
The renewal buds of certain plants occupy a definite position within the
soil or on its surface. This position is mostly obtained by the pull of
special roots, which have been termed contractile roots (Rimbach, 1895,
1899,1929, 1932; Arber, 1925; Bottum, 1941; Davey, 1946; Dittmer, 1948;
The Root 261
Galil, 1958, 1961). Such roots are known to exist on many herbaceous
dicotyledons (e.g. Taraxacum; Medicago sativa; Daucus, Trifolium, Oxalis,
sugar beet) and in many bulbous and cormous monocotyledons (e.g.
Phaedranassa· chloracra, Hypoxis setosa, Bellevalia flexuosa, Gladiolus
segetum, Colchicum steveni, !5do/{don .montanum, Muscari parvijiorum,
Allium neapolitanum). Contractile roots or parts of roots are distinguish-
able from normal roots by their outer, wrinkled appearance.
Centrol cylinder
According to Arber (1925), who studied Hypoxis se(osa, only the outer
cortex is wrinkled whereas the central cylinder and the inner cortex are
unaffected. Rimbach (1899) and later investigators explain the shortening
of the inner core as being due to the change in form of the inner cortical
cells. These cells, according to them, increase in radial and tangential dia-
meter and decrease in length.
Davey (1946) described the histological changes that are involved in the
root contraction of Oxalis hirta seedlings. According to him a small
amount of the contraction is due to the active growth of the phloem paren-
chyma cells in a transverse direction and their shortening in a longitudinal
direction. The main contraction mechanism, however, is as follows: hori-
262 Plant Anatomy
The root systems of trees growing in littoral swamps, in which the soil is
periodically inundated and lacking in oxygen, exhibit various adaptations
to their habitat. These involve features that ensure sufficient aeration and
additional support. /
The Rhizophoraceae arc characteriz~d by stilt-roots which descend from
the stems and whose lower portions only are subterranean. The cortex of
these stilt-roots is spongy due to the development of complex intercellular
spaces (Metcalfe and Chalk, 1950), In Phoenix paiudosa there are, at the
base of the stem, special roots which descend into the ciud and which
contain lenticels and aerenchyma produced by the phellogen,
Aerial, negatively geotropic. root projections, which are termed pneuma~
tophores, are commonly produced in swampy habitats .. These roots serve
for gas exchange. In Avicennia the pneumatophores are erect, peg-like
aerial projections of the lateral subterranean roots. In Bruguiera' erio-
petala knee-like aerial projections, which are part of the horizontal roots,
are produced. The morph~logy and anatomy of the pneumatophores of
Amoora, Carapa and Heritiera were studied in:detiill by Groom and Wilson
(1925). In these genera the aerial projections are wing-like protuberances
which are produced on the upper surface o'fth~ horizont';l roots by inten-
sified cambial activity in these regions. In the· three last-menti~ned genera
it was seen that the pneumatophores all possess lenticels and that they
contain only few xylem vessels and thick-walled fibres. The bulk of the
tissue of the root consists of thin-walled fibres and parenchyma tissue (axial
and ray parenchyma). At certain times this parenchyma was seen to con-
tain much starch, and so it may sometimes act as a storage tissue. However,
most of the cells have no solid contents and therefore it is possible that all
the cells, including the vessels, may act as air reservoirs. In Amoora and
Carapa the intercellular spaces in the wood are no larger than usual.
The Root 263
In certain.plants, for example Beta, in which the root is also diarch, one.
trace of double nature-
enters each
".,."
of the cotyledons.
•.• ~
Each
-f
of these coty-
ledonary traces consists of two bundles which are partially fused along the
protoxylem. In this case the protoxylem is also brought closer to the centre
by the change in the position. of differentiation of the metaxylem in the
lateral
vein
Protoxylem
vein
Melaxylem
(5)
upper part of the hypocotyl and the bundles become completely collateral
oandoendarch,only-in.the cotyledons (Artschwager, 1926; Hayward, 1938).
The double nature of the cotyledonary trace has phylogenetic importance
(Bailey, 1956).
In Medicago sativa the root is usually triarch. In transition, to the hypo-
cotyl one of the xylem strands becomes smaller than the other two. The
two large strands become'situated one opposite the otheLas in a diarch
arrangement. The small third bundle is at right-angles to the two large
ones. Higher up a fourth strand of protoxylem develops opposite the small
third one and so the stele becomes ,tetrarch at the base of the hypocotyL
During this process a fourth strand of phloem is also developed and the
phloem strands alternate with the xylem strands. Higher up in the hypo-
cotyl pith is developed. The phloem strands divide so that they number
eight; these strands become orientated so as to take up a position almost
collateral. to the xylem strands. The differentiation of the metaxylem ele-
I
I
FIG. 119
(coni.)
no. 1 indicate the levels at which the cross-sections were made. 6, An entire seed~
ling at the stage of development corresponding to that depicted in the other
drawings. For further explanations, see text. (Adapted from crooics. 1933.)
Plant Anatomy
ments, a short distance below the cotyledonary node, takes place in such a
position that two V-shaped groups, as seen in cross-section, are formed.
The protoxylem is located in the angle between the twe! arms of the V.
The two triads, i.e. the xylem groups together with their associated phloem,
constitute the 'cotyledonary bundles (Hayward, 1938).
The length of the transition region in dicotyledons differs - in some
plants it is short while in others it is long. In some plants the changesin
orientation are gradual and continue'throughout the entire length of·the
hypocotyl, while in others these changes are restricted to the upper portion
of the hypocotyl alone. In the latter case the hypocotyl is referred to as
being of root structure. The transition region is longest in seedlings with
subterranean cotyledons (hypogeal germination) as it extends for one or
more nodes above the cotyledons.
In monocotyledons the transition bet'veen the vascular tissues of the
root and stem is affected by the'presence of a single cotyledon and by the
shortness of the lowermost internodes (Esau, 1953). In many monocotyle-
dons part of the vascular system of the root is connected'with'the vascular
system of the cotyledon, while part is connected ~ith th~ vascular tissues
of the first foliage leaf. In both these cases the connecting vascular strands
exhibit features typical of iran sit ion. However,,in a small number of mono- \
cotyledons the transition takes place between' the root and cotyledon alone
as is ,common in the dicotyledons (Arber, 1925).
As an example of a transition region in monocotyledons / we shall cite
that of the seedling of Triticum (Fig. 115, no. 3) as described by Boyd and
Avery (1936). The polyarch vascular cylinder of the root is connected to
that of the leaves by the presence of plate-like vascular tissue which is pre-
sent below the insertion of the scutellum. This plate is termed the nodal
plate. Separate bundles arise acropetally from the nodal plate. These bund-
les are irregularly arranged in the basal portions where they exhibiUransi-
tional features. Higher up these bundles continue to branch and produce
a cylinder of bundles with endarch xylem a;rcr~with collateral arrangement
of xylem and phloem. This system consists of bundles that enter the scu-
tellum, coleoptile and the first two foliage leaves.
The transition region of the gymnosperms is generally" similar to that
common in the dicotyledons in which the connection occurs primarily
between the root and (he cotyledons, but ins more complex than in the
dicotyledons because of the increased number of cotyledons (Boureau,
1954).
The complex structure of the transition region between the root and
shoot is apparently, as already suggested by Esau (1961), the result of the
meeting of influences of two morphogenetic centres - the shoot apex and
the root apex. These influences are especially strong in the embryonic stage
of the plant where the two centres (apices) are close to each other. The
differentiation in that region where the two opposite trends meet should
The Root 269
be intermediate between the two. During the growth of the seedling the
two poles move furtherapart'and the influence of each of'them ou'the
region near the other is weakened. Accordingly, the extension of the tran-
sition region into one or more internodes above the cotyledons in seedlings
in which the germination is hypogeal may be explained by the extended
influence of the root apex on the basal internodes'oflhe stem as a result of
,the ret~rded growth of the hypocotyl in such seedlings.
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_TORREY, J. G. 1957. Auxin' control of vascular pattern formation in regenerating 'pea
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TROLL. W. 1948. Allgemeine Botanik. F. Enke, Stuttgart.
<VAN FLEET, D. S. 1942a. The development and distribution of the endodermis and an
associated oxidase system in monocotyledonous plants. Amer. Jour. Bot. 29: 1-15.
VAN FLEET, D. S. 1942b. The significance of oxidation in the endodermis. Amer. Jour.
Bot. 29: 747-755.
·V AN FLEET, D. S. 1950. A comparison of histochemical and anatomical characteristics
of the hypodermis with the endodermis in vascular plants. Amer. Jour. Bot. 37:
721-725. . '
VAN FLEET,- D. S. 1957. "Histochemical studies of phenolase and polyphenols.in the
de~elopment of the endodermis in the genus Smilax, Bull. Torrey Bot. Cl. 84:
9-28.-
WEAVER, J. E. 1920. Root Development of the Grassland Formation. A Correlation of the
Root Systems of Native Vegetation and Crop Plants. Carnegie Inst., Washington
Publ. No. 292, p. 51. .. •
WILCOX, H: 1962a: Growth' studies' of the root of incense cedar Libocedrus decurrens.
'I. The origin and development of primary tissues. Amer~ Jour. Bot. 49: 221-236.
WILCOX, H. 1962b. Growth studies of the root of incense cedar Libocedrus decurrens.
JI: Morphological features of the root system and growth behaviour. Amer. Jour.
Bot. 49: 237-245.
ZOHARY, M. and ORSHAN, G. 1954. Ecological studies in the vegetation of.the Near East
desert. V. The Zygophyl/etum dumosi and its hydro~ecology in the Negev of Israel.
Vegetal/a 5-6: 341-350.
SECONDARY BODY OF THE
PLANT
GROWTH in thickness that occurs distant from the apices is called secondary
growth, and the tissues thus produced are termed secondary tissues. These
tissues constitute the secondary .body of the plant. Secondary tissues devel-
Secondary Primary
xylem phloem
op from secondary meristems, i.e. from the vascular cambium and the
phellogen or cork fambium ..Commonly, _the main.stem, which in.,certain
'plants may reach a diameter of some metres. the branches. roots. and often
even petioles and the main veins -of leaves, consist of secondary tissues
(Fig. 120, no. I).
The development of secondary vascular tissues from the cambium is
characteristic of the dicotyledons and the gymnosperms. In certain. mono-
_ cotyledons the vascular tissues are also increased after the primary growth
is completed, but the cambium of these plants is of a different nature. In
the pteridophytes secondary thickening was more common among those
species that have become extinct. In the living pteridophytes this feature is
\rare but occurs, for example, in [soetes and Botrychium. Certain monocoty-
!edons, as, for instance, some Palmae, exhibit considerable thickening that
is the result of a primary thickening meristem only, but these plants nev-
er reach the diameter of old dicotyledonous trees.
CHAPTER 14
VASCULAR CAMBIUM
THE vascular cambium is a lateral meristem that develops either as longi-
tudinal strands or as a hollow cylinder. In the woody angiosperms and
gymnosperms the primary dssues of the stem 'and root exist for only a rela-
tively short period before they become destroyed or obliterated by the
development of the secondary vascular tissues which are produced by the
cambium. In many herbaceous angiosperms, and also in most of the re-
c~nt lower vascular plants, cambium is absent or vest~gial.
Secondary
xylem
initial
FIG. 121. I, Portion of a cross-section of t-stem of Pinus showing the cambial zone
and neighbouring tissues. 2, As above but i radial section and showing a vascular
ray. (No.2, adapted from Haberlandt, 191,8.)'
cambial zone are seen to be arranged in radial rows (Fig. 121, no. I). The
cells on either side of this zone gradually widen until they acquire the shape
and features of mature phloem and xylem elements. In the narrow sense
of the word the cambium consists only of the single layer of initials, but it
is customary to refer to the entire cambial zone by this term as it is difficult,
in a single section, to distinguish between the initials and the neighbonring
cells that are derived from them. Some authors (Catesson, 1964) doubt
the existence of one layer of cambial initials and consider the cells of the
whole homogeneous part of the cambial zone to have the properties of ini-
tials. When the cambium is active the cambial zone is wide and consists of
many cell layers, but when it is dormant the zone is usually reduced to one
or a few cell layers only.
In conifers, according to Bannan (1962), the cambial zone in the resting
state may consist of five layers, buUUs usually. two- or three-layered. In
the three-layered condition the layer nearest the more or less immature
phloem is recognizable as that of the cambial initials and the inner layers
constitute the xylem mother cells. In these plants the first divisions, on the
renewal of cambial activity, may occur in any of the three layers of the
cambial zone, but the usual site of the first divisions is among the xylem
mother cells closest to the already differentiated xylem and not, as might
be expected, in the cambial initials. The initiation of the divisions closest to
the xylem is of interest and may be connected with the supply of water
as well as with the presence oCgrowth hormones (Bannan, 1962). Accord-
ing to Evert (l963), in Pyr~s malus final differentiation of the phloem ele-
ments from cells produced in the previous season precedes xylem differen-
tiation by about 6 weeks.
I
FlO. 122. Tangential sections of different cambial types. 1, Storied cambium of'
Robinia. 2, Non-storied cambium of Fraxinus.
Vascular Cambium 277
\
I
2
4
thicker than the tangential ones; this featnre is a result of the predominantly
periclinal divisions in the cambial cells during which the thickening of the
radial walls is continuous.
TYPES OF CAMBIUM
CELL DIVISION
The cambial initials and the cells that are derived from them but which
have not yet undergone differentiation divide periclinally and anticlinally
in a longitudinal plane, As a result of t1>e periclinal divisions, which are
the more numerous, new cells are added to the secondary phloem ,and,
xylem, The derivatives of each initial therefore form radial rows, which can
sometimes also be distinguished in the xylem and phloem, Usually, how-
ever, this order is lost in the vascular tissues because of the changes in shape
that take place during the differentiation and maturation (>f their cells,
As a result of the secondary thickening, the circumference of the xylem,
cylinder increases. Together with this the cambium /
also increases in cir-
cumference by the addition of new cells. In.storied cambium the addition
of new fusiform initials is 'brought about by longitudinal aniiclinal divi-
sions (Fig, 123, no. 3) of the existing initials. In non'storied cambium, on
the other hand, the fusiform initials undergo oblique, app/oaching hori-
zontal, anticlinal divisions, after which each of the new cells elongates at
its ends till it is as long as, or even longer than, the cell from which it was
derived (Fig. 123, no. 2).
Because of the great length of ihe fusiform 'initials, the formation of the
cell plate during the process of longitudinai divisio"n is peculiar to these
cells. The ceIL plate begins to form between' the two new nuclei and it
spreads slowly. A relatively long period passes before it reaches'the 'end
walls. While the cell plate is not complete. its free margins are surrounded
by phragmoplast (Fig. 120, no. 2; Fig. 124, nos. 1-4). '
In the conifers (Bannan, 1962) intensively dividing cambial cells divide
once every 4-6 days, whereas apical _meristmatic cells divide 'every 8-18
hr. Possibly, the slower division or'the cambial cells is due to the time
required for the phragmoplasts t<i'reach the ends of the elongated cells,
which may be up to a rew millimetres long. Most divisions, as seen in
radial longitudinal sections, take place among the xylem mother cells. The
rate of division in the cambial initials and the phloem mother cells is
lower than in the xylem mother cells. The relative rates of xylem and
phloem formation have been found to change during the growing season
only in some plants. According to Bannan (1950, 1951a, b), in the conifers
Vascular Cambium 279
there are two processes that take place during the enlargement of the cam-
bial cylinder. Firstly,nearly all t)1e~.neY( ray. initials develop from.special,
transverse divisions offusiform cells. Secondly, single initials are continually
lost from the cambium and are replaced by new ones. Many of the fusiform
initials that.are about to be lost shorten before they are finally obliterated.
Phragmoplast Phragmoplast
2 3 4
FIG. 124. Diagrams showing various stages in the. division of a fusiform initial of
Robinia pseudacacia. l.:. J, Dividing cells as seen in radiafsection .. 4, As seen in
tangential section. (Adapted from Bailey, 1920c.)
I
Seasonal activity of Cambium
~b~1~ a1~ p\an\~ 'Wbose cam.'D1um. 1~ achve tblOUg'nDUt t'ne ent11e \1\" 'X
the plant; i:e., the cambial cells divide continuously'and theresulting cells
undergo gradual differentiation to form the xylem and phloem elements.
This .type of activity is usuallyJound in plants growing in tropical regions ..
Contrary to this, in plants whose origin is in the temperate regions, the
cambium ceases its activity with the onset of unfavourable conditions, usu-
ally the autumn, and it enters a dormant state which may last from the
end of summer till the follewing spring. In spring the cambium again be-
comes active. From an anatomical point of view the commencement of the
cambial activity censists of two stages: (I) the cambial cells become wider
radially; and (2) the cells begin te divide as described abeve. With the
enlargement ,of the cambial cells their radial walls usually become weak-
ened, so that in this stage the bark of the stems and reets may easily be
peeled. In later stages this easy separation of the bark from the xylem is also
pessible because of the increase in number ,of cells in the cambial zene as a
result of the cell divisions. The separatien principally occurs in the regien
of the young xylem cells which have already reached their maximum dia_
meter, but which still have thin primary walls.
280 Plant Anatomy
latter case only the seasons of early and late wood production can be
determined.
'3, Shrubs, such as Anabasis articulata Moq. and Salsola rosmarinus
Solms-Laub., which are intermediate between the first 'two groups in that
the commencement of the growth-ring production is in February.
4. Trees, such as Eucalyptus camaldulensis Dehn. and Tamarix aphyl/a
Karst., in which the formation of the early wood starts in September
(August),.i.e. toward the . end of the dry summer season. In Eucalyptus the
late wood, which consists of one or two bands of flattened fibres two or
three layers thick, is produced during the spring or in early summer, and
the cambium is inactive or almost so during July-August. In some speci-
mens of Tamarix aphylla commencement of growth-ring production was
fbund to be in August-September, while in other specimens two such
periods were seen - one in the- late summer and one at the end of
February-resultingiiHhe production oftwo growth rings annually.
5. Trees and shrubs, such as Acacia tortilis Hayne, A. 'raddiana Savi,
A. cyanophylla Lindl. and Thymelaea hirsuta Endl., in which there are no
growth rings and in which the same type of wood is produced throughout
the year.
In Eucalyptus camaldulensis the annual growth ring was seen to be pro-
duced in September which coincides with the spring of Australia, where
this plant.is indigenous. Therefore it" is seen that the endogenous "growth
rhythm persists in the trunk,of Eucalyptus species and that it withstands
the influence of external factors in a new and different _environment. In the
case of Israel this is "probably possible because of the mildness of the win-
ters. This feature of the growth,rhythm is, however, confined to evergreens,
as in deciduous plants the endogenous rhythm of cambial activity may" be-
come· suppressed under 'the" influence. of sudden changes in climate that
bring about leaf fall and bud burst. In the grapevine a second bud burst,
which was accompanied by the formation of a second growth ring; could
be artificially induced by defoliation (Bernstein and Fahn, 1960).
From the behaviour of tropical woody species and of Eucalyptus intro-
·,duced into an area with a mild climate, it appears that the annual rhythm
of growth-ring production, at least in evergreens, may be considered as a
conservative character. Therefore it may be that the plants of the above-
described types are of different geographic origin. The first group, in which
the growth-ring production commences between November and January,
i.e. at the beginning of the wet winter, and in which the cambium is active
during that period and dormant during the dry season, appears to be the
indigenous type, and it is the best suited to the area under discllssion.
282 Plant Anatomy
The relationship between the cambial activity and ithe 'activity of the
vegetative buds differs in different species. The activity of the cambium
usually starts below the sprouting buds from where it spreads downwards.
The velocity of this spread. also differs. In Acer it was found to be rather
slow (Cockerham, 1930) while in various conifers, in some ring-porous
dicotyledons (Priestley, 1930; Wareing, 195 I) and in some evergreens(Fahn,
1953) the downward spread of the cambial activity is very rarid. For in-'
stance, in the Mediterranean region of Israel the time lag between bud
sprouting and the activation of the cambium is 2-4 weeks in some deci-
duous species, while in evergreens there is no such lag and the two pro-
cesses take place simultaneously (Fahn, 1953).
The stimulus for the reactivation of the cambium is, apparently, a cer-
tain levd or combination of growth-reg"lating'.substances (Gouwentak,
1941). The close relation of cambial and bud activity led Avery el al. (1937)
to conclude that these substances, which are produced by the b_uds, flow
downwards from them along the axis where acti'(ity is thus induced. How-
ever, ,in most plants a certain period of dormancy must be completed
before the cambium can be reactivated by known treatments, such as the
application of growth r~gulators (Gouwentllk and Mass, 1940; Gouwen~
tak, 19,4l) and, increased day length. Thus; the application of such treat'
ments 'Yill induce acth:ity ·only after a certain unknown factor or factors,
which calIse dormancy, have been eliminated. I
According to Wareing (i958) two main groups of regulators, i.e. gibbe-
rellins and auxins, have been shown to affect cambial activity. Under the
influence of gibberellin rapid cell divisions, which are not followed by diffe-
rentiation, are induced in the_c~mbium. Auxins,.on the other hand, cause.
rapid cell differentiation. The simultaneous effect of the above two groups
of substances seems. to result in the appearanceLof normal cambial acti-
vity. .... _-,~ \. *
Very little is as yet known about the factors that cause the cessation of
cambial activity. Wareing (1951) and Wareing and Roberts (1956),have
stressed the role of photoperiodism in the activity of tl1e,cambium.These '
investigators showed that in juvenile plants of Robinia pseudacacia and
Pinus sylvestris cambial dormancy can be induced by the appiication of
short day conditions and that the cambium can be reactivated by long day
conditions. However, other external factors, such as temperature (Waisel
and Fahn, I 965b), as well as internal ones, especially in adult plants, s,eem
to playa major role in the control of the rhythm of canibial activity.
Vascular Cambium 283
References
AVERY, G. S., Jr., BURKHOLDER, P. R. and CREIGHTON, H. B. 1937. Production and
distribution of growth hormone in shoots of Aesculus and Malus, and its probable
role in stimulating cambial activity. Amer. Jour. Bol. 24: 51-58.
BAILEY, I. W. 1920a. The cambium and its derivative tissues. 11. Size variations of
cambial initials in gymnosperms and angiosperms. Amer. Jour. Bot. 7: 355-367.
BAILEY, 1. W. 1920b. The cambium and its derivative tissues. III. A reconnaissance
of c:ytological phenomena in the cambium. Amer. Jour. Bot. 7: 417-434.
284 Plant Anatomy
BAILEY, I. W. 1920c. The formation of cell plate in the cambium of the higher plants.
Proc. Nation. Acad. Sci. 6: 197-200.
BAILEY, I. W. 1923. The cambium and its deriv-ative tissues. IV. The increase in girth
of the cambium. Amer. Jour. Bot. 10: 499-509.
BAILEY, 1. W. 1930. The cambium and its derivative tissues. V. A. reconnaissance of the
'yacuome in living cells. Ztschr. f. ZellJorsch. u. Mikroskop. AnatomielO: 651-682.
BANNAN, M. W. 1950. The frequency of anticlinal divisions in fusiform cambial cells
of Chamaecyparjs. Amer. Jour. Bot. 37: 511-519.
BANNAN, M. W.1951a. The reduction of fusiform cambial cells in Chamaecyparis and
Thuja. Cana'd. Jour. Boi. 29: 57:_67. .
BANNAN, M. W. 1951b. The annual cycle of size changes jn the fusiform cambial 'Cells
of Chamaecyparis and Thuja. Canad. Jour. Bot. 29: 421-437.
BANNAN, M. W. 1962. Tpe vascular cambium and tree-ring development. In: Tree
Growth, ed., T. T. Kozlowski. Ronald Press, New York, pp. 3-21.
BANNAN, M. W. and WHALLEY, B, E. 1950. The elongation of fusiform cambial cells
in Chamaec),paris. Canad. Jour._ Res., Sect. c., Bot. Sci. 28: 341-355.
BERNSTEIN, Z. and FAHN, A. 1960. The effect of annual and bi-annual pruning on the
seasonal changes in xylem formation in the grapevine.- Ann. Bot.,N. S,-24: 159-171.
BUCK, G. J. 1954. The histology of the bud-graft union in roses. Iowa State~ Coil. Jour.
Sci. 28: 587-602.
CATESSON, A. 1964. Origine, fonctionnement et variations ,cytologiques saisonnieres du
cambium de l'Acer pseudoplatanus L. (Aceracecs). Annales des Sciences Nature//es_.
Boral1ique, Paris, 12e serie, 5: 229-498.
COCKEI{HAM, G. 1930. Some observations on cambial activity and seasonal starch
COntent in sycamore (Acer pseudoplatanus). Ppjc. Leeds Phil. Lit, Soc. 2: 61-:80:
EVERT, R. F. 1963. The cambium and seasonal development of the phloem in pyrus
. Malus. Amer. Jour. Bot., 50: "149....:159. -
FAHN, A. 1953. Annual wood ring development in maquis trees of Israel. Palest. Jour.-
Bot" Jerusalem 6: 1-26. /
FAHN, A. 1955. The development of the growth ring in wood of Quercus infectaria and
Pistacia lenliscus in the hill region of Israel. Trop. Woods 101": 52-59.
FAHN, A, 1958a. Xylem structure and annual rhythm of development in trees and
shrubs of the desert. I. Tamarix aphylla, T. jordanis var. negevensis. T, gallica var.
liiaris-mortui. Trop, Woods 109: 81-94.
FAHN, A. 1958b. Xylem structure and annual rhythm of development in trees and
shrubs of the desert. II. Acacia tortihs and A. raddiana, Bull. Res. Counc. Israel
70: 23-28.
F AHN, A. 1959a. Xylem structure and annual rhythm of development' in trees and
shrubs of the desert. III. Eucalyptus comaldulensis and Acacia cyanophylla. Bull.
Res. Counc. lsrael7D: 122-129. ..- , _
FAHN, A. 1959b, Annual rhythm of xylem development in trees \and-shrubs in Israel. \
Proc. IX. Intern, Bot. Congr. Montreal: 110. . ,. .
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species of the East Mediterranean regions-:- News Bull. Internat. Assoc. Wood
Anatomists, 1962/1: 2-6.
FAHN, A. and SARNAT, C. 1963. Xylem structure and annual rhythm of development in
trees and shrubs of the desert. IV. Shrubs. Bull. Res. Counc. IsraelllD: 198-209.
GOUWENTAK, C. A. 1941. Cambial activity as dependent on the presence of growth
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663.
GOUWENTAK, C, A. and MASS, A. L. 1940. Kambiumtatigkeit und Wuchsstoff. II.
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Vascular Cambium 285
HERRERO, J. 1951. Studies of compatible and incompatible graft combinations with
special reference to hardy fruit trees. Jour. Hort. Sci. 26: 186-237.
MENDEL, K. 1936', The anatomy and histology of the' bud-union in citrus. Palest. Jour.
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MOSSE, B. 1955. Symptoms of incompatibility induced in a peach by ring grafting with
an incompatible rootstock variety. Ann. Rep. E. Mailing Res. Sta. 1954, pp. 76-77.
MOSSE, B. and HERRERO, J. 1951. Studies on incompatibility between some pear and
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NEWMAN, I. V. 1956. Pattern in the meristems of vascular plants. 1. Cell partition in
living apices and in the cambial zone in relation to the concepts of initial cells and
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OPPENHEIMER, H. R. 1945. Cambial wood production in stems of Pinus halepensis.
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PRIESTLEY, J. H. 1930. Studies in the physiology of cambial activity. III. The seasonal
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WAISEL, Y. and FAHN, A.1965a. A radiological method for the determination of cambial
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- 'cambial activity in Robinia pseudacacia L. New Phytol. 64: 436-442.
WAREING, P. F. 1951. Growth studies in woody species. IV. The initiation of cambial
- -activity in ring-porous species. Physiol. Plant. 4: 546-562.
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181: 1744-1745.
WAREING, P.- F. and ROBERTS, D. L. 1956. Photoperiodic control of-cambial activity
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WILSON. B. F. 1964. A model fo'r cell production by the cambium of conifers. In:
The Formation of Wood ill Forest Trees, ed., M. H. Zimmermann. Acad .. Press~
New York-London. pp. 19-36.
CHApTER 15
SECONDARY XYLEM
WOOD PARENCHYMA
Parenchyma
I i
wall
I
Vessel lumen 2
Fig. 129, nos. 1, 2). The nucleus. and part of the cytoplasm of the paren-
chyma.cells, .from which the tylosis is formed, ,enter the. tylosis. Tyloses
may divide. I
The number. of xylem rays in a_ trunk incr~ases with the increase in its
girth. The length, width and height of each ray can be measured. Thelength •
of the ray is determined in cross-sections of the wood. The width ofthe
ray is measured in tangential sections and it is usually expressed as the
maximal number of cells in a horizontal direction. The height of the ray,
parallel to the longitudinal axis of the stem or root, is also measured
from tangential sections and it is usual1y expressed in one of two ways-
if itis not very large, in the number of cells, and if it is very large, in microns
or millimetres. The dimensions of the rays'varyin the different plants and
sometimes even in the same plant. When the ray is one ~ell wide, it is said
to be a uniseriate ray (Fig: 127, no. 4);'when twq cells wiele, 'biseriate and
when more than two cells wide, multiseriate (Fig. 130, no. 1). In a tangen-
tial section a multiseriate ray is seen to become narrow towards both its
upper and lower edges, where it is usually uniseriate. \
'In species that have a storied cambium (see Chapter 14) a similar arran-
gement may exist: in the xylem (Fig. no, no. I). Sometimes lhe sloried
arrangement becomes indistinct. because ,of the intrusive growth of the
ends of the developing fibres and tracheids. The blurring/of the arrange-
ment occurs to different extents so that it is possible to distinguish different
degrees of storied arrangement from that where the fibres, tracheids and
axial parenchyma cefIs are equal in length' an~ are arranged in, horizontal
rows, to that in wh'ich arrangement of th:e xylem elemenfs is similar to that
developed from a non-storied cambium. In storied xylem the vessel mem-
bers are usually short. Phylogenetically the storied arningement is thought
to be the more advanced. \
The outer part of the secondary xylem contains living cells and at least
part of it is active in the transp~rt of water. The outer p7~lt is termed the\
sapwood or alburnum. ]n most trees the inner portion of t~e secondary
xylem completely ceases to conduct water and living cells in!it die. This is
accompanied by the disintegration of the protoplast, the loss of the cell sap
and the removal of reserve materials from cells that stored them. In those
species in which tyloses are a characteristic feature of the wood the vessels
in this inner portion become totally blocked, at this stage, by the formation·
of tyloses. The cell walls of the parenchyma cells which were little lignified
may become more heavily lignified. Certain substances, such as oils, gums,
resins, tannins, coloured substances and aromatic compounds, develop in
the cells, or are accumulated in them. In the gymnosperms the flexible
pit membrane becomes rigid and fixed in .such a position that the torus
Secondary Xylem 289
closes the pit aperture. Secondary xylem that has undergone such changes
is' termed -heartwood or duramen (Fi"g. 131, no. '1). The' above-mentioned
changes make the heartwood more-resistant to decay. The accumulation
of coloured substances in this part of the xylem makes it easily recognIz-
able from the sapwood. Heartwood may sometimes be developed as the
result of pathological conditions.
The quantitative relation between the amount of heart- and sapwood, and
the degree of difference between them varies greatly in the different species,
and the differences are influenced by the conditions under which the plants
are grown. In certain trees, e.g. Populus, Salix, and Abies, no distinct
heartwood is developed. In trees such as Robinia and Morus the sapwood
is very narrow, while in Ace,., Fraxinus and Tamarix the sapwood is wide.
There are fundamental differences in the histological structure of the
",ood of dicotyledons and that of gymnosperms, and especially of that of
the conifers. In the 'timber trade the wood of dicotyledons is known as
hardwood and that of gymnosperms as sO[Mood. These terms do not accu-
rately express the degree of hardness, as in both groups wood with both
hard and soft structure can be found.
CROSS SECTION
TANGENTIAL SECTION
RAYS
RESIN DUCTS
Resin ducts are developed in the vertical or both verticil! and horizontal
systems of a large number of gymnosperms. The ducts develop schizo-
genously between resin-producing parenchyma cells which then form the
epithelium of the duct. Sometimes a resin duct mav hecnme hlncke:cI hv
292 · Plant Anatomy
the enlargement of tbe epithelia) cells; such structures are termed tyJo-
soids.,.Dijferences
. .. exist.in-the.thickness and. lignification of, the cdl·wall of
the epithelial cells in the various conifer genera. In those genera, where the
FIG. 127. Micrographs of the secondary xylem of Pinus ha/epensis sectioned in dif..-
ferent planes. 1. Cross-section showing the border between the two growth rings.
X 95.2, Cross-section showing a resin duct . X 70.3 Radial section of a
portion
between the two growth rings. )( 95. 4, Tangential section of a portion incorpo-
rating a resin duct. >< 95.-
resin-secreting cells have thick, lignified walls and in which these cells die
after one season, relatively little resin is prod uced. In those plants where the
epithelial cells are thin walled and function during several seasons a large
amount of resin is produced . Resin ducts, the epithelial cells of which have
lignified waJis, occur in Abies and Cedrus, for example, while in Pinus the
secretory cells are thin walled a nd not lignified (Fig. 127, nos. 2, 4; Fig.
128. no. 2; F ig. 126). ·
Secondary Xylem 293
It is thought that in the secondary xylem of conifers resin ducts are prO-
duced as a .result of injuries, such as wounding, pressure andJrost, among
others. In certain conifers such as Cupress~s, . for example. (Fig.
.
128, nO,
1), resin ducts are never developed in the secondary xylem. The location of
th e resin ducts, when formed in the xylem, depends on the type of injury
a nd on the plant species. For instance, an open wound results in the for-
mation of dense or scanty tangential groups of resin ducts around the
294 . Plant Anatomy
wound. Injuries resulting from pressure or any other factor that acts on a
relatively large area,results in the formation o(scattered"dpcts. The extent
of this scattering depends on the genus: in Pinus, for example, the ducts are
more scattered than in Abies or Cedrus. In the last two the ducts are short
and branched. In Pinus the ducts which develop a great distance from the
.centre oCthe.injury are very long.and.are.not.arranged in groups, but'are
scattered to a great extent. From the results of experiments it has been
F IG. 130. Secondary xylem of tbe stem of Tamarix aphy lla . 1, Tangenti al section.
x 35. 2, Radial sect ion showing heterogeneo us rays. x 35.
sh own that the largest number of resin ducts is pr oduced when the cambium
of the injured branches is intensively active (Bannan, 1933, 19j4, 1936;
Messeri, 1959 and Spurr, 1950).
296 'Plant Anatomy
TANGeNTIAL SECTION
ARRANGEMENT OF VESSELS
FIG. 133. 1-3, Secondary xylem of Populus de/toides. 1, Tangential section. x 80.
2, Radial section. x 80. 3, Cross~section. X 65. 4, Radial section of the secondary
xylem of Pinus halepensis showing tracheids with bordered pits and crassulae.
X 400. 5, Cross-section of the secondary xylem of Zygophy/lum dumosum in
which the initial parenchyma on the border between the adjacent growth rings
can be distinguished. xli O.
Secondary Xylem
299
300 Plant Anatomy
1 , '
- -~
~
FIG. 134, Diffuse-porous secondary xylem. 1 and 2, Secondary
~
" xylem of Salix
babylonica. 1, Cross-section showing the border between two growth rings.' X 100.
\
1940). It has also been recorded that ring-porous wood is relatively com-
mon in plants growing in arid habitats (Huber, 1935). This observation
is supported by the results of the investigations made on woody plants
growing in the Negev. Bailey (1924) suggests that typical ring p orosity
developed in plants already adapted to tropical environments that be-
came subjected to climates with cold winters or with alternating very dry
Secondary Xylem 301
and wet seasons. This developme'1~' "is, in other words, connected. with
seasonal activity.
The pattern of distribution of the vessels is studied in cross-sections
of the wood. Here the vessels can be seen to be single as, for example, in
Eucalyptus (Fig. 137, no. I) and QuerclJs, or in groups of different size
and shape. For instance, the groups may consist of radial, oblique or
tangential rows of two to many vessels, the wa lls of wh ich are in contact
with one another and which are called multiples (Fig. 134, no. 1). Such
ni ctr;h , "+: ,... .... --.. .... . . 1- ..... ___ _ C' _ _____ _
302 Plant Anatomy
and Populus. The groups may be In the fOfm of clusters (Fig. 138, no. 3;
Fig. 140, no. I), i.e. irregular 'groups which c'onSiSt of v~rying numbers of
vessels in both ..radial and tangential directions. Pistacia may be cited as
--
ARRANGEMENT O F THE AXIAL WOOD PARENCHYMA
I
I
fIG. 139. Secondary xylem of Acacia cyafloplrylfa. J, CrOss-section. x 4S. 2, Tan-
. gential sectiO[l. X 100.
pendent of the ve\',\;el\; tho ugh it may come \1\ contact with them here and
t he re; and paratrachea! (Fig . 139, no . I) in whic h t he parenc hym a is
di stinc tly associated with t he vessels. Both these types are subd ivided
306 Plan( Anatomy
FIG. 141. J, Cross-section of the secorldary xylem of Acacia albida, showing broad
bands of parenchyma. x 12.2, Cross-section of tbe secondary xylem of Ochroma.
x 40
308 Plant Anatomy
wood consists' of the two types of cells, i.e. procumbent and square or
vertically elongated cells, it. is said to be"heterogeneous (Fig. 130, no. 2;
Fig. 137, no. 2). It has been proposed by some workers to include this
type of ray with the coniferous rays composed of different cell types in
the term heterocellular ray.
Heterogeneous rays may be uni- or multi seriate. The most common
type of heterogeneous ray is that in which the central portion of the ray
is multi seriate and consists of the radially elongated cells, while the upper
and lower edges contain the square or vertically elongated cells. Sometimes
the radially elongated cells are surrounded by square or vertically elon-
. gated cells. Thert are also a few plants, e.g. Olea, in which the square or
\vertically elongated cells are mingled with the radially elongated cells.
! The above nomenclature concerning the structure and arrangement of
initials. In this manner aggregate rays (Fig. 136, no .. 2) are often formed
from large rays. An aggregate ray is defined as a ray comprising a group
, . _ _ , .~._, w· •
of small and narrow rays which appears to 'the unaiaed eye or under low
magnification to be a single large ray.
2 3 4
I
,
/
Fusiform
initials
5 6
FIG. 142.1-4, Serial tangential sections of the secondary xylem of Viburnum odo-
ratissimum sho~ing the ori'gin of a ray initial at the end of a fusiform initial.
Newly formed ray initial stippled. 5-7. Serial tangen!ial sections of th~ secondary
xylem of Trochodendron aralioides showing the splitfing' of a ray by the apical elon-
gation of a fusiform initial. The entry of the fusiform initial into the ray is made
possible by the loss of the ray initial that is stippled i~_no. 5. (Adapted from
Barghoorn, 1940.) ____
As was mentioned above, the rays may also have increased in size dur..
ing evolution. This enlargement may be brought about by the merging of
rays (Fig. 143), by the anticlinal division of the ray initials, orby the hori-
zontal subdivision of the fusiform initials adjacent to the ray so as to form
additional ray initials. The merging of rays is brought about by the loss
of the fusiform initials that separate the two rays.
Secondary Xylem 311
'-u_
"I ~'
'{
.JI'('
Lr 1
illT 1 \\ \\'
+11
l\
l\
,\dJ
\
FIG. 143. Radial section of the secondary xylem of Casearia nitida illustrating the
fusion of two rays by the vertical elongation ,of the marginal initials and their
derivatives. Cambium on right. (Adapted from Barghoorn, 1940.)
GUM DUCTS
FIG. 144. Diagrams illustrating the relationship of the leaf~trace to th~ expanding
secondary plant body in woody dicotyledons, as seen in radial section. I, Showing
the condition of the leaf-trace in a stem devoid of secondary thickening; the leaf-
gap is still open. 2, A stem after I year of secondary growth showing that the gap
is p.artly closed by the secondary xylem and phloem. 3, Showing, the stem of a
declduollS plant after 2 years of secondary growth; the ruptured ends of the
trace are separated by secondary vascular tissl'es. 4, Showing the extension of the
leaf trace in an evergreen plant after 5 years secondary growth (parts outside the
cambium not represented in th1s diagram). In this case the trace ruptures from
time to time and the gaps thus formed are filled by secondary tissue 'produced by
that part of the cambium on the under side of the trace where it is in contact with
the secondary xylem. In the trace the primary tissue is represented by cross-
hatching and the secondary tissue by solid black areas. In all the figures primary
xylem is represented by widely spaced hatching, secondary xylem by more dense
hatching, primary phloem by fine stippling, secondary phloem by coarse stippling,
the xylem of the diverging part of the leaf-trace by cross-hatching. (Adapted from
Eames and MacDanieis, 1947.)
Secondary Xylem 313
Gt'owth-ring analysis
The problem of the correlation between the amount of annual incre-
ment and the environment arose because of the variations observed in the
width of the annual growth rings (Douglass, 1936: Holmsgaard. 1955:
314 Plant Anatomy
Glock, 1955; Fahn et al., 1963). The many investigations that have been
made,in this respect-suggest that._in arid regions the main factor influenc-
ing the width of. the growth ring is the amount of pre~ipitation, while in
the cooler temperate regions, temperature may play an important r61e. It
has also been suggested tliat there is' a correlation between the width of the
annual rings and the amount of sun-spots. Many authors haye tried to
draw conclusions as to the history of the climate by growth-ring analysis
of very old trees. -
The specific gravity of the wall substance of the secondary xylem of all
plants is more or less the same, and is about 1·53. Therefore the difference:\
in the weight of woods depend on the proportion betwe~n the. amount of
wall substance and lumen. Plants such as Diospyros, for example, in which
the cell walls are thick and the lumina small and which contain many fibres,
have heavy wood. Plants in which the cell walls are thin and in which the
lumina of parenchyma and fibres are large, have light wood. In some ge-
nera, such as Populus and TWa, the specific gravity of the secondary xylem
is low although the fibre walls are no(particularly thin; in these woods the
low specific gravity is due to the presence of numerous thin-walled vessels.
An extremely light wood is that of Ochroma (balsa wood), which belongs
to the type of wood known as "cork-wood". Such wood contains a high
proportion of large, thin-walled parenchyma cells (Fig. 141, no. 2).
Secondary Xylem 315
STRENGTH
DURABILITY
generally darker woods are more resistant to decay. The colour is usually
an indicator of' the amount of·preservative'substances·inlthe'wood.
The ability of wood to remain intact under mechanical strain depends on
the hardness and density of the wood.
PLIABILITY
Gl'uin , \
Border between
, two growth rings
FLu. 145. Diagram iUus.trating the relationship between the secondary tissues of a
branch and main sten'l. (Adapted from Eames and MacDaniels, 1947.)
tors such as light, gravity and the flow of solutions may also influence the
formation of such wo~d,-Stresses also exist in erect trefs buUhey are in-
creased in leaning trees.
COMPRESSION WOOD
TENSION WOOD
.
Tension wood develops on the upper side of leaning dicotyledonous
\
the fibres. In the inner, gelatinous layers, the orientation of the micro-
fibrils is almost parallel to the longitudinal axis of the fibre (MUnch, 1938;
Secondary Xylem 319
Wardrop and Dadswell, 1948, 1955). From these facts some workers have
concluded.that.it.is the gelatinous wall layers thatare mainly responsible
for the contraction of the tension wood. There is -less lignin in tension
wood than in normal wood, but the amount of cellulose and pentosans is
higher (see also p. 86).
Two types of tension wood are distinguishable: compact tension Jl'ood
in which the gelatinous fibres are situated in certain regions of the stem
(e.g .. Acer), and diffuse tension irood in which single or groups of gelati-
nous fibres are scattered among the normal fibres (e.g. Acacia, Fig. 138,
no. I).
2 3
The strength of the tension wood of the different species varies, but in
most cases this wood tends to form horizontal breaks. Some research
workers found that such breaks occur in single fibres. When tension wood
is sawn the cut surface appears woolly due to the tearing of the groups of
gelatinous fibres.
Tension usually exists between the peripheral secondary xylem and that
near the centre of the stem. When a piece of stem is cut longitudinally the
peripheral parts contract more than those portions nearer the centre. In
asymmetric stems containing tension wood the difference in bending is
more obvious. In branches that were bent into loops (Fig. 146, nos. 1-3)
tension wood was seen to develop on those sides of the loop which were
directed upwards. When such a loop was cut, after the formation of the
tension wood, contraction again occurred in those places where the ten-
sion wood had formed. Similar experiments were made with conifer bran-
ches in order to establish the properties oflhe compression wood (Jaccard,
1938).
, Plant Anatomy
320
leads to further alternatives from which the wood can eventually to identi-
fied.
I. Wood lacking vessels (gymnosperms) ........................ 2
-. Wood with vessels (dicotyledons) ........................... 4
2. Resin ducts present in each growth ring .. , .. , ... , ..... ,.. Pin liS
-. Resin ducts not present in all growth rings (only present in case of
injury), or completely absent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3: Resin d'ucts sometimes present; tori with lobed edges ........ Cedrus
-, Resin ducts never present; tori not as above ............ Cupressus
4. (I) Included phloem present., ......................... '..... 5
-. Included phloem absent ................................... 6
S. Rays contain cells that are radially elongated. . . . . . . . . . .. Sa/vac/ora
-. Rays lacking; sometimes the wood parenchyma may be arranged in
short radial rows ,similar to rays ... -, ... '.. _.................... .
Bougainvillea and genera of the Chenopodiaceae
6. (4) Growth rings not distinct. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7
·~~.-Growth rings distinct ...................................... 8
7.- Rays clearly heterogeneolls .................. . Ficus s),co)}lorus
-, Ra,Ys almost homogeneous ....................... Acacia rad(hana
;0,,-.... A. torfiJ;s
8. (6) Wood more or less distinctly ring-porous-the vessels forrued
'at the l5egi'nning of the season are conspicuously larger than those
formed later and this may be seen without the-aid 'of a microscope 9
-. Wood diffuse-porous --'- the diameter of the vessels almost equal
throughout the growth ring. or gradually smaller toward the termi-
nation of the ring ........... '.............................. 13
9. Vessels .single.or arra~'ged in multiples.of 2-4. Rays 2-3 cells wide.
Pafatracheal wood· parenc-hyma around the ,narrow vessels is a}j~
form-confluent ...................... ',' . . . . . . . . . . . . . Fraxinus
-. Vessels and wood pa~enchyma not arranged as above .. , ... , ... , 10
10. Wood rays 1-4 cells wide .............................. , .... II
-. Large wood rays wider than 4 cells .......................... 12
11. Vasicentric parenchyma scanty ....................... Pis/ada
-, Vasicentric parenchyma aliform to aliform~confluent ....... Cerds
12. (10) Large, aggregate, multiseriate rays (more than 20 cells wide)
present; vessels devoid of gum deposits ................. Quercus
-. Rays not aggregate, less than 10 cells wide; vessels filled with
gums .................................. ,. Prunus amygdalus
13. (8) Rays not wider than one cell ........................... 14
-. Rays wider than one cell .......................... '.' . . . . . .. IS
14. Rays homogeneous .................................. Popuilis
-. Rays heterogeneous ................................... Salix
15. (13) Maximum width of the rays exceeds 6 cells ............... 16
-. Maximum width of rays less than 4 cells ..................... 18
322 Plant Anatamy
References
BAILEY, 1. W. 1924. The problem of identifying the wood of cretaceous and later dico-
tyledons: Paraphyllanthoxylon arizonense. Ann. Bot. 38: 439--451.
BAILEY, I. W', I944. The develupment of vessels in -angiosperms and its s'ignificance in
morphological-research. Amer. Jour. -Bot. '31: 421-428.
BANNAN, M. W. 1933. Factors influencing the distribut"ion of vertical resin canals in
the wood of the larch - Larix {aricina (Du Roi) Kodi. Trans. R.S.C. Section 5:
203-218.
BANNAN, M. W: 1934. Seasonal wounding and res'in cyst production i~ the 'hemlock
Tsuga canadensis (L). Carr. Ann. Bot. 48: 857-868.
BANNAN, M. Wi 1936. Vertical resin ducts in the secondary wood.of the Abietineae
New Phytol. 35: 11-46. ' \,
BARGHOORN, E. S., Jr. 1940. The ontogenetic development, and phylogenetic special
ization of rays in the xylem of dicotyledons. I. The primitive ray' structure. Arner
Jour. Bot. 27: 918-928. --
BARGHOORN, E. S., Jr. 1941a. The ontogenetic development and phylogenetic specia-
lization of rays in the xylem of dicotyledons. II. Modifications of the m~ltiseriate
and uniseriate rays. Amer. Jour. Bot. 28: 273-282.
BARGHOORN, E. S., Jr. 1941b. The ontogenetic development and phylogenetic specia-
lization of rays in the xylem of dicotyledons. III. The elimination of rays. Bull.
Torrey Bot. Club 68: 317-325.
BISSET, I. J. W., DADSWELL, H. E. and AMOS, G. L 1950. Changes in fibre length within
one growth ring of certain angiosperms. Nature 165: 348-349.
BRAUN, H. Y. 1961. The organization of the hydrosystem in the stemwood of trees
and shrubs. News Bull. Intern. Assoc. of Wood Anatomists 1961 2: 2-10.
Secondary Xylem 323
BROWN, H. P., PANSHIN, A. J. and FORSAITH, C. C. 1949, 1952. Textbook 0/ Wood Tech-
nology. Vol. I. and II. McGraw~Hi1l. New York.
CHALK, L., MARSTAND, E: B. and WALSH, J. P. C. de. 1955. Fibre length in storied
hardwoods. Acta Bot. Neerlandica. 4: 339-347.
CHATTAWAY, M. M. 1955. Crystals in woody tissue; Part I. Trop: Woods 102:
55-74.
CHATTAWAY, M.-M.J956. Crystals in woody tissue; Part II. Trop. Woods 104: 100-124.
CLARKE, S. H. 1938a. A multiple-entry perforated card-key with special reference to the
identification of hardwoods. New Phytol. 37:_369-374.
"CLARKE;' S:·H. 1938b. The use of perforated cards in multiple-entry identification keys
and in the study of the inter-relation of variable properties. ehron. Bot. 4: 517-518.
DOUGLASS, A. E. 1936. Climatic Cycles and Tree Growth. Vol. 3. A Study of Cycles.
Carnegie lnst., Washington.
EAMES, A. J. and MACDANlELS, L. H. 1947. An Introduction to Plant Anatomy. McGraw.
Hill, New York - London.
ESAU, K. 1953. Plant Anatomy. John Wiley, New York.
FAHN, A.' 1958. Xylem structure and rhythm of development in trees and shrubs of
the desert. 1. Tamarix aphylla, r.- jordanis var. negevensis, T._gaWca var. maris·
mortui. Trap. Woods' 109: 81-94.
FAHN, A. WACHS, N. and _GINZBURG, C. 1963. Dendrochronological studies in the
.Negev. Israel Explor. Jour. 13: 291-299.
:-FREY:WYSSLlNG, A. 1952. Wachstumsleistung des pflanzlichen Zytopiasmas. JJer.
Schweiz. Bot. Ges. 62: 583-591 .
•G_ERRV, E. 1915. Fiber measurement studies. Length variatiom., where they occur, and
"theLr.relation_to the strength and uses of wood. Science 41: 179.
GILBERT, S. G. 1940. Evolutionary significance of ring porosity in woody angiosperms.
Bot. Goz. 102: 105-120.
GLOCK, W. S. 1955. Growth rings and climate. Bot. Rev. 21: -73-188.
HANDLEY, W. R. C. 1936. Some observations on the problem 'of-vessel length deter·
mination in woody dicotyledons. New,Phytol. 35: 456--471.
HOu.rSGAARD, E.1955. Tree-ring analysis of Danish forest trees. Forstl. Forseksv. Danm.
22: 1-246 (with English summary).
HUBER, B. 1935. Die physiologische ,Bedeutung der Ring. und Zerstreutporigkeit.
Ber: Dtsch. Bot. Ges. 53: 711-719, .
INTERNATIONAL ASSOCIATION Of WOOD ANATOMISTS. 1947. International glossary- of
terms used iri wood anatomy. Trop. Woods 1;)7: 1-36.
JACCARD, P. 1938. Exzentrisches Dickenwachstum und anatomisch-histologische
Differenzierung des Holzes. Ber. Schweiz. Bot. Ges. 48: 491-537.
KRIBS, D. A. 1928. Length of tracheids in Jack pine in relation to their position in the
vertical and horizontal axes of the tree. Univ. Minnesota, Agric. Exp. Sta. Bull.
54: 1-14.
KRIBS, D. A. 1937. Salient lines of structural specialization in the wood parenchyma
of dicotyledons. Bull. Torrey Bot. Club. 64: 177-188.
MESSERI, A., 1959. Contributo alla conoscenza dei meccanismi anatomici e fisiologici
della re~inazione. I. Alterazioni anatomiche in legno resinato e tamponato di
Pinus Pil1ea. Ann. Acc. Sc. Forest. 8: 203-225.
METCALFE, C. R. and CHALK, L. 1950. Anatomy 0/ the Dicotyledons. 1-2. Clarendon
Press, Oxford.
MUNCH, E. 1938. Statik und Dynamik des schraubigen Baues der Zellwand, beson-
ders des Druck- und Zugholzes. Flora (Jema), N. S. 32: 357-424.
PHILLIPS, E. W. J. 1948. Identification of Softwoods by their Microscopic STructure.
London.
PILLOW, M. Y. and LUXFORD, R. F. 1937. Structure, occurrence and properties of
compression wood. U.S.D.A. Tech. Publ. 546.
324 Plant Anatomy
PRIESTLEY, J. H. and SCOTT, L. 1. 1936. A note upon summer wood production in the
tree. Proc. Leeds Phil. Lit. Soc. 3: 23S_:_248.
SCURFIELD; O. and WARDROP, A. B. 1963. The nature 'of reaction wood. VlI. Ligni-
fication in reaction wood. Austral, Jour. Bot. 11: 107-116.'
SPACKMAN, W. and SWAMy', B.' O. L. 1949. The nature and occurrence of septate
fibers in dicotyledons. Abst. Amer. Jour. Bot. 36: 804.
SPURR, R. A. 1950. Organization of the procambium and development of the secretory
cells in the embryo of Pinus strobus L. Amer. Jour. Bot. 37: 185-197:
S""ERN, W. L., 1954. A suggesled classification for intercellular spaces. Bull, Torrey
Bot. Club. in: 234-'235. . .. -
STERN-COHEN, S. and FAHN, A. 1964. Structure and variation of the wood fibres of
Eucalyptus gomphocephala DC along and across the stem. La-Yaaran 14: 106-117.
English summary, 1~3-132.
TIEMANN, D. H. 1951. Wood Technology. Pitman, New York.
WARDROP, A. B. 1956. The nature 'of reaction wood. V. The distribution and formation
of tension wood in some species of Eucalyptus. Austral. Journal.' Bot. 4: 152-166.
WARDROP, A. B. 1964. The reaction anatomy of arborescent angiosperms. In;· _The For-
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York.
WARDROP, A. B. and DADSWELL, H. E. 1948. The-nature of reaction wood. 1. The
structure and properties of tension wood fibres. Austral. Jour. Sci. Res. B. 81:
~6. '
WARDROP. A. B. and DADSWELL, H. E. 1950. The nature of reaction wood. II. The cell
wall organization of compression wood trachejds. Austral. Jour. Sci. Res. 3:1-12.
WARDROP, A. B. and DADSWELL, H. E. 1955. The/nature of reaction wood. IV. Vari-
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177-189.
WARDROP, A. B. and DAVIES, G. W. 1964. The nature'of reaction 'wood. VIII. The
structure and differentiation of compression wood. Austral. Jour. Bot. 12: 24-36.
/
CHAPTER 16
SECONDARY PHLOEM
plished solely by the lateral expansion of the existing cells or, as is more
common, by the increase, as a result.of radial cell division, of the number
of'cellS on' die periphery. These widened parts of the ~ays constitute the
expansion tissue which is discussed more fully in the following chapter.
PhloeClJ Periderm
FIG. 147. 1, Micrograph of the outer portion of the stem of Gossyp;um in which
the widening of the vascular ray towards the periderm can be distinguished. X 27.
2, Cross-section of the secondary p hloem of Cupressus sempervirens in which
bands of phloem fib res can be seen to a lternate with bands of phloem parenchy-
ma and sieve cells. X 120.
Secondary Phloem 327
Sometimes only some rays become enlarged, while others do not change.
The outer parts of. the·phloem rays become cut off from the inner portions
by the development of cork tissue, This tissue is produced by the phello-
gen, and. its formation results in the interruption of the. connection be-
tween the inner living layers of the phloem and the outer layers which dry
out.
The arrangement of the sieve tubes and the parenchyma cells differs
in various plants. The sieve tubes and parenchyma cells may form separate
alternating bands as in Robinia and Arist%chia, for example, o~. sieve
tubes may be arranged in radi~l rows as in Prunus. \ ,J " \
In the sieve-tube members, the sieve areas are distinctly better developed
on the sieve plates than on the lateral wall~_However, in soIhe plants, e.g.
the subfamily Pomoideae of the Rosaceae, this difference is relatively
slight. In genera such as Quercus, Jug/ans, Vitis and Populus, the secondary
phloem is not storied and the sieve-tube members are elc,lgated and,bear
mostly compound sieve plates on the o'Slique end walls. In Acer, for ex-
ample, the sieve-tube members are shorter than the above and the end
walls are only slightly o\>lique and they bear simple sieve plates. In Robinia,
Tamarix, Ulmus and Fraxinus the simple sieve plates are horizontal. In
Robinia and Tamarix the phloem is storied and the sieve-tube members
are short. The oblique end walls are usually so orientated that, in a
Secondary Phloem 329
radial longitudinal section of the stem, the surface of the sieve plate is
'seen, 'and in a tangentiai"section; the 'sieve plates are sectioned longitudi-
nally. This arrangement is well demonstrated in Vitis, for example.
References
ABBE, L. B. and CRAFTS, A. S. "1939. Phloem of white pine and other coniferous species.
Bot. Gaz. 100: 695-722.
ARTSCHWAGER, E. 1950. The time factor .in the differentiation of the secondary xylem
and phloem in pecan. Amer. Jour. Bot. 37: 15-24. ,
BERNSTEIN, Z. and FAHN, A. 1960. The effect of annual and bi-annual pruning on the
seasonal changes in xylem formation in the grapevine. Ann. Bot., N. S. 24: 159-171.
ESAU, K. 1948. Phloem structure in the grapevine, and'its seasonal changes. Hilgardia \.
18: 217-296. /
EsAU, K. )950. Development and structure of the phloem tissue. II. Bot. Rev. 16:
67-114.'
ESAU, K. 1953. Plant Anatomy. John Wiley, New York. /
ESAU, K., CHEADLE, V. I. and GIFFORD, E. M. Jr. 1953. Compara?ve structure and
possible trends of specialization of the phloem. Amer. Jour. Bot. 40: 9-19.
EVERT, R. F. 1963. Ontogeny and structure of the secondary phloem in Pyrus mal~s.
Amer. Jour. Bot. 50: 8-37. •
HUBER, B. 1949. Zur Phylogenie des Jahrringbaues der Rinde. Svensk .Bot. Tidskr ..
4k376~382. . . , .
SRIVASTAVA, L. M. 1963. Secondary phloem in the Pinaceae. Univ. Calif ,Publ. Bot.
36: 1-142.
CHAPTER 17
PERIDERM
PHELLOGEN
to its position, the phellogen is a lateral meristem as, like the cambium,
it results in.an increase·of the,diameter·of the axis' by periclinal divisions
in its cells. Histologically the phellogen is simpler than the vascular cam-
bium as it consists of only one type of initial. These cells appear rectangular
in' cros~;.. section with their shorter axis in the radial direction and, in longi-
tudinal tangential section, they are seen to be regular polygons. The
protoplasts of the phellogen cells contain variously sized vacuoles and
they may contain chloroplasts and tannins. There are no intercellular
spaces in the phellogen except in those regions where lenticels develop.
PHELLEM
Like the cells of the phellogeri, the cells of the phellem (the cork cells)
are usually polygonal as seen in"-tangential section and, in cross-section,
they are flattened radially. In cross-section (Fig. 148, nos. I, 2), the cork
cells are usually seen to be arranged in cc·:upact. radia!" rows whIch are
devoid of intercellular spaces. This radial arrangement indicates that the
pbellogen cells divide tangentially.
Cork cells are dead cells. Various types of cork cells can be distinguished'
and in a few plants crystal-containing cells/"nd sclereids may be found
among the cork cells. Sometimes non-suberized cells, which are termed
phelloids,occur in the·phellem. Two common types of cork cells are tbose
which are hollow, thin-walled and somewhat widened rad!illy, ami those
which are thick-walled and radially flattened. The cells of the latter type
may often be filled with dark resiniferous or tanniniferous substances as,
for example, can be seen in Eucalyptus. These two types ,of pheilem cells
may occur together in the same plant as, for example, , in Arbutus and
Betula where they occur in alternating layers. In Betula this fe",ture causes
the cork to peel like sheets of paper.
The primary wall of the phellem cells consists of cellulose and may some-
times also contain lignin or suberin. Internally the primary wall . is lined
by a relatively thick layer of suberin, which is termed',_th~'suberin lamella.
A thin cellulose layer, which in certain plants may;b;' lignified; may be
,Jresent on the inside of this lamella. In the thin-walled phell~m cells this
inner layer of cellulose is absent (Eames and MacDaniels, 1947), The
suberin lamella is impermeable to water and gases, and it withstands the
action of acids. The protoplast of the phellem cells is lost after the various
wall layers have been formed and the cell lumen becomes filled witn air
or the pigmented substances mentioned above.
In the phellem of some plants, 'e.g. species of Haloxylon and Anabasis
(Fig. 148, no. 7), bands or large groups of hollow, thick-walled cells
occur among the usual thin-walled cork cells. These cells have a lignified
primary wall and a thick outer layer of secondary wall on the inside of
Periderm 333
PHELLODERM
The phelloderm cells are living cells with non-suberized walls. They are
similar to the parenchyma cells of the cortex but, if the phelloderm is multi-
seriate, they are usually arranged in radial rows. In certain plants the
phelloderm cells contain chloroplasts and they are photosynthetic. These
cells may also store starch. Sclereids and ot.her special cells are sometimes
present among the phelloderm cells.
Development of periderm
The phellogen may develop in living epidermal (Fig. 148, no. 2), pa-
renchyma or collenchyma cells (Fig. 148, no. I). The cells become meris-
"(ematic and undergo periclinal division. With the commencement of these
divisions starch and tannins. are gradually lost from those cells that con-
tained them. As a result of the first periclinal division two cells, which
are similar in appearance,lare formed. The inner cell is capable of further
division, but often it does not do so. In both cases, however, this cell is
regarded as a phelloderm cell. The outer cell undergoes a periclinal division
resulting in the formation of two cells. The" outer of these two cells differ-
entiates "inio" a cork cell and the inner cell constitutes tlie phellagen initial
and continues to divide. Sometimes the cork and phellogen cells are formed
after the first division and then no phelloderm cell is formed. In addition
to periclinal divisions the initials of the phellogen undergo occasional anti-
clinal divisions, so that the circumference of the cork cylinder is continu~
ously increased.
The number of phellem layers is usually greater than the number of
phelloderm layers. In certain plants the phelloderm is completely absent
but in many plants it consists of one to three layers of cells, while in a few
other plants it may be up to six layers thick. The number of layers in the
phelloderm may also alter with the age of the plant. The number of layers
of pheHem cells produced in a single season varies, in different species,
and may be very large. If the first-formed periderm remains on the
axial organ for many years, the outer layers of cork become cracked and
are shed so that the layer of cork remaining on a plant is of more or less
constant thickness.
In certain plants, such as Quercus suber and Arist%chia, thick layers
334- Plant Ana/omy
Epidermis
As has alread'y been' mentioned 'above, the periderm replaces the primary
protective tissues (epidermis and cortex) of the axial organs. With the
coniinuation of the 'process of secondary thickening the periderms them-
"Seiv-e·s are replaced, from time to time, by new periderrns which are formed
each time deeper in the living tissues of the axis. Therefore, it is necessary
10, distinguish between the first periderm and those that,are formed later.
The development of the first phellogen may take place in different cell
layers -external to the vascular cambium. In many stems, e.g. Solanum
dulcamara, Quercus suber and Nerium oleander, the first phellogen is"formed
in the epidermis itself (Fig.' 148, no. 2). More commonly the first phellogen
develops in the layer of cells immediately below the epidermis. Such de-
velopment can be seen in Populus (Fig. 148, no. I), Juglansand Ulmus,
,among others. In the potalo.tuber, thephellogen develops in the epidermis
as well as 1n the subepidermal cell layer, but the phellogen formed in the
epidermis does not continue to function. after its formation. In th~ stems
of certain plants, e.g. Robinia pseudacacia, species of Aris/o!ochia and
Pinus, the first phellogen forms in the second or third cortical layers. In
Thuja, Punica, Arbutus, Vitis and Anabasis the cambium of the first-formed
periderm develops near the phloem or in the phloem parenchyma itself.
In roots of gymnosperms and dicotyledons, the first phellogen is character-
istically formed in inner layers, usually in the pericycle. In the roots of
- .
.
.
Perlderms
4
5
6
7 8 \
I
FIG. 150. 1-5, Diagrams showing the position, '>hape and extent of the additional
periderms formed in a woody stem on which the first-formed periderm develops as
an entire cylinder close to the epidermis.-l-4, As seen in cross-section of branches
aged from 1-4, years. In nos. 3 and 4 the first-formed periderm toeether with the
tissues external to it have sloughed away. 5, Three-dimensional diagram of an
outer portion of a stem showing the peripheral tissues, which here include a
narrow zone of functioning phloem and a wide rhytidome with deep grooves; a
considerable amount of the latter tissue has been weathered away. 6-8, Protective
layers of monocotyledons. 6, Cross-section of the outer part of the cortex of Cur-
cuma tonga showing storied cork. 7, Radial section of the stem of Cordyline aus-
tralis showing the position of the layers of storied cork, which enclose patches of
suberized undivided cortical cells. 8, Diagram of portion of a cross-section of the
Periderm 339
crust increases in thickness due to the, addition of further cork layers which
enclose. pockets ,of cortical tissue and .dry phloem . .AIl .the. cork layers
together with the cortical and phloem tissues, external to the innermost phel-
logen, are termed rhytidome or outer bd'rk;_while all-_the tissues external
FIG. '151. -I, Photograph of a branch of Cillotropis procera showing the deeply
grooved rhytidome. x 0'6. 2, Branch of Tamarix sp. in which tranSVf'fse lenticels
can be seen. x 0'7.
to the vascular cambium are included in the term bark. The living part of
the bark inside the rhytidome is often termed the inner bark. With the
increase in diameter of the secondary xylem the circumference of the cam-
bial cylinder enlarges. As a result of this, the new-formed layers of second-
ary xylem are larger in circumference than are the outer layers of the
inner bark which are, therefore, brought under strain. This strain is acco m-
stem of Cordyline indivisa showing a superficial layer of crushed cells and alter-
nating tangential bands of suberized, undivided cortical cells, and storied cork.
(Nos. 1- 5, adapted from Eames a nd M acDaniels, 1947; nos. 6.,.8, adapted f rom
Philipp, 1923.)
340 Plant Anatomy
FIG. 152. I and 2. A bott le-stopper made o f cork . J. As seen f rom above (i.e. cross
section of the cork relative to its positi o n on the stem) in which wide growth rings
a nd two lenticels can be seen . x 2 '5. 2. As seen f rom the sidc ( i.e. in tange nti al
section of the cork relat ive to its JJ.osition o n the stcm) in wh ich the lent icels can
be seen as dark patc hes. x 2' 5.3. Raphanu.\' salivils roo t showing each lateral root
to be accompanied by a pair of lenticels.
Periderm 341
,Morphology of bark
The 'outer appearance of stems 'differs in 'different species of plants" and
the type of bark is used in many cases as it taxonomic character. These
differences result from the manner of growth of the periderm, the
structure of the phellem and the nature and amount of tissues that are
separated by the periderm from the stem, Therefore it is possible to con-
.elude that the outer appearance of the stem is determined by the type of
rhytidome,
In plants in which the first periderm forms close to the epidermis a small
amount of primary tissue is cut off from the stem and is eventually shed.
In this case the phellem becomes exposed and then no rhytidome is con-
sidered to be present on the stem. When such a phellem is thin its surface
i~ usually smooth, while if it is thick, the surface is cracked and ridged. In
plants where the first-formed periderm develops deep within the axis,
thicker layers of tissue, which are usually connected to the cork, remain
on the. stem surface, alid therefore these plants exhibit a.rhytidome.
_ Certain rhytidomes, e.g. tho-se of Ulmus americana, Magnolia acuminata
7and Calotropis (Fig. lSI, no. 1), consist mainly of parenchyma tissue and
~soft phellem, while in others, e.g. species of Quercus and Carya, the rhyti-
dome contains large quantities of fibres (mostly phloem fibres) which are
associated with hard cork cells. The manner of formation of the periderm
influences the shape of the bark in general and of the rhytidome in par-
ticular. When ihe subsequent periderms develop in the form of overlapping
scales or shells the outer layers are sloughed accordingly, and so a scaly
bark is formed. This type of bark occurs on relatively young stems of
Pinus, Pyrus communis and others. In Vilis, Lonicera, Clematis and Cu-
pressus,.for example, the subsequent periderms are Jormed as entire'cylin-
ders and so the dead outer tissues are sloughed as hollow cylinders. This
type of bark is termed ring bark. The bark of Platanus, Arbl<tus and spe-
cies of Eucalyptus, for example, is intermediate between the above two
types. In these plants the outer layers of the bark peel off in the form
of relatively large sheets.
The sloughing of the outer layers of the bark is brought about in various
ways. In Arbutus and Platanus the large plates of dead outer tissue sepa-
rate from the inner portions of the bark through a layer of thin-walled
cork cells, and the thick-walled cork cells below them remain attached to
the stem which, therefore, has a smooth surface. In species of Eucalyptus
the sheets of dead outer tissues of the bark exfoliate through layers of pa-
renchyma cells with unthickened walls, which occur on the periphery of
the phellem (Chattaway, 1953). In some trees, e.g. Fagus, the inner bark
grows slowly and therefore much expansion tissue is formed. In this case
the subsequent periderms cut off small amounts of secondary phloem and
the sloughing of the outer bark is slow, resulting in the fall of minute scales
and even powder (Whitmore, 1962b).
342 Plant Anatomy
Conlmercial cor/<
~
is
Commercial cork made from the bark of trees and, in particular, from
that of Quercus suber (Eames and MacDaniels, 1947), In the stem of this
plant the first phellogen forms in the epidermis, This phellogen may remain
on the plant indefinitely but in order to obtain commercial cork this first-
formed periderm is removed when the tree is about 20 years old and about
40 cm in diameter. After the first periderm is removed the' exposed cells
of .the phell_"d..E,!, ~nd cDrtex dry Dut and die, and a new phellDgen is
formed a few millimetres within the cortex, below the first-farmen phello-
gen. This subsequent phellogen produces cork more rapidly and in about
10 years a cork layer thick enough to be of commercial'vaiue is obtained.
This cork is of better v.alue than the virgin cork, which has almost no
commercial value. However, it is of poorer quality than that of cork ob-
tained from periderms that are formed . as aresult of subsequent stripping"
made at about 10 year intervals till the tree 1s about 150 or more years old.
After a few strippings the phellogens are formed in the second'ary phloem.
The pieces 'of cork that are stripped from the tree exhibit surfaces with
different structure-the outer surface is rough because or'weathering and
the presence of remnants of dead tissues outside of the phelloderm, while
the inner surface is smooth. On the radial surfaces and in cross-sections of
,such pieces of cork, bands, which apparently represent annual increments,
can ,be distinguished (Fig. 152, no, I). I '
The dark brown spots that can be seen on tangential surfaces of cork
and the similar stripes seen on radial surfaces and in cross-sections (Fig.
152, nos. 1,2) are lenticels (see p. 344). '
The features that give commercial value to. cork are its imperviousness to
gases and liquids and its strength, elasticity and'lightness.
TVollnd co ..t.
Generally, in such places where living plant tissue is exposed to the air
as a result of wounding wound cork is developed. Usually the outer dead
tissues are separated from the inner intact ones by a layer of cells which
become suberized .. Apart from this,separating layer a phellogen may be
developed in the living undamaged layers. This phellogen produces pheJ-
loderm and ph ell em in the usual manner. The layer of cork thus formed
prevents the loss of water through the wound and it protects the plants
against the entry of fungi and bacteria. Apparwtly wound cork may de-
velop on all parts of the plant, including even fruits and leaves. However,
there are differences in the type and amount of cork developed in different
species, organs and tissues, and under different environmental conditions.
Usually wound cork is more easily developed on woody plants than 01
herbaceous or monocotyledonous ones. Low temperatures and low humid-
ity may delay the development of wound cork even in those places where
it develops readily as, for example, on potato tubers (Kuster, 1925;
-\rtschwager and Starrett, 1933; Bloch, 1941).
Polyderm
In certain species of the Rosaceae, Myrtaceae, Hypericaceae and Ona·
graceae, a special phellogen is formed in the pericycle of the root or under·
ground stern. This phellogen produces, centrifugally, a few layers of thin·
walled non-suberized cells which alternate with a layer of endodermal-lik<
cells. At the start of the differentiation of the latter into cork cells, Caspar·
ian strips appear on the walls, which, with further development, become
entirely lined by a suberin layer. This type of complex tissue is termed poly·
derm. Its inner layers, including the cork cells, are living and may serve as
a storage tissue (Mylius, 1913; Luhan, 1955; Nelson and Wilhelm, 1957).
Lenticels
In the large majority of plants there are restricted areas of relatively
loosely arranged celJs, suberized or non-sul·erized, in the periderm. These
areas are termed lentice/s. Lenticels protrude .above thesurrounding peri-
derm because of the bigger size and loose arrangement of the cells which,
themselves, are usually more numerous in these regions. Because of the
continuity of the intercelfular spaces of the lenticels and those of the inner
tissues of the axial organ, it is assumed thai the function of the' lenticels
is connected with gas exchange, similar to that of the stomata on organs
covered by an epidermis only.
DISTRIBUTION OF LENTlCELS
Externally mature lenticels are usually lens-shaped and they are convex
both towards the exterior and the interior (Fig. 153, no. 2). According to
the orientation of the rupture of the epidermis the lenticels are described·
as being longitudinal or transverse (Fig. 151, no. 2). The lenticels which
are situated ciose1to the outer _ends of the phloem rays enabl~ relatively free
passag~ of gases between the inner hssues of the axis and the atmosphere.
346 Plant Anatomy
In stems with larger vascular rays it may be seen that the lenticels usually
appear directly opposite, the rays.. 'i
In the roots orPhoenix dactylifera lenticellular structures occur which
take part in aeration of the root butwhich differ from the above-descri bed
ordinary lenticels. Here the lenticels formeollar-like stmctures around the
thinner roots. The complementary tissue is, however, of typical structure.
Lenticels begin to form together with the first periderm or shortly before.
The time of the formation of the lenticels differs in various species and it is
dependent on the persistence of the epidermis on the stem. In most species
the development of the lenticels is already commenced during the first
season of 'growth of the organ and sometimes even before the elongation
or the organ is completed.
The first-formed len'ticels generally- appear below a ,stoma or group of
stomata. The cells in these regions begin to div'ide in different directions
and the chlorophyll in them disappears so that a loose colourless tissue is
formed., The division of the cells progresses in the cortex inwards and the
orientation of the divisions becomes mor~ and more periclinal until the
phellogen of the'lenticel is formed. The cells that are derived from the
divisions of the substomatal cells, as well as those produced towards the
exterior by the phellogen of the lenticel, are termed co,riplementary cells.
The increase in number of these cells causes, the ruptu(e of the epidermis
so that masses of complementary cells are pushed out and rise above the
surface of the organ. The exp_ose,d cefls die and weather away and they
are replaced by new cells produced by the phellogen (Fig. '153, no. I).
Complementary cells may be suberized or non:suberized, and they are
usually more or less spherical and thin-walled. Centripetally the phellogen
of the lenticel may produce phelloderm. '
In some species, apart from the complementary cells, the phellogen of
the lenticel also produces closing layers centrifugally ,(Fig. 153, no. 2~.
These layers, which consist of compact tissue, alternate with the comple~
mentary tissue. I
Two types of complementary tissue are-d[stinguished: that in which the
connection between the cells is relatively strong, as, for example, in Sam-
bucus (Fig. 154), Salix and Ginkgo; and that in which the cells have almost
no attachment between them giving the tissue a powder-like appearance,
as, for example, in the stems of Pyrus, Prunus and Robinia and in the roots
of Morus (Eames and MacDaniels, 1947) . .1n the latter type the cells of the
complementary tissue are held in place by the closing layers (Fig. 153,
no. 2). In spite of their compactness, the closing layers contain intercellular
spaces which allow the passage of gases. Similar gaps are found in the
Periderm 347
phellogen itself. The closing layers are ruptured as a result of the continual
production of new complementary ceJls. In the temperate zones the lenti-
Secondary phloem
eels become closed at the end of "the growing season by a closing layer.
This layer is ruptured oilly with the renewal of the activity of the plant
which is accompanied by the rapid and excessive p!oduction of new com-
plementary cells.
DURATION OF LENTICELS
References
ARTSCHWAGER, E. F. and STARRETT, R. C. 1933. Suberization and woundcork forma-
tion in the sugar beet as affected by temperature and relative humidity. Jour. Agric.
Res. 47: 669-674.. .
BLOCH, R. 1941. Wound healing in higher plants. Bot: Rev. 7: 110-146. \
BOUREAU, E. 1954. Anafomie Vegbale, Vol. 1. 'Press~s Universitaires de France, Paris.
CHATTAWAY, M. M. 1953. The anatomy'of bark I. The-genus EucalYPlus. Austral.
Jour.- Bot. 1: 402--433.
EAMES, A. J. and MACDANlELS, L. H: 1947. An IntroduCtion to Plant Anatomy. McGraw-
Hill, New York-London. /
EVERT, R. F. 1963. Ontogeny and structure of the secondary phloem in Pyrus malus.
Amer. Jour. Bot. SO: 8-37.
FLOR£STA., P.la, 1905. Ricerche sui periderma delle Palme. Contr. Biol. Veg. Palermo 3_,
333-354. tav. 18-19.
KOSiER, E. 1925. Pathologische Pjfanzenanatomie, 3rd ed. G. Fischer, lena.
LUHAN, M. 1955. Das Abschlussgewebe der' Wurzeln uhserer Alpenpflanzen. Ber.
Dtsch. Bot. Ges. 68: 87-92.
Moss, E. H. 1940. Interxylary cork in Artemisia with a reference to its taxonomic
significance. Amer. Jour. Bot. 27: 762-768.
Moss, E. H. and GORHAM, A. 1. 1953. Interxy\ary cork and fission of stems and roots.
phytomorphology 3: 285-294. \,
MYLJus, G. )913. Das Polyderm. Eine vergleichende Untersuchung tiber die physio~
logisch~m Scheiden: Polyderm, Periderm und Endodermis. Bibl. ,Bot. 18, 1-119.
NELSON, P. E. and WILHELM, S. 1957. Some aspects of the strawberry root. Hilgardia
26: 631-642.
PHILIPP, M. 1923. Ober die verkorkten Abschlussgewebe der Monokotylen. Bibl.
Bot. Stuttgart 92: 1-28.
TOMLINSON, P. B. 1961. In: Anatomy of the Monocotyledons. II. Pa/mae. Clarendon
Press, Oxford.
TROLL, W. 1948. Allgemeine Botanik. F. Enke, Stuttgart.
WHITMORE, T. C. 1962a. Studies in systematic bark morphology. I. Bark morphology
in Dipterocarpaceae. II. General features of bark construction in Dipterocarpa-
ceae. New Phytol. 61: 191-220.
WHITMORE, T. C.1962b. Why do trees have different sorts of bark ?New Scient. 16: 330-331.
CHAPTER 18
ohyma; these strands increase in number with the increase of the circum-
ference of the~cambium (Fig.,155,.no. I). Jhe,reduction of the, activity. of
the cambium to restricted areas only results in the formation of ridged
stems, which often split. The latter type of secondary growth occurs in
Peganum harmala and Zygophyllum dumosum (Fig. 156, no. 1), among
others. In certain genera, for instance, Serjania, of ~he Sapindaceae, the
cambium first appears in separate strands eachof which surrounds a group
of v!lscular bundles or even a single primary vascular bundle (Fig'. 155, no.
2). Stems that develop in this~ way appear as if they originate from the
fusion of a,number of stems. With the aging of such stems and with the
production of periderm layers the stems split into numerous parts (splits).
A similar structure can result from the excessive development of the xylem
and phloem parenchyma which results in the splitting of both the conduct·
ing tissues and the cambium cylinder.
In Achilleafragrantissima and some species of Artemisia a layer of cork
is produced each year on the border between two growth rings of xylem,
i.e. interxylary cork. This ;[eature,'when accompanied 'by the suberization
_2f.th·e; rays, as in Artemisia herba·alba, for example (Ginzburg, 1963),
or by the cessation of activity in certain portions of the cambium, also re-
:stiltsin the splitting of the stem (Fig. 156, no. 2).
Included phloem
I
F IG. 157. Surface view of cross-sectioned stems of plants belongi ng to the Chen o-
podiaceae showin g different arrangement s of the included phloem a nd its associ-
ated tracheary e lements. 1, Ha/oxylun urrictllatllm. :~ 10. 2, Al1ahasi< articulata.
x 6.
Al1omalous Secondary Groll'th 353
bia, as is the case in Beta, for example (Artschwager, 1926). The cambial
initials divide and the inner cells resulting from this ,division continue to
divide a number of times before they undergo differentiation into xylem
and ph loem elements, while the outer cells form the initials of the next outer
cambium, ,which replaces the former one. The initials of 't his cambium
divide similarly to those of the cambium which it replaces, and this feature
is ,repeated many times. .
In some species, e.g. Salvadora persica (Singh" 1944! and some earlier
authors), Thunbergia mysorensis and T. grandifiora (Mullenders, 1947), no
additional cambia are formed. The included phloem is differentiated from
localized patches of thin-walled cells which are produced centripetally by
the normal cambium. Subsequently the cambium resumes normal activity
i.e. the "centripetal production- of xylem, in- these regions. As a· result of
this the above-mentioned patches of phloem become deeply embedded in
the secondary xylem (see also: Structure of storage roots, p. 258).
In many species of_the Chenopodiaceae, in Bougainvillea and in other
plants with similar secondary thickening, each strand of secondary phloem
is accompanied, on its inner surface, by a group of xylem vessels (Fig.
158, nos. 1-3). In cross-section such stems appear, therefore, to consist
of a fibrous ground tissue in which there are scatter~d "vascular bundles".
The parenchyma associated with the included phloem is termed conjunc-
tive tissue. -This"parenchyma is variously arranged and developed: it may
be ray-like or 'tn bands that connect the phloem strands, or it may surround
and intermingle with the vessel groups. The phloem strands and vessels
of these "vascular bundles" of the secondary, body may anastomose both
tangentially and r;idially, but" tangential connections are.the more common.
I n the perennial species of. the Cheno'podiaceae growing in the,·desert
the anastomosing 'system of included phloem may have, important .adap-
tive value, especially as the phloem in this family may remain active for
many years. Even in cases wh'!re the outer tissues of the '~tem dry out dur~
ing the long summer, the included phloem strands remain viable ana so
can supply nutrients to the buds which, at the onset of the growing sea-
son, can then commence 1'0 develop.
J
embryo the primary thickening meristem constitutes a flat zone below the
leaf and sheath pr.i!,!ordia anq, in the se.edling, a steep. cone. In later stages
of development this meristem again appears as a flat zone and eventually
it becomes concave. Mostof the stem tissues develop from it. During the
maturation Df the palm the primary thickening meristem at first, contri-
butes mainly to the thickening of the stem, but later it is also responsible
,
becomes fully addJtionol
for the increase in stem height. Below the concave meristematic zone of
the mature palm longitudinally orientated rows of cells can be distinguished;
the activity of this zone results in the increased length of the trunk.
The primary thickening meristem develops as the result of periclinal divi-
sions in the cells in the region below the very young leaf and sheath pri-
mordia. This meristem has no special layer of initials, such as is present
in a typical vascular cambium. In the banana corm new cells are added to
the primary thickening meristem from the cortex.
356 Plant Anatomy
In the palms and in the banana corm the provascular strands (the pro-
cambium) are derived from two sources-to a smalh~r extent from ,the
shoot apex, and to a larger extent from the primary thickening meristem.
In addition to the above-mentioned meristem which brings about' pri:"
mary thickening, in palm stems- the ground tissue close to the apex ex-
pands and thus also results in additional thickening of the stem. It has
been reported (Zodda, 1904; Schoute, 1912; Tomlinson, 1961) that in
some palms the expansion of the ground 'tissue continues for a long period
and that it is very obvious in the older parts of the stem which are c~nsi
derably distant from the shoot apex (e.g. in Roystonea and Actinophloeus).
Here the central parenchyma cells and the not yet fully differentiated outer
fibres of the bundle sheath continue to undergo divisions, which are followed
by cell expansion, for a long period. The intercellular spaces also
increase in size proportionally to the increase in size of the parenchyma
cells. This type of secondary thickening has been termed diffuse second-
ary grawth (Tomlinson,~ 1961). The ventricose shape of some palm trees
is, according to ~Torrilinson, probably due to the increased vigour of the
leafy' crown, and such stems are mainly of primary origin, but some
secondary thickening may also take place.
Secondary thicke~ing proper takes plac~n different monocotyledonous
species such as Aloe arborescens, specles-·of Yucca, Dracaena and Sanse-
vieria of the Liliaceae, species of Agave of the Amaryllidaceae, and in
species of Xanthorrhoea, Kingia and Lomandra of the Australian family, the
Xanthorrhoeaceae (Cheadle, 1937; Fahn, 1954). Seconsl6ry thickening in
the monocotyledons is brought about by a special cambIum which appears
in that part of the stem that has already ceased to elongate and which is in
continuation with.the primary thickening meristem (Chouard, 1937). This
cambium develops in the parenchyma of the stem external to the entire
mass of primary vascular bundles. The cells / of this secondary meri- t
stem are, as seen in tangential view, of different shapes-they may be
fusiform (that is, long with tapered ends), rectangular, or with 'one tapered
and one truncate end.
The monocotyledonous cambium functions in the manner described be-
low. At first the initials of the cambium produce cells' towards the centr~'
of the stem and only later a small amoun~. tissue is produ'ced toward the
circumference. The cells produced on the outside of the cambium all devel-
op into parenchyma cells, while those produced on the inside develop, in
part, into parenchyma and, in part, into the vascular bundles (Fig. 160,
nos. 1,2). The inner parenchymatous ground tissue is termed conjunctive
tissue and the walls of its cells may sometimes become thiCkened. The
bundles develop from single cells that are cut off from the cambial initials.
Each of these single cells represents the centre of a future vascular strand.
These cells divide anticlinally to produce two or three rows of cells which
then divide periclinally; later the direction of the divisions becomes hap-
Cambium
Mature Differentiating
secondary secondary Stem
bundle bundles cortex
hazard but still longitudinal. In this manner the secondary vascular bun-
dles,are.formed. The cells of the bundle undergo sliding growth during
their development. The xylem elements elongate fifteen to forty times their
original length, while the xylem parenchyma and the phloem elements
undergo little or no elongation. The thickening of the walls of those
tracheids cl1?sest to the centre of the axis commences, in most cases, prior
to the completion of the cell division in the developing bundle (Cheadle,
1937). "-
The secondary bundles may be amphivasal, i.e. the xylem surrounds
the phloem as is seen in Xanthorrhoea, Lomandra, Dracaena (Fig. 83, no.3)
and Aloi' arborescens, for example, or the xylem may surround the phloem
on three sides and then the bundle appears U-shaped in cross-section
as is seen, for example, in Kingia (Fig. 72, no. I). The tracheary"elements
of these bundles in all the species that have as yet been stud,ied are all
tracheids. The walls of the parenchyma cells in which the vascular bundles
are scattered may be thin or thick and lignified. The parenchyma that devel-
ops externally to the cambium usually remains \thin walled and in many
plants many of the cells of this tissue contain crystals. In XanthorrllOea
resin is secreted in the cells 01 the outer parenchyma so that a resin sheath
is formed around the stem. (It is repor!ed tgat the aborigines of Australia
have almost completely destroyed the (reis of Xanthorrhoea by burning
them in order to enjoy,the beautiful flames that result from the burning of
this resin sheath.) /
The connection between the primary and secondary ,bodies in mono-
cotyledons with secondary thickening is strong, as it is in the dicotyledons.
The union, in monocotyledons, is even more obvious because there are
connections between the secondary bundles and the peripheral foliar ones.
/
References
ARTSCHWAGER, E. 1926. Anatomy of the vegetative organs of the sugar beet. JONr. \
Agric. Res. 33: 143-176. ,.
BALL, E. 1941. The development of the shoot apex and the primary thickening meristem
in Phoenix canariensis Chaub., with comparisons to Washingtonla ./ilifera Wats.
and. Trachycarpus excelsa Wend1. Amer. Jour.-~Bjjt: 28: 820-832.
CHEADLE, V. 1. 1937. Secondary growth by means of a thickening ring in certain mono~
cotyledons. Bot. Gaz. 98: 535-555.
CHOUARD, P. 1937. La nature et Ie role des formation dites "secondaires" dans l'edi·
fication de la tige des monocotyledones. Soc. Bot. France Bull. 83: 819-836:
CLOWES, F. A, L 1961. Apical Meristems. Blackwell Scientific PubL, Oxford.
EAMES, A. J. and MACDANIELS, L. H.1947, An Introduction to Plant Anatomy. McGraw-
Hill, New York - London.
ESAu, K, 1953, Plant Atmtomy. John Wiley, New York,
FAHN, A. 1954. The anatomical structure of the Xanthorrhoeaceae Dumort, Jour.
Linn. Soc. London. Bet. 55: 158-184.
Anomalous Secondary Growth 359
THE FLOWER
Floral organs
The problem of homology and morphological evolution of the flower
has occupied research workers for a"lol1g time. Investigators such as Wolff
and Goethe in the eighteenth century and De Candolle at the beginning
of the nineteenth century, and many others since then, were interested by
this problem (Arber, 1937, 1950). Opinions were expressed that floral organs
are derived directly from foliage leaves. How;,ver, 'in the light of the view
generally accepted today that the leaves aI;d stem constitute a single unit
which is termed the shoot, we can visualize the 'development of the flower as
being parallel to that ofa vegetative branch and not as being d~rived from it.
The flower consists of an axis on which the rest of the. floral organs are
borne. That part of the axis that represents the internode terminated by the
flower is termed the pedicel, The distal end of the pedicel is swollen to
various extents and this portion is termed the floral receptacle or thalamus.
The floral organs are attachedto the receptacle. Niypical flower has four
types of organs. The outermost" organs of the'flow~r~re the separs, whi~h
together constitute the calyx which is usually green and is found lowest
on the receptacle. On the inside of the sepals is the corolla, consisting of
the petals which are generally coloured. These two types of organs together l
form the perianth; however, sometimes one of them. 'may be lacking.
When all the organs of the perianth are similar they are termed tepals.
Within the perianth two kinds of reproductive organs are found: exter-
nally the stamens which together form the androecium, and internally the
carpels which form the gynoecium.
The arrangement of the floral organs on the receptacle may be spiral or
whorled, and both types of arrangement may occur in the same flower.
In most flowers in which the arrangement is whorled, the organs of each
whorl alternate with those of the neighbouring whorl. The floral organs
may be free or fused. Fusion of:organs of the same type is termed cohesion,
and that of different types of organs, adnation.
The Flower 361
7 5 '3 2
12108642
II 9
/ 3
FIG. 161. Four stages in the ontogeny of a flower cluster of Musa; the numerals
irtdicate the order of the appearance of the floral primordia.
. . / .
by a zone of relatively 'small cells which are rich in pr\2toplasm and which
mostly divide anticlinally: With this development the activity 'of the rib
meristem ceases. It will not be discussed here whether in the reproductive
apex a tunica-corpus arrangement is present, but it should be mentioned \
that the number of cell layers in the outer zone is usuall{larger than that .
found in the vegetative apex. The"initiation andthe first stages of histo logi-
cal differentiation of the different floral organs are, in principle, similar to
the early ontogenetic stages of bracts and foliage leaves (Tepfer, 1953).
The morphological and functional differences between the different floral
organs are apparently related to a series of physiological processes which
take place during the different stages of floral differentiation. This assump-
tion is supported by the results of experiments which involved the incising
and dissecting of the primordia of floral organs at different developmental
stages (Cusick, 1956). ..",..
The developing floral organs usually appear in a distinct acropetal order
The Flower 363
(Fig. 162, nos. 1,2), i.e. the youngest organs are closest to the apex. How-
ever, in certain genera and families; e:g. Paeonia, the ''Bixaceae, "Dillenia=
ceae and Tiliaceae, the floral apex stops growing and ,then basipetal or
Stamens 3
5
4
6
FIG. 162. Ontogeny of flowers. 1-3, Ranunculus trilobus. 1 and 2, Two stages in the
development of the entire flower. 3, Developing carpe1. 4-7, Reseda odorata.
4 and 5, Two stages in the development of the entire flower. 6 and 7. Developing
gynoecium at stages later than those depicted in nos. 4 and 5. (Adapted from
Payer, 1857.)
""''i<"'''''''-~''1'
r'1-"" ....., "
" , ' \ ~!\
..
,
\>;'J;\,,~~;~C1; \ it'meo
\::::::)'(s""" . 3 4
2
9 10
FIG. 164. Ontogenyoffiowers.l-4, Development of the a pocarpo us gynoecium of
BUfomus umbel/atus. 5-10, Linum perenne. Stages in the development of the syn~
carpous gynoecium from separate primordia which expand laterally and fuse to
form a single ring. (Adapted from Payer, 1857.)
ginally consisted of the two basal laminar lobes which, as in the onto-
geny,of.the peltate leaf, have fused. The area:\"here the two margins fusp.!
'is termed the' cross~zone. The 'ovule or ov'illes deveiop 'from this zone
(Fig. 163, nos. 5, 6) . .1n most.cascs the dorsal side of the carpel folds and
closes' over the sill. In a few gellera of the monocotyledons, e.g. plants
belonging to the Butomaceae, the. primordia of the carpels are horseshoe-
the stele at a lower' level than do the other traces. The continuation of this
median .trace constitutes the dorsal bundle of. the. carpel and the trace
itself is .termed the dorsal.trace. The dorsal bundle is actually homologous
to the, leaf mid-rib. The two outermost traces on each side are .termed .the
marginal or I'enfrai traces, :as the bundles' that pass along the margins
bf the carpel develop from them. If the carpels are fused to form a syn-
carpous gynoecium these marginal bundles are found lateral to the dorsal
hundle and if the carpels are folded inwards they are founo ventrally
'-...
relative to the dorsal bundle (Fig. 169, nos. I, 2), As a result of the in-
ward-folding ,of the carpel margins the. yentral bundles, .ar,t; inverted-
their phloem i"s directed towards the,cavity of the ovary, and thei(xylem
outwards. If more than three traces are present in a carpel the additional
traces are present between the ventral and dorsal traces, and they are
FIG. 167. Aquilegia caerulea. 1, C ross-section in the receptacle at the level of the
exi t of the traces to the numerous who rls of stamens; Within the receptacle the
stamina! traces are seen to be arranged in radial rows. X 35. 2, Cross-section in
the basal portio n of a sing le carpel in which the single do rsal trace and two ven-
tral traces can be d istinguished. I n the ventral traces (but not in the' branches of
them that appear close to them in the micrograph) it can be seen that the xylem is
directed o utwards and the phloem inwards. X 110.
370 , Plant Anatomy
termed lateral traces, The vascular bundles of the carpels may branch and
may even continue to do so during the development of the fruit,
The vascular bundles that supply the ovules usually :originate from the
ventral carpel bundles, or from branches of them ,that are present in the
placenta:(Fig, 169, no, I), The ovular bundle is single and thin, and reaches
the zone of the chalaLa;' it doe. not penetrate into the nucellm but in some
genera branches of it enter into the integuments,
The structure of the receptacular stele and the, system of vascular bundles
within the floral organs is relatively complex even in the simpler types
of flowers (i.e. those that are phylogenetically relatively primitive). The
interpretation of floral vascularization becomes even more difficult in
flowers in which fusion of the traces has taken place during the process of
evolution. Eames (Eames, 1931; Eames and MacDaniels, 1947) gives
Aquilegia of the Ranunculaceae and Pyrola of the Ericaceae as examples
of flowers with simple vascular systems. In species of Aquilegia the pedicel
contains five thick bundles which alternate with five thin bundles (Fig.
165, no. 2). These bundles fuse at the base of the flower to form an un-
interrupted ring. Above tliis lever five groups or; sepal traces, each group
consisting of three traces per single gap, depart from the stele (Fig. 166,
no. I). A little higher, alternating with the sepal traces, a single trace passes
out to each of the five petals (Fig. 166, no'/2). Above the, petal gaps the
traces to the numerous stamens are fou~d. Each stamen has a single
trace (Fig. 167, no. I). Above the uppermost whorl of stamens the stele
once again becomes an uninterrupted ring. Shortly above this level five
/
compound gaps of the carpels are formed. From the base of each such
gap, the dorsal trace is given off and, from the sides, the pair of ventral
traces (Fig. 170; Fig. 171, no. 2). The ventral traces invert immediately
on their exit from the stele and so enter the carpel wit" the xylem directed
outwards (Fig' 167, no.-2j. Ahovethis levelthe stele consists of fivebundles
which mainly consist of phloem. This vasculat tissue gradually fades
towards the rounded tip of the receptacle. .,
The type of vascular 'system that is exemplified by Pyrola (Fig. 171,
no. I) differs principally from the type of Aquilegia in the following fea,
tures: (a) each sepal has only one trace; (b) above that'level at wh'ich the,
dorsal carpel traces depart from the stele the remaining stelar bundles
fuse in pairs to form five strands, each of which represents the two ventral
traces; (c) no vestigial vascular tissue is found at the top of the receptacle.
/
VARIATIONS IN THE VASCULARIZATION OF THE FLOWE~
that of the inner tissues, and that the fusion of the vascular tissue repre-
sents the last stage in this process, Fusion of vascular bundles involves
those bundles that were close to one another: The fusion may involve
the traces alone or may include part ·of the bundles within the organs,
and not very often does it continue throughout the entire length of the
bundles. Usually there is ontogenetic and histological evidence that
fusion has taken place. Evidence of aborted organs is assumed from the
·presence of rtdimentary organs and persisting traces as 'compared with
related flowers.
Bundle fusion
Staminal
bundle
FIG. 169. Diagrams illustrating the carpellary vascular ~upply. 1, A carpel sup-
.plied by three traces each of which is accompanied by a separate gap; bundles re-
'main unfused within th-e carpel to the stigma. 2, A carpel in which the two ventral
~ rraces are fused from the base of the carpel. 3, A carpel in which the ventral traces
arise fused, but in which they separate in the basal portion of the carpel. (Adapted
from Eames and MacDaniels, 1947,)
(Fig. 172, nos. 16-18). On the other hand, if the carpels become fused
while they are still.open and so form a single common locule with parietal
placentation, the ventral bundles are not inverted and they occur in pairs
along those lines where the cohesion of the carpel margins takes place"
or they may fuse to form common bundles (Fig. 172, no. 11).
Ventral
carpellary
troce
Dorsal
'"' 13 '"' 14 e 15
&?J ~00 17 18
376 Plant Anatomy
the fusion of the staminal bundles with those of the petals (epipetalous
stamens) may be seen, for example, in the Crassulaceae (Fig, 173, no. 7).
In flowers of plants belonging to the Rosaceae (Fig, 173, 'nos. 3-6), it
is possible to discern the fusion of bundles from }nore tban. two whorls
(Jackson, 1934),
FIG; 172. 1-5" Stages -in the fusion of lateral bundles,lin gamosepalous calyces,
I, Calyx of Nepelo veronica cut and spread out; the hiteral bundles are unfused.
2, CaLyx of Afl/ga replans in which the lateraL bundles are fused. 3: ,Diagram of
the cross-sectio-n of the calyx of Monarda didyma in which the lateral bundles are
free. 4, Cross-section of the calyx of Physosregia virginiaI/O in which the'lateral
bundles are fused. 5, Cross-section of the calyx of Salvia patens in which two pairs
of lateral traces are fused. 6-10, Stages in the fusion of lateratbundles in gamo-
petalous corollas. 6, Hamelia patens, in which the lateral traces are unfused. 7,
Senecio [remonai. 8, Anaslrophia ilid/olia. 9, Chrysanthemum leucdnthemum. 10,
XanthiunT orientale. 11-18, Diagrams of cross-sections of carpels showing differ-
ent stages of cohesion. 11, Reseda odorafa in which the placentation is parietal.
12-15, Different stages in the fusion of the ventral bundles in follicles. 16-18,
Different stages in the phylogenetic develcpment of the syncarpous ovary from
the stage where the tissues and vascular bundles of each carpel are unfused
through the stage where the carpeJlary tissues but not the vascular tissues are
fused to the final stage where the carpellary tissues and lateral and ventral bundles
are fused. In nos. }1-18 the xylem is represented by solid black and the phloem is
white. (Nos. I~5 and 11-18, adapted from Eames and MacDanie1s, 1947'; nos.
6-10, adapted from Koch, 1930.)
The Flower 377
F.
378 Plant Anatomy
wall of the inferior ovary (Manning, 1940; Douglas, 1944; Eames and
MacDaniels, 'j 947; Eames, 1961). 'In relation ·to· these plants, therefore,
it is possible to conclude from the vascular anatomy that the wall of the
inferior ovary consists of. the ovary wall proper together with the other
floral organs that are fused with it.
In species belonging to the Cactaceae and Santalaceae (Fig. 174, no. I),
inverted vascular bundles, i.e. those with inwardly directed phloem and
outwardly directed xylem, can be found along the entire length of the
inferior ovary wall (Smith and Smith, 1942; Tiagi, 1955). This feattire
suggests that in these species the inferior ovary has developed as a resuIt
of the involution of the receptacle, as the sinking of the gynoecium into
the receptacle necessitates an inward folding of the upper portion of the
receptacular stele. In Rosa, Jackson (1934) found, according'to the vas-
cular anatomy, tbat the lower portion of lbe Jlesby fflli! consjs!s Df reup-
tacular tissue, while the. upper portion 01' it consists of fused floral. organs
(Fig. 174, nos. 2-4). Jackson also showed that various intermediate stages,
which demonstrate the evolution of the rose fruit, can· be seen in related
genera: The fruit of Pyrus malus vaL paradisiaca and related genera
apparently consists mainly of true ovary wall and of other floral organs
that are fused to it, and the receptacle consdtutes a very small portion,
which is not easily discernible, at the base' of the fruit (Fig. 211, no. 6)
(MacDaniels, 1940). The adnation o('the bundles in an inferior ovary
that is developed from the fusion of the ovary wall prope'j to other floral
organs is not equal in different radii, neither in the number of traces in-
volved nor to the extent to which they are fused.
0
0
Q Q a Q
0 00 •
•
0
a
a 0
CJ •
0
Q " 0
0
v 0
0 o ,
0 0 a~ ,........_ 0
0 ~
0 0
0
0 , '" C
0 " o 0 0 0
0
0
0 0 0 0
~ U
~ 3 4
Tile stamen
PHYLOGENY
the tel orne theory, that is, from primitive dichotomously branched axes.
According to this theory the stamen. has developed as the result of these-.
duction and fusion of a system oraxes that bore spor~ngia at their tips
(Wilson 1937, 1942). New light was thrown on the phylogeny of stamens
and carpels by Bailey and his co-workers from their morphologicaland
comparative anatomical research in many families of the Ranales. From
these investigations it has become apparent that in the woody species of
the Ranales that exist today not only primitive stages of xylem develop-
ment have been preserved, but also primitive types of stamens and carpels
(Bailey and Smith, 1942; Bailey and Nast, 1943a, b; Bailey, Nast and
Smith, 1943; Bailey and Swamy, 1949, 1951; Canright, 1952; Bailey, 1956).
A very primitive type of stamen is found in the genus Degeneria; here no
filament, anther or connective can be distinguished as the stamen is broad
and leaf-like, and has three vascular bundles. The four pollen sacs (the
microsporangia) are deeply sunk,jnto the abaxial side of the stamenJFig.
176, nos. I, 2). The pollen sacs are found between the lateral and median
bundles. Similar stainens are found in other'ranalian genera, e.g. Austro-
baileya, Himantandra and certain genera of th'e Magnoliaceae. Tn the
Magnoliaceae intermediate stages may be found from broad stamens with
three bundles and laminal pollen sacs, i.e/pollen sacs distant from the
margins, as jn Degeneria, to stamens with marginal pollen sacs and dis-
tinct filaments and anthers (Canright, 1952).
from which the wall of the ponen sacs and a large portion of the tapetum
develop, as a.result.of peric1inal and anticlinal cell'di~ision: The tapetum
apparently serves for the nourishment of the developing- pollen mother
cells and microspores (pollen grains). The outermost layer of parietal cells
is located immediately below the epidermis of the anther. Prior to the
liberation of the pollen grains several wall thickenings develop in each of
the cell s of this layer. H owever, no thickening is developed in the outer
"" ,
PrOfoderm
wall closest to the epidermis. Each thickening is V-shaped with the gap
directed. towards .the.epidermis (Fig .. 175, no. 2). This cell layer. is usually
termed the endothecium and the opening of the pollen sacs is brought about
by this layer. The opening mechanism is described as' follows. During the
dehydration of the anther the endothecitim loses water. As the water con-
tent of these cells'decreases the walls of each cell are drawn toward its
centre as a result of the cohesion forces. between the water molecules and
the adhesion forces between the water and the cell walls. Because of the
FIG. 177. Types of anther dehiscence differing from the ordinary. 1, Anther of So~
lanum villosum showing the apical pores. 2 and 3, Anther of Laurus nobilis show-
ing differe~t stages in dehiscence by lateral valves.
anther only (Fig. 177, no. 1). Openings may also develop on the sides of
the anther as, for example, in the Lauraeeae (Fig. 177,(n05. 2, 3).
The cell layer or layers below the endothecium both stretch and become
compressed during the development of the pollen-sac and in many plants
they are obliterated so that it is difficult to distinguish them in mature an-
thers immediately prior to dehiscence (Fig. 175, no. 2).
The formation of the tapetum takes place as a result of gradual differen-
tiation in the anther wall. In those cases where additional tapetal layers
@®w~~/ 2
FIG. 178. 1, Diagram of portion of across~section of the poilen sac of Symphoricar-
pus racemOSJiS showing an amoeboid tapetum. 2, Different types of arrangement
of the pollen grains in the tetrad. From left to right -tetrahedral, isobilateral.
decussate, T-shaped,
. ,
linear. (Adapted from Maheshwari, 1950.)
develop, they arise from cell division in other cells of the anther, and
especially of those on the inner side of the sporogenous
. tissue (Fig. 176,
\.
nos. 7, 8). In some cases the sporogenous tissue itself may take part III
tapetum formation (Eames, 1961). The tapetal celkare distinctly enlarged,
rich in protoplasm and they may be multinucleate or polyploid. Various
irregular mitotic divisions and nuclear fusion take place in these cells
(Maheshwari, 1950). Two types of tapetum are distinguished-glandular
or secretory tapetum (Fig. 168, no. 2) in which the cells remain' in their
original position where they later disintegrate and their contents are ab-
sorbed by the pollen mother cells and the developing pollen grains: and
amoeboid tapetum (Fig. 178, no. I) in which the protoplasts of the tapetal
cells penetrate between the pollen mother cells and the developing pollen
grains where they fuse among themselves to form a tapetal periplasmodium.
The Flower 387
tain species, fatty substances which are apparently absorbed from the
tapetum. In many plants the starch disappears from thefollen grains dur-
ing the ripening of the anther, while in others the starch disintegrates only
in the pollen tube. It is assumed that there is a connection between the
disintegration of the starch and the high osmotic pressure of · the pollen
FIG. 179. 1-9, Different types of wall formation in pollen mother cells. 1-5,
Successive type in Z ea. 6-9, Simultaneous type in M e/ilotus alba. 10, Diagram of
the paired pollinia of Asclepias which are joined by the adhesive body. I I, Dia-
gram of a group of massulae from a pollinium of a plant belonging to the Orchida·
ceae showing the connecting viscin threads. (Nos. 1- 9, adapted from Foster and
Gifford, 1959; nos. 10 and 11, adapted from Schoenichen, 1922.)
The Flower 389
tubes which is higher than that of the cells of the style through which the
tube passes.
The chemical analysis of mature pollen grains shows the following
composition (McLean and Ivimey-Cook. 1956):
proteins 7,0-26·0 % ash 0·9-5·4 %
carbohydrates 24,0-48·0 % water 7·0-16·0%
fats 0'9-14·5%
Mature pollen grains usually have two walls-the outer wall or exine,
and the inner wall or intine. The intine participates in the formation of
the pollen tube. The exine at first appears within the special wall as a thin
membrane. With further development this membrane thickens discernibly,
and two layers become distinguishable in it; the outer of these two layers
is termed the sexine and the inner; the nexine (Erdtman, 1952). The sexine
is thin and has a high refractive index and, therefore, it is not easily seen.
The surfaceof the sexine is at first smooth but later, after the formation
of the nexine, many types of projections may develop on it. In the aper-
:ryres,-'regions of characteristic shape from which the pollen tubes emerge,
JIi·~i'le.may be completely absent or it may be represented by the nexine
only (Erdtman, 1952). The nexine is relatively thick and is impregnated
with cutins. The specific cutin present in the exine has been termed
sporopoUenin (Frey'Wyssling, 1959). This substance is more stable than
other cutins and suberin. 'lIt is also found in the walls of fungal spores.
The preservation of pollen grains in peat and in older deposits is apparently
due to the presence of sporopollenin.
The intine is not of constant thickness on the circumference of the
pollen grain and it is always thicker at the apertures. The inner part of
the intine apparently contains cellulose and the outer part, pectin. Callose
apparently also occurs in the intine at the apertures. The intine readily
absorbs water, as a result of which it swells greatly, especially at the aper-
tures. Because of this, the nexine, if present in the apertures, is ruptured
and the intine emerges.
With the development of the pollen grain walls, the mother cell walls
and special walls are destroyed and dissolved. The substances of these
walls mix with those of the tapetum, and the young grains thus become
~uspended in a colloidal liquid from which they absorb nutrients. Accord-
Ing to Strasburger (1889) the entire exine is secreted from the protoplast
of the pollen grain, while according to some workers (Mezzetti-Bambacioni,
1941) it is possible that the tapetum participates in the formation of the
exme.
Numerous characteristic projections and sculpturings develop on the
outer ,urface of the pollen grain. Only in a few plants, such as species
of the Gramineae, are the poJlen grains smooth. In most cases the sexine
constitutes a layer of drumstick-shaped rods, which are termed pila. In
390 Plant Anatomy
many plants the heads ofthese rods fuse to form the tegil/um (a roof-like
layer) which may also be formed by a membrane covering the heads
of the ,rods. The tegillum is generally perforated by Iriinute pores, and
spines, warts or other structures may deyelop externally on it. The forma·
tion of air sacs is brought about by the separation of the pila from th,
2 3 4
5 6 7
-~ r \ 8,
(V\ /
8 IOQ f
,/6
FIG. 180. 1-8, Sculpturing on pollen grains. -},-&;ttered pita. 2-4, Adherent pila
arranged in ch<lracteristic reticulate patterns or in rows, 5, Cross~section of a pol-
Jen-grain wall with a thick sexine. 6, As in no. 5, but with thin sexine. 7, Cross-
section of pollen-grain wall on which the heads of the pila are fused to form a
tegillum. 8, Different types of pila fusion, and the resulting types of structure.
On the left, disappearance of the pila bases; on the right, development of spines,
etc. 9-16, Arrangement and types of apertures. 9 and 10, Unisulcate pollen grain.
9, Lateral view. 10, Polar view. 11 and 12, Tricolpate pollen grain. 11, Lateral
view. 12, Polar view. 13, Rugate pollen grain. 14, Porate pol1en grain. 15 and 16,
Pollen grain with three pores. 15, Lateral view. 16, Polar view. (Nos. 1-8, adapted
from Erdtman, 1952.) - ,
The Flower 391
FIG. 181. Micrograp5s of pollen grains. 1, Pollen grain of Sonchus oleraceus show-
ing the well developed ribs and spines on the sexine. x 940. 2, Po llen grain of
Lavatera cretica in which numerous pori can be distinguished. x 405. 3, Polar
view of a pollen grain of S~necio joppensis with three apertu res. X 810. 4, Arbutus
andrachne, tetrad of pollen grains which forms a single dispersal unit. x 770.
The FlolI'er 393
FIG. 182. Micrographs of pollen grains. 1, Acacia, a single disper~l unit consist-
ing or many pollen grains. X 840. 2, Single pollen grain of Nuphar. x 860. 3,
Single pollen grain of Hordeum spontaneul1l showin g the single ape rture. X 1150.
4, Th~ee pollen grains of Oenothera drummondii ; each grain has three apert ures.
x 210.
394 Plant Anatomy
A circular porus, in which the two layers of the exine are absent, may
sometimes be present in the middle of the elongated, nexine-covered
furrow (e.g. in, Centaurea and other genera of the Compo sitae).
In Zostera, which is marine, the pollen grains are thread-like. This
feature is apparently connected with the hydrophilous mode of pollination.
Pollen grains of most plants that are of typically wirid-pollinated familie's
are smooth and dry. If most of the genera of a familY,are insect pollinated
and. only c~riain genera of it. are wind pollinated, as, for example the
genera Artemisia and Ambrosia of the Compo sitae, the wind-pollinated
genera retain the sculptured structure typical. of the entire family, but it
may be developed to a lesser extent. /
The size of pollen grains also varies/very greatly; Erdtman classifies
them, according to size, into the following ,groups: perminuta, in_ which
the diameter is less than 10 1'; minuta, in which the diameter is 10-25 1';
media, 25-50 1'; magna, 50-100 f'; permagna, 100--200/1'; giganta, the
diameter of which is greater than 200 1'. Very small grains may be found
in Myosotis alpestris (2·5-3·51') and Echium vulgare (10-141'); very large
grains_occur)n Cucurbita pepo,.(230 I') and Mirabilis jalapa (250 it).
I
The carpel
FIG. J 83. 1-3, CondupJicate carpel of Drimys piperita. I, Lateral view showing the
stigmatic margins. 2, Cross-section showing the position of the ovule3 and the
penetration of a pollen tube. 3, Cleared, unfolded carpel showing the general pat-
tern of the vascularization and the source of the ovular supply. 4-7. Diagram-
matic representation of the development of the present-day carpel from the condu-
plicate carpel. 8- 10, Diagrams of cross-sections of syncarpous gynoecia showing
the different ways in which fusion may take place. 8, Lateral cohesion in a whorl
of open conduplicate carpels. 9, Adnation of the free margins of the conduplicate
carpel to the torus. 10, Cohesion of ventral surfaces of the carpels. (Adapted from
R-:.ilF1o'\/ ~n~ <::l1r:JITHI 1 Q.c;: l ...
The Flower 397
At the time of anthesis, i,e, the maturation of the anthers and ovules,
or prior to it, only slight histological differentiation is observable in the
ovary wall. It then consists mainly of parenchyma and 'vascular tissues
and is covered by a cuticle-bearing epider';'is, A~ the ovary develops into
a fruit, striking histological changes take place in the ovary wall (see
Chapter 2 0 ) , ' /
The stigma and style have special structures and.physiological character-
istics that enable the pollen grains to germinate 9n the stigma and the
pollen tube to penetrate to the ovules, The protoderm of the stignia differ-
entiates into a glandular epidermis, the cells of which are rich in proto-
plasm, This epidermis usually is papillate and covered with a cuticle
(Schnarf, 1928), and it secretes a sugar-containing solution. Sometimes
other cell layers below the' epidermis form a glandular tissue which func-
tions similarly to ·the epidermis. In many' plants (e:g, Phaseolus, Lilium,
Papaver and Lupinus) the epidermal cells of the stigma develop into
short dense hairs (Fig, 184; no,.2; Fig,J85, nos. 1,2), or they·may develop
into long, branched hairs, e,g, the Oramineae and other wind-pollinated
plants (Fig, 184, no. I). "".'
Between the tissue of the stigma andhhe ovary, there is a specialized
tissue through which the germinating pollen'tube penefrates. This tissue'
provides a nutrient substrate which aids tlie pollen tub" to/grow through
the style into the ovary. This tissue was-termed transmitting tissue by
Arber (1937) and this is the term that will be used here; however, this
tissue is also known by various other terms. As has already been mentioned,
in the most primitive dicotyledons (e,g, in ranalian Jamilies such as the
Winteraceae and Degeneriaceae) the carpels do not develop styles and
the pollen tubes reach the ovules by growing through the hairs present
on the unfused margins of the carpels, In phylogenetically more advanced
forms the carpel margins fuse, a style is developed and the stigmatic tissue
is reduced to the upper portion of the style only, This tissue however,
The Flower 399
several canals, the number of which is equal to that of the carpels. The
canals are lined entirely or in longitudinal strips by gland ular transmitting
tissue, which may be papillose (Fig. 186, nos. 1, 2). In the canal the cells
of tra nsmitting tissue are covered by cuticle. In ma ny plants (e.g. Cucur-
hila and Datura) this tissue is several cell layers thick. The transmitting
400 Plant Anatomy
tissue also covers the placenta and in certain species it is even present on
the funiculus. In some plants the transmitting tissue i~ brought.closer to
the micr~pyl~ by the development of outgrowths of the placenta or stylar
canal; these outgrowths .h~ve been termed obturators (Schnarf, 1928).
F IG . 185. 1, Diag ram of a lon gitudinal sect ion of the stigma of Papaver rhoeas in
which the germinatio n of pollen grains amon g the un icellular hairs on the surface
of the stigma can be seen. 2, Tip of the style o f L upinus {uleus showin g pollen
grains amo ng the hairs on the st igma. (Adapted fr om Schoenichen ; 1922.)
The Flower 401
Ontogenetic research on the styles of Cucurb"ita and Daiura has shown that
the multiseriate transmitting tissue and the multiseriate glandular· tissue
of the stigma develop from the epidermal cells by peric1inal division
(Kirkwood, 1906; Satina, i 944). Tn most angiosperms. the style. is· solid
(Fig. 187, nos. 1, 2), a nd then the transmitti ng tiss ue constitutes strands of
elongated cells rich in cytopI.lsm . The middle lamellae o f these cells swell
to produce a mu cilaginous substance in the intercellular spaces through
which the pollen tubes grow. In a syncarpous gynoecium with a sin1?le,
402 Plant Anatomy
solid style several strands of transmitting tissue develop and these are
connected to the diiferent"placentae .of the oyary.- f _
Different opinions have been expressed as to the factors directing the
growth of the germinating pollen tube. There are investigators who suggest
that a chemotactic attraction exists between the pollen tube and the tissues
of the stigma and ovule. According to other workers the very structure
and arrangement of the transmitting tissue in the style direct the growth
of the pollen tube (Schnarf, 1928 ; Renner and Preuss-Herzog, 1943).
In hollow styles the tubes of the germinating pollen grains grow between
the papillae of the transmitting tissue, ;tnd, if t)ley are absent,~ on the outer
surfaces of the epidermal cells. In many plants the cuticle on the trans-
mitting tissue disappears before pollination and the walls ofthe glandular
tissue soften and swell. The pollen tubes may sometimes penetrate deeper
into the transmitting tissue and grow between the cells. In solid styles
the pollen tubes grow between the cells of the transmitting tissue. In
grasses the pollen tube may even grow between the cells already in the
stigma. On the stigma of grasses there are large multicellular hairs, con-
sisting of several longitudinal rows of cells. The pollen tube penetrates
between the inner cell rows of these hairs and from there to the transmit-
ting tissue of the style. In the ovary the pollen tube penetrates via the trans-
mitting tissue, which lines the ovary wall and the placenta, and eventually
it reaches the ovule (Pope, 1946; Kiesselbach, 1949). Before the pollen
tubes penetrate the transmitting tissue the walls of its cells swell, so that
the tissue appears collenchymatous with mucilaginous walls and the con-
nections between the cells weaken. As a result of these changes it is easy
!9~macerate the transmitting tissue at this stage of development. The pol-
len tubes pass through the swollen, mucilaginous parts of the walls which
they-apparently digest (Schoch-Bodmer and Huber, 1947). Proofs have
also been given that pollen tubes contain enzymes that are capable of
disintegrating pectic substances (Paton, 1921). The protoplasts of the
transmitting tissue may also be utilized by the developing pollen tube,
but in many plants they may contract and die. As a result of this the style
does not increase-in width even when.it contains very many pollen tubes.
Apart from the. transmitting tissue and vascular bundles, the style
consists of thin-walled parenchyma and a typical cuticle-covered epidermis,
in which stomata~ may sometimes be found.
Tile ovule
The ovule consists of the nucellus which is surrounded by one or two
integuments, and it is attached to the placenta by a stalk, i.e. the funiculus.
At the free end of the ovule a small gap is left by the integuments; this
opening is termed the micropyle. The region where the integuments fuse
with the funiculus is termed the chalaza. A nucellar cell, usually one of
those below the outermost layer at the micropylar end, differentiates into
the macro- or megaspore mother cell. The nucellus is, therefore, considered
to be the megasporangium.
Ovules may be of different form. The following two main types may be
distinguished: (I) orthotropous or atropous in 'which the nucellar apex is
In a straight line with the funiculus and is continuous with it; (2) anatropous
OVUle in which the apex of the nucellus is directed backwards toward the
404 Plant Ana/omy
u'nicu\us
cells
nucleus
cell
integument
integument
; cells
5
4 ; I
12
/
\
base of the funiculus (Fig. 188, nos. 6, 7). Between these two extreme forms
there are. different.intermediate stages in which the ovule axisjs variously
bent (Fig. 188, nos. 8-10). A detailed terminology has been developed
for all these forms i.e. hemianatropous, campylotropous and amphitropous
(Schnarf, 1927; Maheshwari, 1950). In the Plumbaginaceae, Opuntia and
some other genera of the Cactaceae the funiculus is very long and it surrounds
,he ovule; this type of ovule is termed circinotropous (Fig. 188, no. II).
'Tlie ovules develop from the placentae of the ovary. The ovule pri-
mordium originates by periclinal division of cells below the surface layer
)f the placenta. At first the primordium appears as a conical projection
",ith a rounded tip. The first sporogenous cell is already distinguishable
n the primordial nucellus in that it is larger than the neighbouring cells
llld it has a larger nucleus and denser cytoplasm. The inner integument,
Nhich is sometimes also the only one, begins to develop some distance
from the nucellar apex. The initiation of this integument takes place by
periclinal divisions in the protoderm. At first the integument appears as
ill !lllDUlar ridge, which later grows toward the nucellar apex and so en-
,elops the nucellus, except for the micropyle left at the free end of the
:;;;-ule (Fig. 188, nos. 1-4). The initiation of the outer integument, if it is
)fesent, takes place in the protoderm a little lower than that of the inner
ntegument and it develops similiuly to 'the latter. In many plants the
Juter integument does not reach the micropyle. In anatropous and bent
ovules the growth of the integuments is asymmetric. In plants with sym-
petalous flowers the nucellus'is usually enveloped by a single integument,
while in more primitive dicotyledons and in many monocotyledons the
ovule has two integuments.
The nucellus is usually considered to be the megasporangium, but the
homology of the integuments is still an unsolved problem. At the chalaza
there is no differentiation between the tissues. of the integuments and the
funiculus:
In certain plants the structure of the ovules differs from that described
above. There are ovules that lack integuments and those in which the
number of integuments is greater than two. The nucellus may be fused
entirely to the integuments, In some ovules the integuments grow more
than usual and may even close the micropyle, while in others the integ-
uments do not reach the nucellar tip. In certain plants, e.g. species of
Asphodelus, a third integument develops from the base of the ovule; this
structure is termed aril (Fig. 188, no. 12). (See also Chapter 21.)
The thickness of the nucellus in a mature ovule differs in various plants.
h may be very thin-one to two cell layers surrounding the embryo sac-
Or it may consist of numerous cell layers. Also the integuments may vary
In thickness, and the thinnest may consist of the two epidermal layers
only. However, in such integuments the part closest to the micropyle may
be somewhat thicker.
Plant Anatomy
The entire surfaces of all the ovular parts are covered with cuticle.
Thus ·it" is possible to distinguish an outer cuticle whic!, covers the funi-
culusand the outer integument externally, a middle cuticle which is double
and" is present between the two ·integuments, and an inner cuticle which
is also double and is present between the inner integument and the" nu-
cellus.
During the development of the embryo sac the vegetative tissue of the
nucellus is completely or partly destroyed, and its content is absorbed
by the other parts of the ovule. In certain plants, e.g: the Centrospermae,
the nucellus may, in the seed, produce a nntritious tissue which is termed
perisperm. With the maturation of the ovule the histological structure of
the integuments alters. In many plants the inner epidermis of the inte-
gument d"velops into a nutritious layer which,is termed the· integumental
tapetum. This layer consists of tall, dark-staining cells. This feature is
characteristic of those families in which the 'nucellus is destroyed' early
so that the integument is brought into contact with the embryo sac. It
is a common feature in the Sympetalae.
/
MEGASPOROGENESIS
/'
There are plants in which several megaspore mother cells appear in a
single oyule, but usually only a single mother cell develops in each nucel-
lu,. Generally the sporogenous cell develops directly from a hypodermal
nucellar cell (Fig. 189, no. 1). This cell is distinguishable from the neigh-
bouring cells by its size, the size of its nucleus and the density of its cyto-
plasm. In certain phints indirect developmen't of the sporogenous cell has
been observed; the" hypodermal cell first divides into an outer parietal
cell, which is usually smaller, and a larger inn~r cell, which constitutes
the primary sporogenous ceil. The latter usually develops into the mega-
spore mother cell, and 'the parietal cell may divide in different planes to
form numerous parietal cells. As a result of these divisions the,megaspore
mother cell is pushed deep into the nucellus in many, plants (Fig. 189,
nos. 2-7; Fig. 190, no. 1). On the other hand, in ovules w~ere no parietal
cells develop the nucellus is thin (Fig. 19Q"no. ·3). The phyiogenetic signi-
ficance of the parietal cells is not clear, but it is thought that the trend,
during the development of the angiosperm ovule, has been towards their
loss.
The megaspore mother cell undergoes a meiotic division which is accom-
panied by the formation of a separate wall on each of the four 'megaspores.
The megaspores are arranged in one row, and generally the three closest
to the micropyle degenerate, and the remaining one enlarges.
The Flower 407
Parietal Megaspore
cel's 'mother cell Tetrad of
megaspores
6
FlG.·189. Development of the megaspore in Hydrilla verticillata. (Adapted from
Maheshwari, 1950.)
two male gametes before the opening of the anther (Fig. 191, nos. 7, 8).
In other plants it has been seen (Maheshwari, 1950) that the generative
cell divides only after it penetrates into the developing pollen tube (Fig.
191, nos. 9, 10).
In the past it was believed that the two male gametes move passively
in the cytoplasmic stream of the pollen tube, but recently the opinion has
been"advanced that they move independently because it was seen that their
egetative
O
cell
,
3 ,
I
'='.~~'
~
._c'" · .~''Ci-"-,-",'\$;'10','
Male gametes
movement is not identical with that of the cytoplasm. The opinion also
existed that the vegetative nucleus, which is also termed the pollen-tube nu-
cleus, controls, to some extent, the penetration of the pollen tube into the
ovary. However, this opinion has been questioned (Maheshwari, 1950). In
certain plants the vegetative nucleus begins to degenerate a short while
after its formation and, even when it is present for a long time, it does not
always precede the male gametes and it may be found behind them.
The inner lamella of the wall of the pollen tube consists of callose in
addition to cellulose (Tupy, 1959). The protoplast is present only in the
distal part of the tube and it becomes separated from the proximal part of
the tube by the formation of callose plugs, which are formed from time
to time by the protoplast and, as a result, it is possible to distinguish many
such olulls in a lonu nnllpn tnhp
410 Plant Anatomy
MEGASPOROGENESIS MEGAGAMETOGENESIS
TYPE MEGASPORE DIVISION DIVISION DiVISION DIVISION DIVISION· MATURE
MOTHER CELL I 1I m N V EMBRYO SAC
POLyGONUM 8 ®Cj 0 0 GJ
DENOTHERA 8 ffi (!) 0 0J
,LCI"" G@) (J 0 W
PEPERDMIA 8 (J C3 0
PEN'" 8000~
DRUSA 8'0 0 Q Q
F"T'LLARIA 0) 0 0 Q 0
PLUMaMELLA () 00Q
PLUMBAGO () 000
ADOXA 8000
FIG. 192. Diagram showing important types of embryo sacs and their develop-
ment in angiosperms. (Adapted from Maheshwari, 19S0.)
412 Plant Anaiomy
ratus and the fourth forms a single polar nucleus. Fertilization in this type
results in the formation of a diploid endosperm nucleus,and not a triploid
one as in the above type.
Allium type
/
TETRASPORIC E¥BRY9 SAC
Adoxa type
Fritillaria type
;
This type is found in many genera among which are Lilium and Fritil-
laria. Here three of the four nuclei obtained from the meiotic division
move to the chalazal pole of the young embryo sac, and the fourth is
found at the micropylar pole. The latter nucleus divides in the usual man-
ner, and the other three nuclei fuse to form a single triploid nucleus," which
immediately divides into two. As a result of this, a secolld four-nucleate
stage is obtained in which there are two haploid nuclei at the micropylar
pole and two triploid ones at the chalazal pole. Later a third and last di-
vision takes place, which gives rise to four haploid nuclei at the micro-
pylar pole and four triploid ones at the chalazal pole. The final arrange-
The Flower 413
ment in the mature embryo sac is a normal haploid egg apparatus, three
triploid antipodal cells, and a tetraploid secondary nucleus which results
from the fusion of~a haploid and a triploid polar nucleus,
Nectarics
Nectar, a sugar-containing solution, is secreted by nectaries which most
frequently occur on insect- and bird-pollinated plants. Nectaries may con-
sist of specialized tissue which differs histologically from the neighbouring
tissues, i.e. structural nee/aries, or they may consist of non-specialized
tissues, i.e. non-structural nectaries (Zimmermann, 1932; Frey-Wyssling
and Hiiusermann, 1960). Non-structural nectaries have been observed in
many plants and on various organs, e.g. on the leaves of Pteridium aquili-
num and Dracaena refleXa;!)ll the floral bracts of Sanse~ieria zeylanica; on
the sepals ofPaeonia albiflora; and on. the tepals of Caltleya percivaliana.
Struct~ral nectaries may form special outgrowths or they may occupy
._d_elimrteci-~egions of the surface layers of the various plant organs on which
~y~jire formed. Such nectaries usually consist of excretory epidermis Of
·trichomes·and a differentiated nectariferous tissue below it. At the nectari-
ferous tissue there is a well developed vascular supply (Frei, 1955; Frey-
Wyssling, 1955).-Sometimes no nectariferous tissue is developed and then
only a secretory epidermis is present. Which of the above two types of
nectaries is the more advadced is still to be clarified.
LOCATION OF NECTARIES
Nectaries may develop on all parts of the plant. Nectaries that are con-
nected with the floral organs are.termedfloral nectaries, and those developc-
ing on the vegetative parts of the plant, extrafloral nectaries. Extrafloral
nectaries may be found on different organs such as petioles (Passiflora)
stipules (Vicia faba), teeth of leaves (Ailanthus aitissima, Prunus and
Impatiens), and on the margins of the cyathia of Euphorbia. Extraftoral
nectaries are regarded, phylogenetically, as more primitive than floral
nectaries (Frey-Wyssling, 1933). This chapter will deal only with floral
nectaries.
Many workers used the form and location of the nectary as taxonomic
characteristics by which they attempted to support their theories regarding
the phylogenetic relationship between species, genera and families (Bon-
llIer, 1879; Schniewind-Thies, 1897; Knuth, 1898-1905; Porsch, 1913;
Daumann, 1928, 1930a, b, c, 1931a, b; Brown, 1938). It has been shown
(Fahn, 1953b) that a phylogenetic trend of development, expressed by the
acrocentripetal change of position of the nectaries within the flower, i.e.
from the sepals towards the ovary and uo to the stvl!'"_ f'yjo;:h: Thp rA'L... ",; ... ~
414 Plant Anatomy
STRUCTURE OF NECTARIES
Garidella unguicularis
The nectariferous tissue in this flower is found at the petal knee, i.e.
in that place where the blade passes into the claw (Fig. 194, nos. 1-4).
This knee forms a type of spur, the aperture of which is closed by a cover
connected to the claw of the petal (Fig. 194, nos. 2, 3). Within the aperture
of the spur, on the ,joe of t,oe pet~l-blsde' sad close to the caver margins,
are brushes of unicellular hairs, which thus block the spur aperture more
tightly. The tongue of the bee is PU'hed into the spur, which contains the
The Flower 417
FIG. 194. Nectaries. 1-4, Garidella unguicularis. I, An entire petal. 2, Median lon-
gitudinal section of a petal. 3, Knee portion, in longitudinal section, enlarged to
show the position of the nectariferous tissue. 4, Outer portion of nectariferous
tissue showing the thick cuticle on the epidermis. 5-7, Capparis sicuia. 5, General
aspect of the flower after the removal of the sepals, in which the nectar collects,
to show the triangular nectary. 6, Median longitudinal section of the receptacle
in the region of the nectary (stippled). 7, A portion of the epidermis over the nec-
tary showing the modified stomata through which the nectar is secreted. 8, Cistus
villosus. Flower from which the petals and stamens have been removed to expose
the nectary.
are covered by cuticle and form low papillae. The contents of the epidermal
cells do not differ from those of the other nectariferous cells, which are
granular and yellow in colour. The vascular bundles come into contact
with the nectariferous tissue (Fig. 194. no. 3).
418 Plant Anatomy
Capparis sicula
Colchicum ritchii
The nectariferous tissue is found on the Ifa'sal portion of the filaments
(Fig. 195, no. I), which are adnated to tn;; tepals. External1y the nectari-
lF 4
o
I
~ 6
FIG. 195. Nectaries. 1-3, Colchicum rjlchii.I, An entire tepal with adnated stamen.
Nectariferous tissue at the base of the filament, stippled. 2, Cross-section of the
filament in the region of the nectary. 3, Portion of the epidermis of the nectary in
surface view. 4-6, Citrus limon. 4, Median longitudinal section of an entire flower,
showing the position of the nectary. 5, Portion of the epidermis of the nectary.
6, Portion of a cross-section of the nectary showing a modified stoma, and canal
below it through which the nectar is secreted.
The Flower 419
ferous tissue appears as a yellow band surrounding' the base of the fila-
ment. A, swelling of the' petal is present on each side of the stamen; the
nectar accumulates between these swellings. In a cross-section of the
filament transparent epidermal cells overlying a thick region of rounded
cells with granular contents can be seen in the region of the nectary (Fig.
195, no. 2). The central portion of the section is occupied by vascular
bundles and parenchymatous tissue in which there are m~ny large inter-
cellular ·spaces. Small intercellular spaces are also preseut in the nectari- ,
ferous tissue. The nectariferous tissue on that side of the filament facing
the petal is much thicker aud its colour is darker (yellow-brown) than that
on the free side. There is no discernible development of the cuticle on the
epidermal cell walls. Large, circular stomata are present on the surface of
the nectary (Fig. 195, no. 3).
Citrus limon
=The nectary in Citrus forms a ring around the base of the ovary (Fig
'195, nos. ~6). Stomata with wide apertures are present on raised portions
'OCthe ,ring. In tangential section of the nectary the stomata are seen to be
strikingly rounded in shape (Fig. 195, no. 5), and in cross-section it is
seen that the substomatal chambers are fairly deep and that the cells
below the epidermis are small and compact. The epidermis itself is seen
to consist of small .cubi~al, thick-walled cells which are covered by a
relatively thin cuticle. All the cells of the nectary, including those of the
epidermis, have a granular colourless content. Sometimes these cells may
also contain crystals, as i's a common featureinall other tissues of Citrus.
Cistus villosus
Bauhinia purpurea
In this plant the nectariferous tissue lines a tubular cavity which has
been formed either by the depression of the receptacle on one side of the
gynophore, or by the fusion of the basal portions of the perianth and
420 Plant Anat(Jmy
stamens (Fig. 196, ·no. 2). In a cross-section of the receptacle (Fig. 196,
no. I) two whorls of vascular bundles can be 'seen as well as the large
central bundle of ' the gynophore. The epidermis lining the above cavity'
, \
FIG. 196. Nectaries. 1 and 2, Bauhinia purpurea. 1, Cross-section of the flower in
the region of the nectary. The vascular bundles of the various floral organs are
shown in the tissues around the nectary. 2, Median longitudinal section of the flow-
er showing the position of the tube-like nectary_ 3,'Diagram of a cross-section
of the ovary of Muscari racemosum. 4, Diagram of a cross-section of the ovary
of Asphodeline lutea. 5-7, Diagrams.-cf lo-ngitudinal sections of flowers. 5,
Muscari. 6, Allium. 7. Asphodeills. 8 and 9, Bupleurum subovatum. 8, Portion of a,
cross-section of the epidermis and underlying nectariferous tissue. 9, Surface view
of portion of the epidermis. /
Bupleurum subovatum
As in all the Umbelliferae, the nectariferous tissue in this plant is found
on the upper portion of the inferior ovary, i.e. on the stylopodium. In a
cross-section of the nectary (Fig. 196, no. 8) the epidermis is seen to be
transparent and consists of thick-walled cells covered by a thick ridged
cuticle (Fig. 196, no. 9). This epidermis contains very small, sunken sto-
mata. Below the epidermis there is a thick region of compact cells with
grariular contents. These nectariferous cells are strikingly different from
the neighbouring parenchyma cells below them.
NECTAR SECRETION
. . . I '
Sucrose, glucose and fructose have been found to be' among the most
common constituents of nectar. ,Apart from these, mucilages, proteins
and organic acids are also sometimes found in nectar. The acids are res-
ponsible for the low pH- found in the nectar of some plants (Beutler,
1930; Fahn, 1949). Zimmermann (1954) found transglucosidases in the
nectar of Impatiens. The concentration of nectar varies from 3 to 87.%.
It was found that the output of fresh nectar, and the amount of the dry
matter in it, produced by different s~cies in a 24 hr period varies consid~r
ably. The variation per flower may ~~ between 0·13 mg fresh nectar
and 0·10 mg dry matter, to 268 mg fre,\h nectar and 47 mg dry matter
(Fahn, 1948, 1949). In unisexual flowers there are striking differences in
the amount of nectar secreted by the male and female flowers. Among the
external factors that increase the amount of nectar secretion, temperature
and soil moisture should be mentioned; these f~ctors influence the general
physiological activity of the plant (Fahn, 1948" 1949). The sugar content
in the plant is the most important internal factbr that influences nectar
secretion (Helder, 1958).
From the work of some investigators (Frey-Wyssling and Agthe, 1950;
Agthe, 1951 ; Zimmermann, 1953), it became clear that tilere is a correlation
between the translocation of substances'i~ the phloem and the secretion
by the nectariferous tissue. The above workers attempt to explain the
mechanism of secretion on the basis of this relation~hip. According to
them the contents of the sieve elements is extruded through the nectari-
ferous tissue by the excess pressure in the sieve elements. The second pos-
sibility, that the nectar solution is secreted by an active.mechanism which
takes place in the nectariferous tissue, appeared less feasible to the above
workers, but today, in the light of the most recent research on sugar up-
take, it seems that,this second possibility is 'more correctCHelder, ,1958).
\
9
424' Plant Anatomy
As has already been described above, the pollen grain germinates and
produces a pollen tube on the stigma. The pollen tube, which carries within
it the two male gametes, passes through the style and reaches the ovule.
In most plants the pollen tube penetrates into 'the ovule via the micro-
pyle. In some plants the pollen tube penetrates through the chalazal region,
i.e. chalazogamy. This feature occurs, for example, in Casuarina and species
of Pistacia. After its entry into the ovule the pollen tube penetrates into
the embryo sac where it may pass between the synergids and the embryo-
sac wall or between the. egg cell and the synergids. Usually one of the
synergids is destroyed as a result of the penetration of the pollen tube.
Later the tip of the pollen tube ruptures and the two male gametes, some-
times together with remnants of the vegetative cell, enter into the cyto-
plasm of the embryo sac (the 'female gametophyte). One of the male
gametes fuses with the egg cell, and the second fuses with the two polar
nuclei or with the secondary nucleus if the latter.two have fused previously.
This process of fertilization is termed double fertilization. As a result of
the fusion of a male gamete with the egg cell~ zygote, which later develops
into the embryo, is formed and, as a result ofthe fusion of the second male
gamete with the poiar nuclei or with the secondary nucleus, the endosperm
is formed. /
FIG. 197. 1. Embryo sac of Musa errans which has a nuclear endosperm. 2-5,
Embryo sac of Eremurus himalaicus showing the development of a helobial
endosperm. 6-8. Development of the cellular endosperm in Villarsia reniformis
in which the first divisions are transverse. 9-11, Cellular endosperms in which
the first divisions are longitudinal. 9 and 10, Adoxa moschatellina. 11, Centran-
thus macrosiphon. (Adapted from Maheshwari. 1950.)
The Flower 425
polar nuclei or with the secondary nucleus. This division usually precedes
that of the zygote. The further development oCthe er\oosperm differs in
the various groups of plants, and the following main types can be distin-
guished according to the manner of development.
Nuclear endosperm
In this type the first divisions are not followed by wall formation, and
the nuclei usually take up a parietal position and a large vacuole forms in
the centre of the embryo sac. These nuclei may remain free in the cyto-
plasm of the embryo sac throughout the entire development, or later
walls may develop in at least certaiil parts of the embryo sac, as in Capsefla
bursa-pastoris. Sometimes a few nuclei may divide at a faster rate than
the others and so isolated groups or "nodules'! are formed, These "no-
dules" become surrounded by a distinct cytoplasmic membrane (Fig. 190,
~no: 4; Fii-i')7, no. 1). In this type, as in the following types, there
:"are-many variations which are discussed in detail by Maheshwari (\950).
~-
Cellularendosperm
!
In this type the first division of the endosperm nucleus is accompanied
by the formation of a wall which is usually horizontal, but which may some-
times be longitudinal or diagonal (Fig. 197, nos. 6-'-11). The planes of the
following divisions may be parallel to that of the first division, but a short
while afterwards walls develop in different planes, so that the mature en-
dosperm consists of a tissue the cells of which are orientated in different
directions.
In certain plants (e.g. Thesium Impatiens, Acanthus, Lobelia and Lobu-
laria) haustoria of peculiar structure develop at one or both poles of the
endosperm (Fig. 198, nos. 1,2). These haustoria may penetrate deep into
the neighbouring tissues of the ovule and from these tissues they transfer
nutrients to the developing endosperm. In certain plants secondary haus-
toria develop laterally from endospermal cells close to the micropyle or
chalaza.
Helohial endosperm
FIG.' 198. 1, Upper portion of the ovule of Impatiens roylei showing the hypha~
like branches of the haustorium which penetrate into the tissue of the funiculus.
2" Longitudinal section of the ovule of Lobelia' amoena showing the cellular endo-
sperm, and rnkropylar and chalazal haus-toria. (Adapted from Mab'eshwari,
1950.)
,
y
,/
the micropylar chamber where the resulting nuclei remain rree, while in
the chalazal chamber the nucleus does not divide or undergoes only a few
divisions. In the course of further development the amount of cytoplasm
in the chalazal chamber is reduced and the nuclei begin to disintegrate.
Simultaneously, in many species, cell walls may appear in the micropylar
chamber.
No doubt intermediate forms exist between the three above-mentioned
endosperm types. It is as yet not clear whether the course of phylogenetic
de'lel'lllment has been fr'lm the nuclear t'l the cellular tYlle, or 'lice '1ersa
(Maheshwari, 1950).
The Flower 427
After its formation, the zygote commonly enters a dormant stage for a
certain.period. At the same time the large vacuole which was still present
in the egg cell disappears and the cytoplasm becomes more homogeneous.
Usually the zygote begins to divide after the division of the endosperm
nucleus. There are plants, e.g. Oryza and Crepis, in which the zygote be-
gins to divide a few hours after fertilization, while in others the first divi-
'sion takes place only much later. In Pistacia vera, for example, the first
division takes place about twa months after fertilization.
The plane of the first division o(the zygote is almost always transverse.
As a result of this division two cells are obtained; that closer to the micro-
pyle is termed the basal cell, and the other, the terminal cell. During the
course of further development the terminal cell may divide transversely
or longitudinally. The basal cell usually divides transversely. However, in
certain genera this cell does'not divide but enlarges to form a large, sac-
like cell. (Wardlaw, 1955.)
~Dic7ityledonous embryo
(b) The basal and terminal cells participate in the formation of the em-
bryo. . . asterad type (Fig. 200, nos. 1-6).
12 13 14
The Flower 429
FIG. 200.1-6, Development of the embryo of Geum urbanum (asterad type). 7-14,
Development of the embryo of Nicotiana (solanad type). 15-21, Development of
the embryo of Muscari comosum. 22-30, Development of the embryo of Poa
annua, Explanations in text. (Adapted from Maheshwari, 1950.)
G
,b
'0
'-.
8
f
®
,
7 8 9
·10 II
13 14
16 17 18 19 20 21
/
Q'b @rdjij n'
~" ~ ~ m t--C:__..--<
22 23
24 25
7n
The Flower 431
199, noS. 19, 20). During still later stages of development the cotyledons
bend. to conform to the shape of.the embryo sac (Fig. 199, no .. 21).
In some plants, e.g. Sedum acre, the sac-like suspensor cell closest to the
micropyle may develop a branched haustorium.
Monocotyledonous embryo
5
FIG. 201. 1-2, Different types ofsuspensors in the Papilionaceae. 1, Orobus angusti-
Jolius in which the suspensor consists of large multinucleate cells. 2, Cleer arie-
tinum in which the suspensor consists of two rows of relatively large uninuc1eate
cells. 3, Asperula showing young embryo with suspensor haustoria. 4-6, Develop-
ment of adventitious embryos in Poncirus tn/oliata. 4, Micropylar portion of the
embryo sac showing a fertilized egg cell, pollen tube, endosperm nuclei, and
certain nucellar ceIls (stippled) which are enlarged, rich in cytoplasm and which
have large nuclei. 5, As in no. 4, but at a later stage of development. ,6, Still later
stage, showing numerous embryos in the endosperm. Only the embryo that devel-
oped from the zygote has a suspensor. (Adapted from Maheshwari, 1950.)
432 Plant Anatomy
In most plants the function of the suspensor is only to push the embryo
into the endosperm, but in some plants it may develop into a large'haus-
torium which penetrates between the cells of the endosperm, and, to a
certain extent, even between the cells of the tissue surrounding 'the endo-
sperm. In many genera of the Papilionaceae the suspensor is of the latter
type (Guignard, 1882). In Pisum and Orobus for example, the suspensor con-
sists of two pairs of multinucleate cells; the cells of the pair closer to the
micropyle are large and very elongated, and those of the second pair are
almost spherical (Fig. 201, no. 1). In Cicer the suspensor consists of two
rows of uninucleate cells (Fig. 201, no. 2). In.the Rubiaceae (Lloyd, 1902;
Sou"ges, 1925) the suspensor at first develops 'as a multicellular thread,
and later the cells closest to the micropyle develop lateral projections. These
projections penetrate into the endosperm and their tips swell (Fig.
201, no. 3). /'.
APOMIXIS
I
Apomixis is a process of asexual reproduction in which. no nuclear fu-
sion takes place and which occurs in place of sexual repr~duction. Mahesh-
wari (1950) distinguished four types of apomixis.
I. Non-recurrent apomixis. The megaspore mother cell undergoes the
regular meiotic division to form the haploid embryo sac. The new embryo
developsfrom.the egg cell, i.e. hap/oidparthenog~nesis, or from any' other
cell of the embryo sac, i.e. hap/Did apogamy. As piants thus formed con-
tain only one set of chromosomes they are usually sterile and 'so the pro-
cess is not repeated in the next generation. /' .
2. Recurrent apomixis. The embryo sac may develop from a sporogenous\
cell-:- generative apospory- or from other ?elk oVthe /~ucellus -so-
matic apospory. In·thls case all the cells are diplOid and,the embryo may
develop from the egg cell (diplOid parthenog;;esis) or froin any other cell of
the embryo sac (diploid apogamy).
3. Adventive embryony. The embryo does not develop from the cells of
the embryo sac but from a cell of the nucellus or the integuments. This type
of development is also known as sporophytic budding.
4. Vegetative reproduction. In this type of development the flowers, or
parts of them, are replaced by bulbi!s or other vegetative propagules,
which may germinate while still on the plant.
The Flower 435
POLYEMBRYONY
FAHN, A. 1952. On the structure of floral nectaries. Bot. Gaz. 113: 464-470:
FAHN, A. 1953a. The origin of the 'banana inflorescence. Kew Bull.: 299-306.
FAHN, A. 1953b. The topography of the nectary in the flower and its phyIogeneticaI
trend. Phytomorpho[ogy 3: 424-426.
FAHN, A., STOLER, S. and FIRST, T. 1963. The histology of the vegetative and repro~
ductive shoot apex of the Dwarf Cavendish banana. Bot. Gaz. 124: 246-250.
FOSTER, A. S. and GIFFORD, ~E. M. Jr. 1959. Comparative MorphOlogy of Vascular
Plants. W. H. Freeman, San Francisco. ,
FREI, E. 1955. Die Innervierung der fioralen Nektarien dikotyler Pflanzenfamilien.
Ber. Schweiz. Bot. Ges. 65: 60-114.
FREY-WYSSLING, A. 1933. Ober die physiologiscbe Bedeutung der extraftoralen Nek-
taden von Hevea brasiliensis Muell. Ber. Schweiz. BOl. Ges. 42: 109-122.
FREY-WYSSLING, A.1955. The phloem supply to the nectaries. ActaBot. Neerl. 4: 358-369.
FREY-WYSSLING, A. 1959. Die pjianzliche Zellwand. Springer-Verlag. Berlin.
FREY-WYSSLING, A. and AOTHE, C. 1950. Nektar ist ausgeschiedener Phloemsaft. Verh.
Schweiz. Naturforsch. Ges. 4: 175-176.
FREY-WYSSLINn, A. and HXUSERMANN, E. 1960. Deutung der gestaltlosen Nektarien.
Be;. Schweiz.- BOl. Ges. 70: 151-162.
GOEBEL, K. 1928-33. Organogr~phi~ der Pflanzen, Vols. 1-3. G. Fischer, Jena.
GUIGNARD, L. 1882. Recherches anatomiques et physiologiques sur de legumineuses.
Diss. Paris.
HANSTEIN, J. 1870. Entwickhingsgeschichte der Keime der Monokotylc und Dikotyle.
Bot. Abhandl. Bonn 1: 1-112. _ L
HAUPT, A._ W. 1953. Piant Morphology. McGraw-Hill, New York.
HELDER, R". - J. 1958. The excretion of carbohydrates (nectaries). In: Han-dbtlck del'
Pjlanzenphysioiogie 4: 978-990.
HENSLOW, G. 1891. On the vascular system of floral organs, and their importance in
the interpretation of the morphology of flowers. lour. ,Linn/Soc. London, Bot.
28: 151-197.
HILLER, G. H. 1884. Untersuchungen tiber die Epidermis der Bli.ithenblatter. lahrb.
Wiss. Bot. 15: 411-451. .
JAcK-SCJN, 'G. 1934. The morphology of the flowers of.Rosa and certain closely related
genera. Amer. Jour. Rot. 21: 453-466. /
JOHANSEN, D. A. 1945. A critical survey of the present status of plant <!moryology. Bot.
Rev. 11: 87-107. .
JOSHI, A. C. 1947. The morpnology of the gynoecium. Presidential address: 34th Indian
Sci. Congr. Delhi.
KiESSELBACH, T. A. 1949. The structure and reproduction of corn. Nebr. Agr. Exp. Sla.
Res. Bull. 161. It, \
KIRKWOOD, J. E. 1906. The pollen tube in some of the Cucurbitaceae. Bull. Torrey Bot.
Club 37: 327-341. I
KNUTH, P.1898-1905. Halldbuch der Bliitenbio!ogie.(II 1 , H 2 , III l , IH~. W. Engelmann
Leipzig.
KOCH, M. F. 1930. Studies in 'the anatomy and morphoIogy of the composite flower.
I. The corolla. Amer. Jour. Bot. 17: 938-952.
LEINFELLNER. W. 1950. Der Bauplan des synkarpen Gynoezeums. Osterr. Bot. Ztschr.
97: 403-436.
LEROY, Y. F. 1955. Etudes sur les Juglandaceae. A la recherche d'une conception
morphologique tle la Reur femelle et du fruit. Mem. Musel(m Nation. d'Hist.
Naturelle Ser . .E,) Bot. 6: 1-246.
LLOYD, F. E. 1902. The comparative morphology of Rubiaceae. Mem. Torrey Bot.
Club 9: 1-112.
CHAPTER 20
THE fRUIT
THE fruit generally' develops from the gynoecium, but in many fruits
other organs also participate. Sucb organs may be the tepals (Morus),
the receptacle (Fragaria), bracts lAmmas), the 1\.Of"\ tube, which i~ formea
by the floral organs together with lhe receptacle (Pyrus malus), or the
enlarged axis of the inflores,~ence (FiCUS). In cases where organs other than
the gynoecium participate in the formation of the fruit, the fruit is termed
a false fruit or pseudocarp.
It is generally accepted that the fruit develops after fertilization, but
Jhis is not always so. Fruits of many plants, such as certain varieties of
CMusa, Citrus and Vitis, develop without the formation of seeds. This
Dry fruits
DEHISCE~T FRUITS
(i) Si/iqua: a pod-like fruit COJlsisting of two carpels which is con sid-
,««<1 b~ ma,,~ to b« a '1'",0...1 t~?'< ,,( ,,",,~mte. (w:. bet"w). The.
suture between the carpels' margins forms a thick rib, termed
replum, around the fruit. From these ,utures, which bear the
438 Plant Anatomy
SCHIZQCARPIC FRUITS
Fleshy fruits
Berry or bacca. A fruit in which the pericarp is usually thick and juicy
.and in which three strata can be distinguished: the outer stratum which
usually contains the pigment orthe fruit-exocarp; the relatively thick
i,fraium below it-mesocarp; and the membranous inner ~!ratum
endocarp. This fleshy pericarp may enclose' one or many seeds (e.g. grape,
tomato):' ,The fruit of Citrus, which is also a berry, has been specially termed
an hesperidium. The fruits of Coffea, Sambucus, Hedera, Cucumis and
Musa are also. berries but, theoretically, they are false fruits as they develop
from inferior ovaries; they differ from tYl'ical false fruits in that the extra-
carpellary parts contribute a'nlY a small part in the construction of their
pericarp.
Drupe. This fruit differs from the berry in that the endocarp is thick and
hard (Prunus, Mangijera,.Pistacia, Juglans). The "nut" of Cocos is a drupe
in which the mesocarp consists of fibrous matter.
Aggregate fruits are obtained when the carpels of an apocarpous gyn-
oecium ripen individually but in the course of ripening the individual fruits
of a flower aggregate to form a single unit, as, for example, in Rubus.
There are also indehiscent dry fruits consisting of several carpels and
containing one or more seeds, i.e. carcerulus (in a few Cruciferae, as, for
example, Crambe).
The Fruit 443
O'te,~
epidermis
Inner
= c
=~
epklermisE~ - -:_. !~
6-
lI
i:g_":·",_~~,,-o:,:: ~
,
.
9 10 II
442 Plant Anatomy
many fruits the exo- and endocarps may consist only of epidermal tissues
and the peri carp then consists mainly of mesocarp. The; division into these
pericarp layers is only one of convenience to facilitate the anatomical
description of "mature fruits and these layers do not represent separate
tissues from the point of view of their origin. In this hook this division will
also be used in the description of false fruits.
In large fruits vascular bundles, which develop in the ground tissue, are
added to the vascular system of the gynoecium (as described in the pre-
vious chapter) so that the supply of water and other substances to all parts
of the fruit is made possible. .
Usually there is a relationship between the manner of fruit and seed
dispersal, and the histological structure of the peri carp.
Dehiscent fruits
Follicle. In the follicle of Delphinium, for example, the exocarp (outer
epidermis and sometimes also a hypodermal iayer) consists of thick-walled
cells; the meso carp is parenchymatous and the endocarp (the inner epider-
mis) consists of thick-walled cells (Fig. 202, no. I). The vascular bundles
have sc1erenchymatous sheaths. The pericarp dries out with the maturation
of the fruit and the dehiscence of the follicle, along the'line of marginal
fusion of the carpel, results from this drying process. '
Legume. The following is a description of the basic structure of a legume
of the Leguminosae. The exocarp usually consists of epidermis only, the
mesocarp-of relatively thick parenchyma, the endocarp""":of sClere-
chyma on the inside of which is usually a thin-~alled epidermis or a few
parenchyma layers and an epidermis. The vascuiar bundles ~re situated-in
the parenchyma of the mesocarp and they are accompanied by sderen-
chyma. Although the general structure of the legume in most species of
the Leguminosae is uniform, there may bedifferencesin.the relative thick-
ness of the different pericarp tissues, the structure of their cells, the orien-
tation of the different elements and, som~mes, also in the submicroscopic
structure (Monsi, 1943; Fahn and Zohary, 1955). Thus in Astragalus
rnacrocarpus, the outer epidermal cells are thin walled (Fig. 202, no. II),
while in Lupinus hirsutus (Fig. 202, no. 6) and species of Vieia they are
thick walled. The epidermis may be typically uniseriate or a hypodermis
may be present, as, for example, in Lupinus hirsutus. The tissue of the meso-
carp may not always be parenchymatous and in certain legumes it is en-
tirely collenchymatous (Calyeotorne vi/losa, Fig. 202, no. 4) or it may be
partly collenchymatous and partly parenchymatous (Retama raetam, Fig.
202, no" 9). In some legumes there are sclereids scattered in the collenchyma
The Fruit. 445
7 8 9 10
444 Plant Anatomy
(R. raetam and Anagyris foetida, Fig. 202, nos. 9, 10). The sclerenchyma,
which constitutes most of the endocarp, consists either of one zone of
fibres which are all arranged in one direction (Astragdlus macro carpus,
Fig. 202, no.' II ; Acacia raddiana, Fig. 202, no. 7; Lupinus hirsutus, Fig.
202, no. 6) or of two zones in which the longitudinal orientation of their
ceJIs differs (Astragalus hamosus, Fig. ,202, no. 2; Hymenocafpos circinna-
tus). In the legumes of certain species there is no sclerenchymatous region
at all (G/ycyrrhiza echinata; Trifolium subterraneum; M elilotus, Fig. 2()3,
no. 3). In some species the sclerenchymatous stratum of the endocarp'is
lined on the inside not by parenchyma but by collenchyma (Retama
raetam, Fig. 202, no. 9; Anagyris foetida, Fig. 202, no. 10). The cell walls
of the inner epidermis are usually thin,. but in the legumes of certain spe-
cies they may be slightly thickened (Trifolium stellatum) or the cells may
be fibre-like (Trigonella arabica). Spines, which usually consist of scleren-
chyma, develop on certain legumes (Scorpiurus muiicata, Fig. 203, no. 4;
Hedysarumpallens, Fig. 203, no. I).
The two valves ofa dried legume usmilly twlst,(Fig. 210, no. 2). This is
brought about by the anisotropic shrinkage of the thickened walls of the
pericarp cells. This feature is a result of the orientation of the micro-
nbrils and cellulose crystals in the walls. The greatest swelling of the cell
walls takes place in the direction at right,angles to the longitudinal axis of
the Il1icrofibrils. Because of this the greatest shrinkage; during the drying
out of the valves, is also in this direction. In Vicia and in many species of
other genera (Fahn and Zohary, 1955) the 'sclerenchymi cells of the en-
docarp are orientated at an angle of about 45° to the lo'ngitudinal aXIS of
the legume, While the elongated, thick-walled epidermal or epidermal and
hypodermal cells are orientated in a similar angle but in the opposite di-
rection. In the valves of these legumes the microfibrillar orientation relative
to the cell axis is the same in both the endo- and exocarp, but as the cell
axes in these two strata of the pericarp are, themselves, differently orien-
tated, tension develops during the drying out of the valves. This tension
results in the twisting of the valves after the forces that keep the cells to-
gether in the mature abscission zone are overcome. The legume then de-.
hisces explosively, the valves contort and the seeds are/e~pelled.
I
--j.
termine the direction of the bending of the teeth or valves. The thinner
walls of the epidermis, or of the sclerenchymatoustissue below the epider-
mis, or of the sclerenchyma accompanying the vascular bundles, usually
Valve
constitute the resistance tissues. The swelling and, therefore, the shrinkage
?fthesetissues along the axis of ben ding is relatively small. These differences
In swelling and shrinkage cause the characteristic opening movements. In
capsules abscission tissue is developed between the teeth or valves. In capsules
that open by means of a lid, e.g. the fruits of Hyoscyamus, Plantago, Anagallis
and Portulaca, the absclssion tissue develops as a ring around the capsule.
446 Plant Anatomy
There are many variations in the structure connected with the opening
mechanism that has been described .above. In the legumes of Wisteria
sinensis (Monsi, 1943) and Lupinus angustifolius (Fahn and Zohary,
1955), for example, all the sclerenchymatous cells of the endocarp have
uniform orientation, but two zones, in" which the orientation of the cellu:-
lose crystals differ, can be distinguished. These zones, therefore, also differ
in the direction of greatest swelling. There are also legumes in which the
endocarp sclerenchyma consists of two Jayers that are distinguished by cell
orientation (Astragalus !ruticosus, Fig. 202, no. 8; A. harnosus, Fig.·202,
no. 2; Hedysarum pallens, Fig. 203, no. 1). In the legumes of other species,
e.g. Ornithopus compressus, the endocarp sclerenchyma consists of two
zones, as mentioned above, but in one zone the orientation of the fibrils is
parallel to the longitudinal cell axis and in the second zone, at right-
angles to it. As a result of this the fibrillar orientation, relative to the axis
of the legume, is the same in the two zones. In these plants, in addition,
the orientation of the fibrils of the epidermal cells is parallel to that of the
fibrils in the sclerenchyma and therefore the fruit does not dehisce when
dry. Abscission tissue is also not developed in such legumes.
Siliqua. In the peri carp of the siJiq ua the cells of the exo- and mesocarp
are usually thin walled' and the endocarp tissue is sclerenchymatous. An
abscission zone usually develops between the replum and the valves
(Fig. 204, no. 2). ./
Capsule. The epidermal cells of many capsules, which open by teeth or
valves, have very thick outer walls while the mesocarp'tissue is paren-
chymatous. Elongated, thick-walled cells may sometimes be present below
the epidermis (Fig. 205, no. 1), The inner epidermal cells may also be thick
walled. In the capsules of certain species of the Primulaceae, such as Lysi-
machia rnauritiana, the cells of the inner epidermis have particularly thick
walls on the side closest to the mesocarp (Guttenberg, 1926). The dehis-
cence ortbe capsuleis also brought about by the anisotropic swelling of the
cell walls. The walls that bring about the dehiscence in this case are mainly
the very thick walls of the outer or inner epidermis, and it is they that de-
I
FIG. 203. 1-4, Diagrams of cross-sections of legumes of different species of the
Leguminosae. In those places where the sclerenchyma cells are seen in longitudi-
nal section, the sclerenchyma is indicated by parallel lines, and in those places
where the cells are cross-sectioned, it is indicated by cross-hatching. 1, Hedysa-
rum pallens. 2, Astragalus fruticosus. 3, Melilo/us sp. 4, Scorpiurus muricata. 5-10,
Hygroscopic movements. 5 and 6, Capitula of Anvillea garcini, showing the move-
ment of the involucre bracts. 5, Dry. 6, Moist. 7 and 8, Salvia horminum, showing
the movement of the pedicel and the mericarp-containing calyx. 7, Dry, showing
the pedicel bent <lownwards bringing the calyx close to the axis of the, inflores·
cence. 8, Moist. 9and 10, Cichoriumpumi!um, capitula showing movement of the
involucre. 9, Dry. 10. Moist.
The Fruit 449
In the dispersal of the seeds of dry fruits, sepals and extrafloral organs
may sometimes participate, e.g. the bracts of the capitulum (Cichorium,
Fig. 203, nos. 9, 10; Anvil/ea, Fig. 203, nos. 5, 6), floral pedicels (Salvia
iwrminum, Fig. 203, nos. 7, 8), axes of inflorescences (Plantago cretica,
Fig. 204, nos. 3, 4), and entire branches (Anastatica hierochuntica) (Stein-
brinck and Schinz, 1908; Zohary and Fahn, 1941; Fahn, 1947). The mecha-
nisms of dispersal in these cases are also usually based on the differences
in direction of excessive swelling in the different zones of the sclerenchy-
matous tissues. This swelling mechanism also causes the twisting of the
"beak" of the mericarp of Erodium (Fig. 210, no. I).
In addition to the swelling mechanism there is the cohesion mechanism
that is responsible for the movement of organs connected with the dispersal
of fruits and seeds. Cohesion tissue occurs, for example, on the outer side
of the bracts of many of the Compo sitae (Senecio, Tragopogon and Gero-
pogon, Fig. 205, nos. 2, 3), between the bases of the rays of the umbel, e.g.
Ammi visnaga (Fig. 205, nos. 4-6), between the tepals, e.g. Plantago ere-
.tica (Fig. 204, nos. 5, 6), etc. The cells of cohesion tissues are usually elon-
"gated and thin walled; the longitudinal axis of these cells is at right-
angles to that of the shrinkage axis of the tissue, As a result of the loss of
;;;'water·the cell walls fold or wrinkle, the cell volume is reduced and the tis-
"'sile;-iil shrinking, draws with it the organ that is attached to it. If water is
again absorbed by the cells, they swell and the volume of the tissue increases
(Guttenberg, 1926; Zohary.and Fahn, 1941),
. I
Indehiscent fruits
The rupture between the lid and the rest of the capsule, along this abscission
line, is apparently brought about by the fact that the maturing pericarp
I
tissue
....Involucre
bract
3
2.
4 5
FIG. '205. i, Vaccaria pyramTdata:longitudinal section through a -tooth oflhe cap·
sule. The outer epidermis (directed upwards) has very thick walls in which the
lamellae are orientated almost perpendicularly to the surface of the teeth. These
walls, when wet, are capable of extensive swelling in a longitudinal direction, and
the elongated cells below this epidermis and the cells of the inner epidermis, \
itself, form a tissue that resists this movement. As a result the valves open out-
wards when they are dry and close when they are moist. 2 and 3, Geropogon, show-
ing action of cohesion tissue - hatched areas:2;Ripe capitulum in dry condition.
3, In moist condition showing that the cohesion tissue, when saturated with water,
raises the involucre bracts. 4-6, Ammi visnaga 4, Diagram of a cross-section
through the base of the compound umbel, showing the bases of the partial umbels
to be embedded in coheson tissue (stippled). 5 and 6, Tangential sections through
the cohesion tissue. 5, Dry. 6, Moist. (Nos. 4-6, adapted from Guttenberg, 1926.)
dries out and shrinks while the volume of the seeds, which fill the entire
cavity of the fruit, does not change (Rethke, 1946; Subramanyam and
Raju, 1953).
The Fruit 451
3). The integumental celIs of the mature seed are compressed and partiy
obliterated, but the outer epidermiscmay remain as a layer of thick-walled
cells.
(nner layers
of nuceUus
Endosperm
_Starchy
endosperm
4
FIG. 207. 1-4, Portions of cross·section through the pericarp of the caryopsis of
Triticum at different stages of development. (Adapted from Hayward, 1938.)
Caryopsis. The pericarp and the remains of the integuments of the single
seed of the caryopsis are completely fused (Krauss, 1933; Bradbury,
MacMasters and Cull, 1956). In the caryopsis of Triticum (Fig. 207, no.
450 Plant Anatomy
\
i
FIG. 206. Development of the pericarp in LactYell satlva. 1, Portion of a cross-Sec-
tion of an ovary 2 hr after fertilization. 2, Portion of a cross-section of the cypsela
about a week after fertilization. 3, Mature pericarp. 4, Diagram of a cross-section
of an entire cypsela. (Adapted from Borthwick and Robbins, 1928 and Hayward,
1938.)
rest of the pericarp, apart from the ribs, consists of one or two eel! layers
as a result of the disintegration of the parenchyma cells. Cypselae differ
greatly in colour. These differences are partly due to the presence or ab-
sence of pigment in the outer epidermal cells of the pericarp (Fig. 206, no.
The Fruit 453
Vas,"'"
Rib
Oil
7
Aleurone
grains
FIG. 208. 1-4, Fruit of Apium graveolens. I, Drawing of the entire cremocarp. 2,
Cross-section of a single mericarp. 3, Portion of a cross-section of a mericarp
showing the structure of a rib. 4, As in No.3, in the region of an oil cavity. 5, A
mature cremocarp with separated mericarps. 6, Distal portion of spine of the
nutlet of Ranunculus arvensis showing the pointed, curve-d cell at its apex by which
it becomes attached to the dispersal agents. 7, Cypsela of the Cornpositae show-
ing the feathery pappus. (Nos. 1-4, adapted from Hayward, 1938.)
452 Plant Ana/omy
4; Fig. 220,'nos. 1,2), for example, three main parts can be distinguished:
(1) the caryopsis coat which includes the pericarp, the seed coat and the
nucellus; (2) the endosperm; (3) the embryo. Five layers can be distinguished
in the pericarp: the outer epidermis, the hypodermis, a zone of thin-
walled cells, cross cells and tube cells .. The outer epidermis and the hypo-
dermis together form the exocarp. The 'cells of the exocarp are elongated
in a direction parallel to the longitudinal axis of the caryopsis, and they
become compressed and their walls thicken considerably so that, when
the caryopsis is ripe, cell lumina cannot easily be distinguished in them.
The cross cells are found below the parenchymatous layer and they have
thick walls with pits which are elongated transversly to the cell. The longi-
tudinal axis of these cells is at right-angles to that of the exocarp cells.
The tube cells constitute the inner epidermis of the pericarp and they
occur on the inside of the cross cells. There are large intercellular spaces
between the tube cells, the walls of which are pitted and thinner than those
of the cross cells. The longitudinal axis of the tube cells is parallel to that
of the eKocarp cells. The tube cells are only clearly visible on certain parts
of the caryopsis (Bradbury e1 al., 1956). The seed coat, which is adnated to
the pericarp, is crushed in the mature caryopsis and therefore it is difficult
to discern cells in this zone. When fully mature the seed coat consists only
of the inner integument, the outer being cbmpletelY destroyed. In a section
of the caryposis stained with Sudan.IV two cuticles are, discernible: an
outer thick one of the single remaining integument, and a thin one which
constitutes that of the nucellus together with thai of the' inner side of the
integument. The nucellus tissue may be partially or enti~ely digested during
the development afthe caryopsis. In those parts where nucellus is still pres-
ent it is discernible as one or two layers of thin-walled cells between the
aleurone layer and the seed coat. In the mature caryopsis these nucellar
cells are usually crushed and they appear as a ,thin, glassy zone, which is
bright and colourless (Fig. 207, nos. 1-4).
Cremocarp. The fruit of Apium graveolens is described here as an example
of a cremocarp (Hayward, 1938). Each mericarp has five ribs in which the
vascular bundles, accompanied by scierenchyma, are situated (Fig. 208,
nos. 1-3). Between the ribs one to three oil ducts m,ay be present (Fig. 208,
nos. 2,4). The exocarp cells (i.e. the outer epidermis) are small and isodi-
ametric. In surface view they appear lolled: The mesocarp is parenchyma-
tous and its cells have small papillae and finely striated walls. The oil
ducts in the mesocarp are surrounded by polyhedral cells which become
brown as the fruit ripens. The slightly elongated cells of the innermost
layer of the mesocarp are wider than those of the endocarp. The endocarp
(the inner epidermis) consists of narrow cells of which the longitudinal
axis is parallel to the transverse axis of the mericarp (Fig. 208, nos. 3, 4),
or their arrangement is mosaic. Inside the pericarp is the thin seed coat
which surrounds the endosperm in which the embryo is completely enclosed.
The Fruit 455
The dispersal of indehiscent, dry fruits and their seeds is brought about
in various ways, and here also a correlation can be found between the man-
ner of dispersal and the anatomical structure of the peri carp. Thus, for
instance, on the akenes of Ranunculus arvensis strong, hook-like hairs,
which consist of extremely thick-walled cells (Fig. 208, no. 6), develop;
these hairs enable the fruit to cling to the coats of animals. Cypselae of
many of the Compositae develop a characteristic pappus of hairs or bris-
tles which may be branched (Fig. 208, no. 7). The outer cypselae of Calen-
dula officinalis are boat-shaped and the margins are widened to form back-
wardly directed wings. In a cross section of such a cypsela (Fig. 209, no.
1), it is possible to see that the endocarp consists of a thick sclerenchyma-
tous tissue. The centre of the keel, the wings and the ribs on the side oppo-
site the keel consist oflarge parenchyma cells whose pitted walls are slightly
thickened. Most of the region between the epidermis and the inner tissues
of the cypsela consists of very long palisade-like gells which have large
intercellular spaces between them (Fig. 209, no. 2). This tissue together
with the thick-walled large parenchyma cells, which are filled with air,
aid'in the dispersal of these cypselae by wind.
:.... An outgrowth of large oil-storing cells occurs on the basal part of the
'fruit' of some plants, e.g. Hepatica triloba, Ranunculus ficaria, Anemone
nemorosa, Adanisvernalls and Fumarla a/fieinalls. Such growths are termed
eiaiosomes and they may also occur on seeds (see following chapter). In
Baraga o/ficinalis, Anchusa ,pp., Ajuga spp. and other species the elaio-
some of the fruit or the mehcarp develops from a portion of the pedicel
(Schoenichen, 1924). Elaiosomes are thought to be an adaptation to fruit
and seed dispersal by ants.
There are many other modifications in the histological structure of dry
fruits which are adapted to dispersal by air, water and animals, but it is
not within the scope of this book to discuss all 9f them.
Berry. In a berry all of the ground tissue of the ovary wall develops
into a fleshy or juicy tissue, and sometimes other organs may also contrib-
ute to the formation of this tissue. In Lycopersicon, for example, the
greater part of the juicy tissue develops from the placenta. In certain
berries the locules of the fruit become filled with growths of the pericarp
and placenta (Physalis) or of the septa (Bryonia diaica) (Kraus, 1949).
Below, the structure and development of some special fruits which are
considered to be berries, in the wide sense of the term, are described .
. Lycopersicon esculenturn. This fruit consists of a pericarp and placental
tJSsue on which the seeds are borne. The exocarp consists of an epidermis
and three or four layers of collenchyma cells. The epidermal cells are
454 Plant Anatomy
remain attached to the branched carpophore (Fig. 208, no. 5). There are
different opinions as to the origin of the carpophore-+there .are investi-
gators who believe that it arises from the floral axis, while others believe
that it develops from the carpels. Jackson (1933) claims that usually only
cell
~
2
FIG. 209. Calendula officinalis.
/' .
1, Diagram of a cross-section of the cypsela. 2,
Epidermal cells and palisade-like cells, between which there are large air spaces,
grea tly enlarged. (Adapted from Schoenichen, 1924.)
the basal portion is of receptacular origin while the greater portion of the
carpophoreis carpellary and contains the ventral carpel bundles. The abscis-
sion zone occurs partly between the two mericarps and partly between the
mericarps and the carpophore. With the maturation of the cremocarp,
an abscission zone also appears within the carpophore, in the lignified tissue
between the two ventral veins. This abscission zone splits the carpophore
into two.
The Fruit 457
,/ 3
-
Chromoplosts
~-~
.. Sepal
5
bundle
Petal
bundle ,
"I
o
? ---:o.,_Hypanthium
Bundles conneCTing
the dorsal and
ventral bundles
1
456 Plant Anatomy
polyhedral and are covered by a thin cuticle. The ' number of epidermal
cells does not increase greatly with the development and growth of the
fruit, and so the epidermal cells of the mature fruit are much larger than
..
Flo. 210. 1, Erodium, photograph of a mature mericarp in a dry condition. 2,
Spartillm jUllceclm, photograph of mature legumes which have dehicsed as a
result of the twisting of the valves. (Photographs, courtesy of D. Koller.)
those in the young fruit. The glandular and other hairs that are usually
present on the young fruit are shed as it matures. There are no stomata on
the epidermis of the fruit (Rosenbaum and Sando, 1920). The mesocarp
The Fruit 459
the pericarp, take place before fertilization or immediately after it. The
growth of the fruit is mainly accomplished by cell enlargement (Ragland,
1934). At first all the cells enlarge almost equally in all directions, ,but with
FIG. 212. 1 and 2, M icrog raphs of portio ns of the pericarp of Citrus. x 40. 1,
Outer portion . 2, I nner portion. 3, M icrograph of a section of the endosperm of
Phoenix dactyli/era. x 430. c-'
458 Plant Anatomy
consists of a thick layer of large thin walled cells which enclose many
intercellular spaces. In the early stages of development, so\>n after pollina-
tion, the number of layers in the mesocarp increases rapidly, but the main
increase in thickness of the peri carp results from an enormous increase
in cell volume in later stages of development. During the process of fruit
ripening, some of the cells of the inner and central portion of the carpels
may disintegrate. With the development of the ovules, after pollination,
the parenchymatous tissue of the placenta grows around the funiculi.
This parenchyma continues to grow until it completely encloses the dev~l
oping seeds (Fig. 211, no. 5). The cells of this tissue are thin-walled and
they form a homogeneous tissue; they do not fuse with the pericarp but
they adhere to it as well as to the seeds. At first this parenchymatous
tissue is firm, but as the fruit ripens the cell walls become thinner and the
cells are partly destroyed (Hayward, 1918).
Citrus. In this genus the fruit develops from a syncarpous gynoecium
with axile placentation. With the development of the fruit the number of
cells throughout the ovary increases and, finally, \ three strata (Fig. 212,
nos. 1,2) can be distinguished (Schoenichcn, 1924; Ford, 1942; Scott and
Baker, 1947). The eXDcarp (flavedo) consists of small, dense collenchyma
cells which contain ch~ornoplasts. This tissue/contains essential oil cavi-
ties (Fig. 212, no. 1). The epidermis consists of very small, thick-walled
cells, and in surface view it _resembles a 'Cobbled surface; they contain
chromoplasts and oil droplets. A rew scattered stomata can be found.in
the epidermis. The mesocarp (albedo) consists ofloosely conhected, colour-
less cells; this tissue has a spongy nature and is white becal{se of the numer-
ous air spaces in it. The endocarp is relatively thin and consists of very
elongated, thick-walled cells which form a compact tissue. The stalked,
spindle-shaped juice vesicles, which fill the loculesjwhen the fruit ripens,
develop from the cells of the inner epidermis ·and subepidermal layers
(Hartl, 1957). Each juice vesicle is covered externally by a 'Iayer of elongated
cells which enclose very large, extremely thin-walled juice cells '(Fig. 211,
nos. 1, 2).
Drupe. In the drupe of Prunus persica (Add oms, Nightingale and Blake, \
1930) most of the cell divisions, which occur during.the development of '
j
_____ ---
FIG. 211. 1 and 2, CiJrlls. J, A single juice vesic1e. 2, Portion of the vesicle as seen
under the microscope. 3 and 4, Ecballium eiaferium. 3, Diagram of a longitudinally
sectioned fruit. 4, A few cells from the inner, white portion 'of the pericarp. 5,
Diagram of a cross -section of the ovary of Lycopersicon esculentum after fertili-
zation in which the enlargement of the parenchymatous tissue of the placenta is
shown. This tissue forms the fleshy tissue of the fruit. 6, Diagram of a cross-
section of the fruit of Pyrus malus var. paradisiaca. (No.2, adapted from
Schoenichen, 1924; nos. 3 and 4. adapted from Guttenberg, 1926; no. 5, adap-
ted from Hayward, 1938; no. 6, ada~ted from MacDaniels. 1940.)
The Fruit 461
vascular bundles are embedded. Further inwards the pericarp is white and
consists of elliptical cells which are rich in pectic substances and have
thick, pitted walls. The longer axis of these cells is at right-angles to the
longitudinal axis of the. fruit and large intercellular spaces are present
between them (Fig. 21"1, no.4). Still further inwards is the tissue that en-
velops the seeds, and which consists of large, vesicle-like, extremely thin-
walled cells between which there are no intercellular spaces. These cells
have a very thin layer of cytoplasm, and their cell sap contains the glucoside,
elaterinidin. This substance is present in such large amounts that in the
ripe fruit the osmotic pressure of the sap reaches about 27 atmospheres.
As a result of the turgor pressure of the elaterinidin-containing cells, the
elastic cells of the white portion of the pericarp expand, and this occurs
mainly in the direction at right angles to the longitudinal axis of the fruit.
Abscission' tissue develops, as the fruit matures, around that part of the
stalk that is within the pericarp (Fig. 211, no. 3). At the instant when
the pressure which develops in the inner juicy tissue surrounding the
seeds exceeds that of the force that keeps the cells of the separation layer
together, the stalk is expelled. Simultaneously the pericarp, and especially
the white portion of it, contracts and the fruit content-the large
juicy~cells together with the seeds-is ejected with great force. It was
found that the amount of contraction of the peri carp in a transverse
direction is 17·3 % and in a longitudinal direction 10·8 % (Guttenberg,
1926). . I
The fruit of Impatiens is a fleshy capsule in which the septa are extremely
delicate. It is cylindrical but somewhat swonen in the upper portion in
which the seeds develop (Fig. 213, no. I). This upper part of the fruit re-
mains inactive, as far as the opening mechanism is concerned, while in
the lower portion tension is developed between the outer tissue, which
has an expansion potential, and the inner tissue, which offers_resistance.
When the fruit is mature, the abscission tissue between the carpeIs rup-
tures and each valve abruptly curls inwards and, as a result of this, the
seeds are expelled (Fig. 213, no. 2). The expansion tissue is located below
the outer epidermis which consists of thick-walled cells. This tissue con-
sists of radially elongated parenchyma cells and lacks intercellular spaces
(Fig. 213, nos. 3, 4). The cells have a rich sugar content when the fruit
ripens and the osmotic pressure in their cell sap reaches 25 to 26 atmospheres.
This pressure would result in the rounding of the cells were it not for
the resistance offered by the inner portion of the pericarp, which consists
of two or three layers of collenchyma cells, the longitudinal axes of which
are parallel to that of the fruit (Fig. 213, no. 3). These cells elongate by
10% as a result of the turgor pressure in the outer tissue, and they contract
again to the same extent with the opening of the fruit. The outer tissue
elongates parallel to the longitudinal axis of the fruit by 32·25 % with the
opening of the fruit (Guttenberg, 1926).
462 Plant Anatomy
• I
••
2
Outer
The Fruit 465
SCOTT, F. M. and BAKER, K. C. 1947. Anatomy of Washington navel orange rind in
relation to water spot. Bot. Gaz. 108: 459-475.
SMITH, W. H. 1940. The histological structure of the flesh of the apple in relation to
growth and senescence. Jour. Pornol. and Hart. Sci. 18: 249-260.
SMITH, W. H. 1950. Cell-multiplication and cell-enlargement in the development of
the flesh ~f the apple fruit. Ann. Bot., N. S. 14: 23-..:38.
STEINBRINCK, C. and SCHINZ, H. 1908. Uber die anatomische Ursache dec hygro-
chastischen Bewegungen def sog. Jerichorosen, usw. Flora 98: 471-500.
SUBRAMANYAM, K. and RAJU, M. V. S. 1953. Circumscissile dehiscence in some angio-
sperms. Amer. Jour. Bot. 40: 571-574.
TUKEY, H. B. and YOUNG, J. O. 1942. Gross morphology and histology of developing
fruit of the apple. Bot. Gaz. 104: 3~25.
WINKLER, H. 1939. Versuch eines "natilrlichen" Systems der Frtichte. Beitr. Bioi.
P/I. 26: 201-220.
WINKLER, H. 1940. Zur Einigung und WeiterfUhrung in der Frage des Fruchtsystems
Beil'. Bioi. P/I. 27: 92-130.
ZOHARY, M. and FAHN, A. 1941. Anatomical-carpological observations in some hygro-
~hastic 'P~'l>.nt'5 of the oriental flora. Palest. Jour. EoL, Jerusalem 2~ l1S~13S.
464 Plant Anatomy
References
,
ADDOMS, R. M., NIGHTINGALE, G. T. and BLAKE, M. A. 193Q. Development and
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BORTHWICK, H. A.- and ROBBrNs, W. W. 1928. Lettuce seed and its germination.
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BRADBURY, D., MACMASTERS, M. M. and CULL, J. M. 1956. Structure of mature wheat
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HN/WARD, H. E. 1938. The-Structure of Economic Plants. Macmillan, New York.
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KRAUSS, L. 1933. EntwickIungsgeschichte der .Fruchte von Hordeum, Triticum, Bromus
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MACARTHUR, M. and WETMORE; R. H. 1939. Developmental studies in the apple fruit
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MACARTHUR, M. and WnMc1RE, R. H. 1941. Developmental studies of the apple fruit
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(Cornell) Agr. Exp. Sta. Mem. 230. . ~ {
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split~pit and gumming. Proc. Amer. Soc. Hort. Sci. 31: F-21. '
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677-683.
ROSENBAUM, J. and SANDO, C. E, 1920. Cor;elation between size of the fruit and the
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SCHOENICHEN, W. 1924. Biologie der BliitenpJlanzen. T. Fisher, Freiburg i. Br.
The Seed 467
(5'0'0°',0:-0_.0
0- 0_o,_0 " _'00
90,(2:<,,9-:0"Q
-T'Y~=?~}outer
integument 3
0:'0 0 0 0 1
.~l ~ijOiOr:;:~"m'"t
Jii
::2= I Cuticle
between
~ Integument
Endosperm/ 4 and nucellus
6
FIG. 214. 1~7, Portions of cross-sections of testae. 1, Viola tricolor. 2, Magnolia
macrophy'Ua._ 3, -Sinapis alba. 4, Malva silvestris. 5, Plantago Ian ceo/ala. 6. Lyth-
rum salicaria, showing the invaginated hairs which contain substances that become
mucilaginous when moistened, and then the whole structure is pushed out like a
finger of a glove. 7, Ceratonta siliqua. 8, An entire seed of Orchis in which the
embryo (stippled) can be seen through the tran.')parent testa. (Nos. 1 and 2,
adapted from Eames and MacDaniels,1947; nos. 3-6, adapted from Netolitzky,
CHAPTER 21
THE SEED
THE seed develops from the ovule. In the mature seed the following parts
can be distinguished: the testa, which is the seed coat and which develops
from one or two integuments; endosperm, which may be present in a large
or small amount; the embryo, which constitutes the partially developed,
young sporophyte. In some seeds the endosperm is completely absent and
such seeds, as well as those that contain a very small amaun't of endosperm,
are termed exalbuminous seeds. In the seeds of certain plants, e.g. Beta, the
,nucellar tissue persists and increases in volume\ to form the perisperm.
Several features can be distinguished on the outer' surface of the seed. The
micropyle may be completely obliterated or it may remain as a distinct
pore. In the place where the seed was attached to the funiculus a scar,
termed the hilum, is present. Water can penet~ate with relative ease through
the hilum. In anatropous ovules, in which part. of the funiculus is fused to
the integument, the seed becomes detached together with the fused part of
the funiculus which forms a characteristic ridge termed th,/aphe. After the
fertilIzatIOn of the ovule, growths, termed arils, develop on the surface of
the seeds of certain plants. These growths when they occur on the funicu-
lus, e.-g., in Euonymus and A-cada spp., are ot1en termed stropnfoles and when
occurring around the micropyle, e.g. ih Ricinus, caruncles. Arils are very
common in tropical and sUbtropical plants, e.g. inllntsia bijuga, Pithecello-
bium" dulce, Durio· zibethinits~ Acacia retivenea, Bocciafrutesceris and Myris-
tica fragrans. Arils are organs that are well adapted to seed dispersal by
animals (Corner, 1949). Arils that contain oil, such as those of Chelido-
nium majus, Luzula vi/losa, Gagea lutea, Reseda odorata, Galanthus nivalis,
and Ricinus communis, are elaiosomes' (see preceding,chapter). In Cheli- .
donium majus (S~emes, 1943) the elaiosome (Fig. 217, no. 3) consists of
small basal cells and large, sac-like outer-cells which store oil, proteins
and starch-like grains. Elaiosomes are thought to be connected with seed
dispersal by ants. ....,;.,
The testa
Angiosperm ovules have one or two integuments (see Chapter 19). In
order to clarify which parts of the integuments take part in the formation of
the testa, ontogenetic investigation is necessary. All parts of one or both
468 Plant Anatomy
integuments may take part in the formation of the seed coat, e.g. Viola
tricolor (Fig. 214, no. I), but in most seeds much of the integumental tissue
is destroyed and absorbed by the other developing tiss~es of the seed. and
then the seed coat develops only from the remaining parts of the integu-
ments. The parts that are destroyed are usually the innermost or intermediate
middle layers of the integument. The nucellus may also take part in
the construction of the seed coat (Fig. 214, nos. 2, 6). However, in the devel-
opment of most seeds the nucellus is apparently completely destroyed
(Netolitzky, 1926; Eames and MacDaniels, 1947). '
In certain seeds, especially those of indehiscent fruits, only the two or
three outermost layers of the integument persist. In the mature seed of the
Umbelliferae only the outer epidermis of the outer integument remains.
In the seeds of certain genera of the Compositaei e.g. Lactuca (Fig. 206,
nos. 3, 4), the integuments are represented only by a thin layer of oblite-
rated cells which persists beneath the cypsela coat. The innermost layers
of the latter coat also disintegrate (see .Chapter 20). In the seeds of certain
monocotyledons, e.g. Zea, the integuments are, completely destroyed. In
seeds· developing from ovules with two integuments, the two integuments
may be present in the testa or only two or three of the QuteJ::most layers
of the outer integument may remain. In such seeds the inner integument
may constitute the major portion of the se(d coat (Fig. 214, no. 4), or the
outer one may be the better developed and be especially adapted for pro-
tection while the inner is non-specialized (Fig. 214, no. 3). The former type
is characteristic of the Maivaceae, Violaceae, Hypericaceie , and Ti1iaceae,
and the latter of the Cruciferae, Papaveraceae, Berberidaceae and certain
genera of the Liliaceae, Iridaceae and Araceae. In the Onagraceae, Lyth-
raceae,. Aristolochiaceae and others, protective layers develop from both
integuments, and the-entire nucellus or its outermost layers only also take
part in the formation of the seed coat (Fig. 214, no. 6). In mostof the Legu-
rninosae, RanuncuJaceae'and certain'genera of the" LiJiaceae and Amarylli-
daceae the inner integument and the nucellus are completely obliterated.
In the seeds of only a few plants in wpich the ovule is surrounded by a
single integument does the entire integument form the.seed coat. Usually
only a few of the outermost cell layers and the inner epidermis persist; e.g~
in the Plantaginaceae (Fig. 214, no. 5) and Polemoniaceae.1
differentiated embryo. This feature enables the dispersal of the seeds, which
are about a quarter of a millimetre long, by wind.
The structure of seed coatnhat are very hard or fleshy, and especially
of those that are sculptured, is complicated.
In the seeds of some plants, e.g. certain genera of the Leguminosae, the
testa is covered by a very thick cuticle which prevents the passage of water
and air as long as it is undamaged.
In the seed coats of certain plants, a layer of radially elongated cells,
which are'palisade-like but devoid of intercellular spaces, may be present.
These cells have been termed Malpighian cells after the investigator who first
described them. Because of their shape and the thickness of their walls,
these cells are also termed macrosclereids. The thickness of the walls is
characteristically not equal throughout, and the walls may consist of
cellulose only or of lignin or cutin as well. Malpighian cells are especially
characteristic of the Leguminosae where they constitute the outer epider-
mis. In seeds of the Leguminosae, the cell lumen of the Malpighian cells
is usually widest at the base of the cell. In Cercidiumjloridum, the walls of
rthese epidermal cells have been observed to be traversed by plasm odes-
_matd (Scott et al., 1962). In a cross-section of the seed coat, a thin line
-which runs across the cells and parallel to the surface of the seed close to
the cuticle can be distinguished. This is due to the fact that, along this
lille, the light refraction differs from that in other parts of the cells. This
line is termed the light line or linea lucida(Fig. 214, no. 7). In the seeds of
certain species the light line is a result of deposition of wax globules in the
cells (Eames ,and MacDaniels, 1947). Non-epidermal Malpighian cells are
mainly found in families other than the Leguminosae, e.g. in the seeds of
dossypium (Fig. 216, no. 4) iwd Malva(Fig. 214, no. 4).
)n the seeds of many of the Leguminosae one or more layers of cells with
unusual shape are found below the Malpighian cells; these cells may be
funnel- or bone-shaped, for instance, and because they are also thick-
walled, they are termed osteosclereids. They may, like the epidermal cells,
contain pigments or be devoid of them. In certain species of Phaseolus
these cells also contain crystals of oxalate salts (Netolitzky, 1926). The
greater portion of the inner tissue of the integuments disintegrates and
the outer cells develop at the expense of the contents of this tissue. The
cells that remain after this process of disintegration may sometimes be
thick-walled. In the testa cf many seeds the inner epidermis may be pres-
ent and sometimes a cuticle can be distinguished between the. testa and
the remains of the nucellus or endosperm (Fig. 214, nos. 4--7). Relatively
well-developed vascular bundles may be found in the testa of certain plants
(Arachis), while in others (Pisum and Lathyrus) they can hardly be distin-
guished. In leguminous seeds the nucellus apparently disappears completely.
Corner (1951) has drawn attention to the value of the structure of the
seed coat in the taxonomy of the Leguminosae. The cells of the testae
4io Plant Anatomy
of the seeds of other families also have different characteristic shapes, wall
thickenings, etc. The testa of Phoenix consists of thin;walled cells only.
During the course of seed development, the number of cell layers of the
integuments, and especially of the inner integument, decreases.
The structure and development of the seed of Lycopersicon escu/enlum
has been described by Soueges (1907) and is of great interest. In the thick
integument of the young Lycopersicon seed the following four parts can be
distinguished: an outer epidermis; an intermediate parenchymatous tissue
in which inner and outer zones can be distinguished; an inner epidermis
which contains pigment (Fig. 215, no. 1). With the development and enlarge-
ment of the seed, the cells of the outer parenchymatous zone increase
in number, and thickenings develop in the inner tangential walls and at
the base of the radial walls of the outer epidermal cells (Fig. 215, nos. 2,
3). When the seeds are partially mature the outer epidermal cells are seen
Outer zone
of thln-
walled cells
4
FIG. 215. Pottions of cross-sections of the testa of Lycopersicon esculentum show-
, ing different stages of development. (Adapted from Hayward, 1938.)
The Seed 471
Endosperm
Testa
Membranous
nucellus
Hypocofyl
Radicle
-Hypocotyl
" Outer
integument
'- Inner
integument
to a very large extent in a radial direction. The sap of these cells develops
a turgor pressure which preserves the characteristic external shape of these
seeds (Fig. 216, no. 3).
:;)
J.
).
FIG. 217. 1 and 2, Tamar-ix. 1, An entire seed, with tuft of hairs. 2, The base of a
single hairof the tuft, enlarged to show the characteristic structure of the- abaxial
wall. 3, Elaiosome of Chelidonium majus. (No.3, adapted from Szemes, 1943.)
The endosperm
stitute the reserve substance. In the former type mainly starch grains and
proteins are the stored substances. There are two main forms in which
proteins may be stored in the endosperm-in an amoi-phous form (glu-
tens), or in the form of aleurone grains. Aleurone grains consist of a pro-
tein crystalloid and a spherical body (the globoid) which contains salts of
calcium, magnesium and phosphorus in an organic compound. In the
caryopses of cereals glutens are found in the starch cells and aleurone
grains are restricted to the outermost layer of endosperm cells (the aleu-
rone layer). In Ricinus, however, aleurone grains occur throughout'the
entire endosperm. In endosperm in which there is no starch, oils and fats
may be present as reserve substances. (See also Chapter 2.)
Cell walls that constitute reserve material, e.g. in the seeds of Phoenix
(Fig. 212, no. 3) and Diospyros, usually consist of hemicelluloses and other
similar carbohydrates. In Phoenix, Meier (1958) recorded that these walls
apparently contain, in addition, about 6 % of cellulose. These cells have a
very thick secondary wall.
Some seeds, e.g. those of Ceratonia, have a \ mucilaginous endosperm
(Fig. 214, no. 7). A stratified thickening of the walls of these endospermal
cells can be distinguished. These walls become more or less mucilaginous
when in contact with water. In the dry seeds' these endospermal cells are
hard and, during germination, they function both as a swelling and nutri-
tional tissue (Netolitzky, 1926).
Seedlings
With the germination of the seed, the testa ruptures at the 'micropylar
end and the radicle emerges. GeneraJly, the radicle penetrates into the
soil, ,develops root hairs and often lateral ro'ots. After this, further
rupturing of the testa takes place. •
In many seeds the cotyledons and shoot apex emerge while ihe hypoco-
tyl elongates as a result of intercalary growth. This type of germination is
termed epigea/ germination; examples of such germinatkm can be seen in,
Helianthus, Raphanus, Phaseolus and Ricinus (Fig. 2l8(nos. +-6). '
The cotyledons of plants in which the germination is epigeal may vary
in form and function. The cotyledons of Phaseolus vulgaris, for example,
are very thick and function as storage organs (Fig. 218, no. 3). These coty-
ledons wither early and are shed. The cotyledons of many other dicotyle-
donous plants in Vlhich the germination is epigeal are thinner structures,
which, when appearing above ground, more closely resemble foliage leaves
(Fig. 218, no. 6).
In many otber plants, e.g. Vida, Pisum sativum and Quercus, the thick
cotyledons, which contain reserve materials, remain within the testa and
the hypocotyl elongates only slightly or not at all. This type of germination
The Seed 479
embryonic organs, such as the colearrhiza, far example (Rath, 1955; Brawn,
1960; Eames, 1961; Foard and Haber, 1962; Negbi and Koller, 1962).
A provascular system can be distinguished within the embryo, and it has
been described in detail by Avery (1930).
References
AVERY, G. S., Jr. 1930. Comparative anatomy and morphology of embryos and seed~
lings of maize, oats, and wheat. Bot., Gaz: 89: 1-39. '
BROWN, W. V. 1?60. The morphology of the grass embryo. Phylomorpho!ogy 10:
215-234.
CORNER, E. J. H. 1949. The Durian theory or the origin of the modem tree. Ann. Bot.
N. S. 13: 367-414.
CoRNER, E. J. H. 1951. The leguminous seed. Phytomorph%gy 1: 117-150.
EAMES, A. J. 1961. Morphology of the Angiosperms. McGraw-Hill, New York.
EAMES, A. 1. and MACDANIELS, L. H. 1947. An Introduction 'to Plant Anatomy. 2nd
ed. McGraw-Hill, New York.
-ESAU;~K. 1953. Plant Anatomy. John Wiley, New York.
'" FOARD, D. E. and HABER, A. H. 1962. Use of growth characteristics in studies of morpho-
logic relations. 1. Similarities between epiblast and coleorhiza. Amer. Jour. Bot.
49: 520-523.
-GUTTENBERG, H. v. 1926. Die Bewegungsgewebe. In: K. Linsbauer. Handbuch der
Pjlanzenanatomie. B-d. 5: Gebr. Borntraeger, Berlin.
HAYWARD, H. E. 1938. The Structure of Economic Plants. Macmillan, New York.
MEIER; H. 1958. On the structure of cell walls aT}d cell wall mannans from ivory nuts
and from dates. Biochem. Biophys. Acta. 28: 229-240.
NEGBI, M. and KOLLER, D. 1962. Homologies in the grass embryo-a re-evaluation.
Phytomorphology 12: 289-296.
NETOLlTZKY, F. 1926. Anato~ie der Angiaspermen-Samen. In: K. Linsbauer, Hand·
buch der Pjlanzenanatom;'e, Abt. 2, Teil 2, Band 10, Gebr. 'Borntraeger, Berlin.
ROTH, 1. 1955. Zur morphologischen Deutung des Grasembryos und verwandter
Embryophyten. Flora 142: 564--600. , _ /
SARGANT, E. and ROBERTSON, A. 1905. The anatomy of the scutellum in Zea mays.
Ann. Bot. 19: 115-124.
SCOTT, F. M., BYSTROM, B. G. and BOWLER, E. 1962. Cercidium jloridum seed coat,
light and electron microscope study. Amer. JOllr. BOl. 49: 821-833.
SOUEGES, E. C. R. 1907. Deveioppement et structure du tegument seminal chez les
Solanacees. Ann. Sci. Nat., Bot., Ser. 9, 6: 1-124.
SZEMES, G. 1943. Zur Entwicklung des Elaiosoms von Chelidonium majus. Wiener Bot.
Zeit. 92: 215-219.
TROLL, W. 1948. Allgemeine Bo/anik. F. Enke, Stuttgart.
TROLL, W. 1954-7. Praktische Ein!iihrung in die Pjlanzenmorphologie. Vals. 1, 2.
O. Fischer, Jena.
476 . Plant Anatomy'
HYPOcotY'f~' Endosperm
~
co;,y~et~OIOPhYli T"lo COlyledoo-
Epicoty[ Cotyledon_ 3
o
Testo\ Silt-shaped
~~:~~~Tn~f
portion of
Endosperm
2
First foliage-i / '
\eof
Testa
f" ,
Houyonum
Cotyledon /
I
I
rpnmory
Sheathing \ \ .'M'
portion at,
Embryo
cotyledon 10
, . 8
Radicle
Umbiliform 6
structure
overlying the
embryo
7
I
'\ "
FIG. 219.1 and 2, Seedlings of Vida/aba.I, Longitudinally sectioned germinating
seed. 2, A young plant. 3-5, Seed and seedling of Allium cepa. 3, Longitudinally
sectioned seed. 4, Germinating seed sectioned longitudinally. 5, Developing seed-
ling. 6-11, Seed and seedling of Phoenix dactyli/era. 6, An entire seed. 7, Longi-
tudinally sectioned seed. 8, Germinating seed. 9, Mature seedling which has al-
ready produced foliage leaves. 10, Longitudinally sectioned seed which has ger-
minated and has already produced a mature seedling. II, Cross-section of seed-
ling at the same stage as in No. 10. (Adapted from Troll, 1948.)
Glossary of Terms 481
apical cell, the single initial present in the apical meristem of some roots and shoots;
typical of many lower vascular plants.
apocarpy, that condition in a flower and ovary where the carpels are free.
apomixis, proe'ess of reproduction in the ovule without fertilization.
apposition (of cell wall), wall growth as a result of successive addition, layer on layer, of
wall materia1.
aril, a ftes1_1y growth on the seed developing from the base of the ovule. Sometimes also
used to refer to outgrowths developing from other parts of the ovule.
asterosclereid, a branched sclereid.
atactosteIe, a stele which consists of vascular bundles scattered throughout the ground
tissue as- in the Monocotyledoneae.
autochory, seed dispersal by means of a self~dispersal mechanism.
axial organ, the root, stem, inflorescence Ot flower axis without their appendages.
back wall, that part of the guard-cell wall which is adjacent to the subsidiary or ordinary
neighbouring epidermal cells.
bark, a collective term f9f all the tissues outside the vascular cambium.
bark, outer, see rhytidome.
basipetal, proceeding towards the base.
'bif~c'ialleaf or dorsiventralleaf, a leaf in which palisade parenchyma is present on one
- s{de of the bJade and spongy parenchyma on the other.
body, primary, that part of the plant which develops from the primary meristems, api-
""--cal and intercalary.
body, 'secondary, that part of the plant, comprising the secondary vascular tissues and
periderm, which is added to the primary body as a result of the activity of the lateral
meristems, i.c'. the _cambium and phellog~n.
brachysclereid or stone cell, a short, more Or less isodiametric sclereid.
bulliform, cell, an enlarged epidermal cell common in the leaf of the Gramineae; rows
of such cells occur along the leaf,
bundle sheath, a layer or layers of cells surrounding the vascular bundles of leaves; may
conslst of parenchyma or sclerenchyma.
bundle sheath extension, a strip o(ground tissue present along the leaf v~ins and extend-
ing from the bundle sheath to the-epidermis; m'ay be present on both sides of the vein
or on one side only and may consist of parenchyma or sclerenchyma.
callose, a polysaccharide present in sieve areas, walls of pollen tubes, wans of fungal
celIs, etc.
callus, (1) a layer of callose which forms on sieve areas; (2) the tissue formed as a result
of wounding or a tissue developing in tissue culture.
calyptrogen; a term arising out of the histogen theory. In the root apex that meristem
from which the root cap develops independently of all other initials of the apical
meristem.
cambium, non-storied, a cambium in which the fusiform initials, as seen in tangential
section, partially overlap one another and are not arranged in horizontal rows.
cambium, storied, a cambium in which the fusiform initials, as seen in tangential section,
are arranged tn horizontal rows.
cambium, vascullr, a lateral meristem from which the secondary vascular tissues, i.e.
secondary xylem and phloem, develop.
cambium-like transitional zone, a cyto-histological zone visible in some shoot apices
(see pp. 55-59).
carpopbore, the split axis of a cremocarp to which the mericarps remain attached after
they become separated from each other.
478 Plant Anatomy
,\
cuticle, a layer of cutin, a fatty substance which is almost impermeable to water, on the
outer walls of the epjdermaJ cells.
(utic1e layer, the outer portions of the epidermal walls which are impregnated with cutin.
cutinization, the deposition of cutin in cell walls.
cylinder, central or vascular, that part of the axis of the plant consisting of vascular tissue
and the associated parenchyma. Equivalent to the term stele. but without the evolu-
tionary connotation' associated with this term.
cystolith, a specific outgrowth of the cell wall on which calcium carbonate is deposited.
'Characteristic of certain families, e.g. the Moraceae.
cytochimera, a combination, in a single plant organ, of tissues the cells of which are of
different chromosome number.
cytokinesis, the process of cell division which results in the formation of two separate
cells.
grollth, intrusive, that type of growth in which the growing cell penetrates between exist-
ing cells and in which new areas of contact are formed between the penetrating and
neighbouring cells.
growth, mosaic, a theory concerning primary wall growth-(see p; 30).
growth, multinet, a theory concerning primary wall growth (see p. 30).
growth, symplastic,_the process of uniform growth of _neighbouring cells so that the
adjacent walls do not alter position relative to each other and no new areas of contact
are formed.
growth ring, a clearly distinguishable region in the secondary xylem or phloem which
:is formed during a single growth season.
guard cells, a pair of specialized epidermal cells which, together with the aperture be-
tween them, form the stoma.
gum, a general term for the substances formed on the disintegration of cells, mainly
carbohydrates.
gummosis, a pathological condition which is expressed by the formation of gum.
guttation, the secretion of water, in a liquid form, from plants.
gynoecium, all the carpels of a single flower.
gynophore, an elongation of the floral axis between the stamens and _the carpels, thus
forming a stalk which 'elevates the gynoecium.
-_haplostele, a protostcle in which the xylem, as seen in crossMsection, is more or less cirM
cular.
~hacdwood, a common name for wood of the Dicotyledoneae.
haustorium, a specialized organ that draws nutriment from another organ or tissue .
. heartwood or duramen, the inner layers of wood in the growing tree or shrub, which have
lost the ability to conduct and no longer contain living cells. Generally darker in COM
lour. than sapwood. '1
hilum, (1) that portion of a starch grain around which the starch is laid down in layers;
(2) the scar present on a seed resulting from its abscission from the funiculus.
histogen, a term used to refer to those initials in the apical mcristem of the root and shoot
which are predestinated to give rise to a particular and constant tissue system of the
organ concerned. Hanstein distinguished three histogens: dermatogen which gives
rise to the epidermis; periblem which gives rise to the cortex; plerome, which gives rise
to the vascular cylinder.
hydathode, a structure sometimes of glandular nature, through which water is secreted
in liquid form; found mainly on leaves.
hygrochastic process, manner of fruit opening or of the movement of other organs,
as a result of water uptake; usually connected with the dispersal of seeds or spores.
hypoblast, a term used for the suspensor of the mature grass embryo.
bypodermis, a specific layer or layers of cells beneath the epidermis, which differ strUCM
turally from the tissue below them. In the narrow sense of the term, refers only to
such layers which arise from a meristem other than the protoderm.
hypophysis, one of the cells of the embryo in its early stages of development (see p. 432).
bypsophyll, an inflorescence bract; a vestigial leaf or any other leaf having a structure
different from that of a foliage leaf and occurring near the top of the shoot.
idioblast, a specific cell which is clearly distinguished from the other cells of the tissue in
which it appears, either by size, structure or content.
initial, (1) in meristems;-a cell which remains within a meristem and which adds cells
to the plant body as a result of cell division; (2) of an element; a meristematic cell
which differentiates into a mature specialized element.
integument, an envelope surrounding the nucellus of the ovule.
A82 . Plant Anatomy
epidermis, the outermost cell layer of primary tissues of the plant; sometimes comprising
more than one layer-multiseriate epidermis. t
epipetalous stamen, a stamen which is adnated to a petal.
epithem, the' tissue between the vein ending and the secretory pore ora hydathode.
ergastic matter, the non-protoplasmic products of metabolic processes of the proto-
plasm; starch grains, oil droplets, crystals and certain liquids; found in the cytoplasm,
vacuoles and cell walls.
eustele, phylogenetically the most advanced type of stele, the vascular tissue of which
forms a hollow reticulate cylinder built of collateral or bicollateral vascular bundles.
exalbuminous seed, a seed which is devoid of endosperm when mature. ~
exarch xylem, in reference to the direction of maturation of the elements in a strand of
primary xylem; a strand in which the first-formed elements (the protoxylem) are fur-
thest from the centre of the axis, i.e. the maturation is centripetal.
exine, the outer wall of a mature pollen grain.
exocarp, the outermost layer of the pericarp (fruit waH); also termed epicarp.
exodermis, in some roots the outermost layer or layers of cells of the cortex, the struc-
tUIe of which i'3 ':,\mihI to that of the enUOUe1'ID\'5, Le. the ce\\ wa\\o:. ale more 01' \e'5'5
thickened and contain suberin lamellae. A type of hypodermis.
exogenous, ,developing from external tissues.
gametophyte, that plant generation which gives rise to the gam~tes from which, after,,-
fertilization, the sporophyte develops. ./ ~
gap, branch, in a siphonostele the parenchymatous region in the vascular cylinder above
the position where a branch-trace enters a branch.-
gap. leaf, in a siphonostele, the parenchymatous region in the vascular cylinder above
the position where the leaf-trace enters a leaf.
germination, epigeal, the process of germination in which the cotyledons and epicotyl
emerge from the seed as a result of the elongation of the hypocotyl.
germination, bypogeaJ, the p,rocess of germination in which the hypocotyl elongates
very little or not at all and the thick cotyledons, which store reserve materials, remain
within the testa.
gonophyll, according to the gonophyll theory, a sterile leaf together with an ovule-bear-
ing branch from which it i:"l assumed that the carpel has been derived.
growth, gliding or sliding, that type of growth in which the walls of neighbouring cells
slide over one another. -
48" Plant Anatomy
acuna, (1) an intercellular space; (2) an interruption in the vascular tissue of the central
cylinder.
laticifer, a cell or row of cells containing latex, a substance specific to such cells.
lenticel, an isolated area in the periderm consisting of suberized or non-suberized cells
with numerous intercellular spaces between them.
leucoplastid, a plastid devoid of pigment.
lignin, a mixed polymer containing phenolic derivatives of phenyl_propane. Commonly
found in secondarily thickened cell walls. /
lithocyst, a -cell containing a cystolith.
Iysigenous, the manner of formation of an intercellular space as a result of the disintegra-
tion of cells.
/
maceration, the artificia1 separation of the individual cens of a tissue by disintegration
of tbe middle lamella,
macrosdereid, a sorrrewlril( elongated sclereid the secondary wall of which is un~uan,.
thickened. Common in the seeds of Leguminosae where they represent the epidermis
of the testa and where they are termed Malpighian cells. /
.Malpighian cell, see macrosclereid.
mantle, all'those"outer cell layers of the shoot apex of the Angiospermae which can be
distinguished by their layered arrangement from the cells of the inner portion of the
shoot apex.
massula, a large group of adherent pollen grains, which participates in the formation
of a po11inium, ' /
matrix, a substance in which another substance is deposited or embedded.
median, situated in the middle. - f
megaspore, the femalespore from which the female-gametophyte develops; also called
macrospore.
meristele, one of the bundles of a dictyostele; see also vascular bundle.
meristem, a tissue which produces cells that undergo differentiation to form mature
tissues.
meristem, apical, a meristem situated in the apical region of the shoot or root which, as
a result of divisions, gives rise to those cells which form the primary tissues of the shoot
or root.
meristem, flank, one of the cyto-histological regions of the shoot apex (see pp.53-57).
meristem, ground, a meristematic tissue which originates in the apical meristem and
which produces tissues other than epidermis and vascular tissues.
Glossary of Terms 487
meristelfl, intercalary, meristematic tissue derived from the apical meristem and which
becomes separated from the apex in the course of develoment of the plant by regions
of more or less mature tissues.
meristem, lateral, a meristem which is situated parallel to the circumference of the plant
organ in which it, occurs.
meristenl, plate, a parallel layered meristem in which the planes of cell divisions in each
layer are perpendicular to the surface of the organ which is usually a fiat one.
meristero, rib, (1) one of the cyto~histologka\ regions of the shoot apex (see p. 53), (2) a
meristemcharacterized by parallel series of cells in which transverse divisions take place.
mesarch xylem, in reference to the direction of maturation of elements in a strand of
primary xylem; a strand in \yhich the first-formed elements (the protoxylem) occur in
the'centre of the strand; i.e. the maturation is both-centripetal and centrifugal.
mesocarp, the middle layer of the pericarp (fruit wall).
mesocotyl, often refers to the internode betwen the scutellar node and the coleoptile in
the Grarnineae.
mesomorphic, having structure characteristic of mesophytes.
mesophyll, the photosynthetic parenchymatous tissue situated between the two epider..
mal layers of the leaf.
mesophyte, a plant suited to a fairly and continuously moist climate.
metaphloem, that part of the primary phloem which undergoes differentiation after the
. protophloem.
-metaxylem, that part of the primary xylem which undergoes final differentiation after
the protoxylem.
;micella, the present usage refers to a unit of cellulose in which the molecules are ar-
ranged parallel to one another so that the atoms form a crystalline lattice structure.
microfibril, a submicroscopic thread-like constituent of the cen wall; composed in most
plants of cellulose molecules.
micropyle, the opening at the free end of the ovule, between the integuments.
microspore, the male spore from which the male gametophyte develops.
microsporocyte, a cell which differentiates into a microspore.
middle lamella, the lamella present between the walls of two adjacent cells.
mimchQ.tldriotl (Qlutal mit(lch_(lo._d(ia)~ a ve(~ small QrctoQlas.mic bod~ in thec~toQ-las.m;
contains enzymes involved in respiration.
morphogenesis, the total expression of the ~orphological phenomena of differentiation
and development of tissues and organs.
mother cell, a cell which gives rise to other cells as a result of its division.
multiple (of vessels), referring to the arrangement of vessels as seen in cross~section of
the secondary xylem; a group of two or more vessels arranged in radial, oblique or
tangential rows.
mycorrhiza, the symbiosis between fungi and the roots of higher plants.
myrosin cell, a cell containing myrosin; present in the vegetative parts and seeds of
certain Cruciferae.
obturator, an outgrowth of the placenta or stylar canal which brings the transmitting
tissue closer to the micropyle.
488 Plant Anatomy
paracytic type (of stomata), a specific pattern of arrangement of the guard cells and other
epidermal cells adjacent -to them (see p. 150).
parenchyma, a ground tissue composed of living cells which may differ in size, shape and
wall structure_
parenchyma, apotracheal, axial parenchyma of the secondary xylem, typically indepe~:
dent of the vessels although may occasionally be in contact with them. Divided into
the following types according to the distribution as seen in cross-section of the secon-
dary xylem: banded or metatracheal-concentric uni- or multiseriate bands, arcs or
entire rings; diffuse -single cells distributed irregularly among fibres; initial- bands
of parenchyma produced at the beginning of a growth ring; terminal-bands of
parenchyma produced at the end of a growth ring.
parenchyma, axial, parenchyma of the vertical system of secondary xylem, i.e. paren-
chyma cells derived from fusiform cambial initials.
parenchyma, paratrache31, axial parenchyma of the secondary xylem associated with
the vessels or vascular tracheids. Divided into the following types' according to the
distribution as seen in cross-section: aliform-paratracheal -parenchyma which is
expanded tangentially in the form of wings; conflue:at-groups of aliform parenchy-
ma which become corttinuous so as to form irregular tangential or diagonal bands;
scanty-an incomplete sheath or a few parenchyma cells present around the vessels;
vasicentric-parenchyrna forming an entire sheath of variable width around indi-
vidual vessels or groups of vessels. /
parenchyma, wood, see parenchyma, xylem.
parenchyma,-'xylem, parenchyma occurring in the secondary xylem, usually in two sys-
tems: (I) axial, and (2) radial (ray parenchyma). /
parthenocarpy, the production of fruit without fertilization.
pectic compounds, a group of polymers of galacturonic acid and its derivatives; occurr-
ing in three types of compounds -protopectin, pectin and pectic adds. Constituting
the most important component of the middle lamella.
perforation (in stele), interruption in the vascular tissue of a siphonostele, other than
leaf- or branch-gap; developed as a- result of secondary reduction.
perforation plate, that portion of the cell wall of a vessel member which is perforated.
The following types of perforation plates are distinguished: (l) foraminate perforation
plate-a plate with numerous, more or less circular perforations; (2), reticulate
perforation plate -a plate in which the remnants of the wall between the perforations
form a net-like structure; (3) scalariform perforation plate-a plate' with numerouS
elongated pores which are arranged parallel one near the other; (4) simple perforation
plate-a plate having one large perforation. /~
peribtem, see b.\st~ge\\..
pericarp, the fruit wall. ___ -
periclinal, parallel to the surface.
pericycle, that portion of the ground tissue of the vascular cylinder between the conduct~
ing tissues and the endodermis.
periderm, the secondary protective tissue which replaces the epidermis; consists of
phel/em, phellogen and phe/{oderm. .
perisperm, a nutrient tissue of the seed, similar to the endosperm, but of nucellar
origin.
pheIJogen, the cork cam bium; a secondary lateral meristem which produces the phellelll
and phelloderm.
phloem, the principal tissue responsible for the transport of assimilates in the vascular
plants; consists mainly of sieve elements, parenchyma cells, fibres and sclereids.
Glossary of Terms 489
phloem, interxylary, secondary phloem which occurs within the secondary xylem as is
the case in certain Dicotyledoneae.
phloem, intraxylary, primary phloem occurring on the inner side of the primary xylem.
phragmoplast, a fibrous' structure which appears during mitotic telophase between the
two daughter nuclei; participates in the formation of the cell plate which divides the
mother cell into two.
phyllome, a collective term referring to all types of leaves.
phylogeny, the history of a species or a' larger taxonomical group from an evolutionary
viewpoint. I
piliferous cell, see trichoblast.
pit, a depression in a cell wall with secondary thickening; in such an area only primary
wall and middle lamella are present.
pit, bordered, a pit ir which the aperture in the secondary wall is small and conceals
below it a dome-shaped chamber which is situated above the pit-membrane.
pit cavity, the cavity of a single pit extending from the pit membrane to the aperture
bordering the cell lumen.
pitmembrane, the middle lamella and primary wall closing the pit cavity on its outer side.
pit-pair, two complementary pits of neighbouring cells.
pit, vestured, a bordered pit having projections, which may be.simple or branched, on
that part o(the secondary wall which forms the border of the pit chamber or the pit
_ aperture; found in certain Dicotyledoneae. .
-pith, the ground tissue in the centre of the stem and root.
pitting, the type and arrangement of pits in the cell wall.
~placenta, the region of attachment of the ovules to the carpel.
placentation, the position of the placenta in the ovary.
plasmalemma, the membrane on the outer surface of the cytoplasm; adjacent to celI
walLAlso known as ectoplast.
plasmodesma, a thin, cytoplasmic strand which passes through a pore in the cell wall,
and which usually connects the pro top lasts of two adjacent cells.
plastid, a protoplasmic body separated from the cytoplasm by a membrane; fulfilling
a definite function.
piastochron, that period of time between the commencement' of two successive and repe-
titive phenomena, for example, between the initiation of two successive leaf primordia.
plectostele, a protostele in which the xylem is arranged in longitudinal plates which may
be in terconnected.
plerome, see bistogen.
~plumule, the bud or shoot apex of the embryo.
pneumatode, in a velamen, a group of cells with very dense spiral wall thickenings;
enables gas exchange when the root is saturated with moisture.
pneumatopbore, an aerial, negatively geotropic root projection serving for gas exchange;
produced in swampy habitats.
pollen tube, an elongated projection, covered only by intine, of the vegetative cell of a
pollen grain.
pollinium, the entire pollen grain complement of a single pollen sac, when the grains
adhere together to form one mass.
polyarch, the primary xylem of root in which the number of protoxylem strands is large
(usually more than five).
polyderm, a special type of. protective tissue composed of alternating bands of endo-
dermis-like cells and -non-suberized parenchyma cells.
polyembryony, the presence of more than one embryo in an ovule.
primary pit-field, a thin portion of the primary waH in which the pores, through which
plasmodesmata pass, are concentrated.
primordium, an organ, cell or organized group of cells in the earliest stage of differen-
tiation.
Glossary oj Terms 491
raphe, a ridge along the seed formed by that part of the funiculus which was fused to the
ovule. \
raphide, a needle-shaped crystal usually occurring in dense bundles.
ray, phloem, that part of the vascular ray which passes through the secondary phloem.
ray, pith, an interfascicular region in a stem. /
ray, vascular, a strip of tissue running radially thrDugh the secondary xylem and
phloem; "formed 'by the cambium. /
ray, xylem, that part of the vascular ray which passes through the secondary xylem.
replum, the ridge surrounding the siliqua of the Cruciferae which 're-main's attached to
the false septum, as a frame, on the dehiscence of the fruit. / .
rhytidome, that part of the bark comprising the periderm and tissues external to it
which are cut off by it. Also called outer bark. '
ribosome, a minute protoplasmic body in the cytoplasm playing an important role in
protein synthesis.
ring-porous wood, see-wood.
rod cell, see macrosclereid. /
root-cap. a- thimble-shaped structure which covers the root"apex.
root, contractile, a special root which has th-e ability to contract and thus bring the
developing renewal buds to a definite position in relation to the soil surfaCe.
root hair, a type of trichome developing on the epidermis of roots; absorbs solutions
from the soil.
/
sapwood or alburnum, that portion of the wood that in the living tf;e and shrub contains
living cells and reserve materials. _____ 'l()
scalariform, the parallel arrangement, One near- the other, of elongated structures in
the cell wall of an element.
schizogenous, the manner of formation of intercellular spaces by the separation of cells
along their middle lamellae.
schizo-Iysigenous, the manner of formation of intercellular spaces by both cell separation
along their middle lamellae and cell disintegration.
sclereid, a sc1erenchymatous cell of variolls shape. but usually not much elongated; has
thjck, Jjgnified secondary wall which often contains many pits.
scierenchyma, a supporting tissue composed of fibres and/or sclereids.
sclerification, the process of changing into sclerenchyma by the formation of secondary
walls. .
492 . Plant Anatomy
tapetum. the innermost layer of the pollen sac wall; the contents of its cells are absorbed
by the pollen grains during their development.
tapetum, amoeboid, tapetum in which the protopiasts of its cells pedetrate between the
pollen mother cells "and developing pollen grains; glandular, tapetum in which the
cells remain in their original position until their disintegration.
telome, the ultimate terminal portion of a dichotomously branching axis bearing a spo-
rangium, (fertile telorne) or lacking a sporangium (sterile telome). According to the
Ie/ome theory the most primitive vascular plants were composed entirely of telome
systems.
testa, the seed coat.
tetrarch, the primary xylem of root in which the number of protoxylem strands is four.
tissue, complementary, see cells, complementary.
tissue, conjunctive, (I) a special type of parenchyma associated with included phloem
in Dicotyledoneae with anomalous thickening; (2) the parenchyma present between
the secondary vascular bundles of MonocotyJedoneae with secondary thickening.
tissue element, each individual cell of a tissue.
tissue, expansion, an intercalary tissue in the outer portion of the inner bark formed
mainly by the phloem rays; accommodates the expansion in circumference.
iissue, ground, all the mature plant tissues except the epidermis; periderm and vascular
tissues.
tissue, mature, a tissue which has undergone differentiation.\
tissue, mechanical, a tissue comprised of celis, the walls of which are more or less thick·
eoed; such tissues give support to the plant body. Also referred to as supporting tissues.
tissue, proliferation, a tissue which develops from phloem parenchyma in the outer pOf·
tion of-the inner bark aceomodating the expansion/in circumference.
tissue, transfusion, that tissue which, in the leaves~o{ the Gymnospermae, surrounds or
is associated in some other way with -the vascular bundles. Comprised of dead tra·
cheids and living parenchyma cells.
tissue, transmitting, a tis~ue in the style similar to the tissue of the stig91a in both strue·
ture and physiological properties; connects, the stigma and the inside of the ovary.
tonoplast, the cytoplasmic membrane whieh borders the vacuole.
torus, the thickened central portion of- the pit membrane in a bordered pit.
trabecula, a rod·like projection of the cell wan which· crosses the cell lumen, usually in
a radial direction.
trace, branch, that part of a vascular bundle in the stem extending from the position
where it joins the vascular system of the branch to where it joins the vascular system
of the majn stem.
trace, leaf, that part of a vascuJar bundle/ in the stem from the position where it enters
the leaf to where it joins the vascular system of the stem.
trachea, see fessel. \
tracheary element, that type of xylem element which takes part in water transport. \.
Classified into two types-tracheid and vessel member. ;' I ..
tracheid, a tracheary element of the xylem, which unlike the vessel member, is not
perforated. ---v;;:
triarch, the primary xylem of root in which the number of protoxylem strands is three.
trichoblast, a specialized cell in the root epidermis which gives rise to a root hair.
trichome, an epidermal appendage; may be of various shapes, structures, size and func·
tion; includes hairs, scales, etc. /
tunica, the outermost layer or layers in the apical shoot meristem of the Angiospermae
in which the plane of division is almost entirely anticlinal.
tylosis, an outgrowth of a ray cell or of an axial parenchyma cell into the lumen of a
vessel; such outgrowths partially or completely block the vessels.
tylo'loid, a prolif.eration of an epithelial cell into an intercellular cavity such as resin or
gum duct.
Glossary of Terms 493
unifacialleaf, a leaf in which the structure of both sides is alike and in which each side
forms a mirror-image of the other side. Ontogenetically, a leaf which develops from a
centre of growth situated on one of the sides of a leaf primordium.
l'3cuoiation, the shape, amount and size of the vacuome. Also the process of forming
vacuoles.
vacuole, a cavity in the cytoplasrTl containing an aqueous solution, the cell sap.
vacuome, the collective term for all the vacuoles of a single cell.
vascular, an adjective referring to the xylem or phloem or both.
vascular bundle, a strand of conducting tissue. The following types of vascular bundles
are recognized: (1) bicollateral vascular bundle-vascular bundle in which phloem
is present both on the outside and inside of the xylem; (2) collateral vascular bundle
-vascular bundle in which phloem is present on one side of the xylem only, common-
ly external to it; (3) concentric ~ascular bundle-vascular bundle in which the phloem
surrounds the xylem (i.e. amphicribral) or the xylem surrounds the phloem (i.e.
amphivasaJ).
vein, a strand of vascular tissue in a flat organ, such as a leaf.
velamen, the multiseriate epidermis present on the aerial roots of some ropical epiphytic
species of the Orchidaceae and Araceae; also present on some terrestrial roots.
venation, the arrangement of the veins in the leaf blade.
~ vessel, a series of vessel members joined end to end by their perforated end walls.
Also termed trachea.
_vessel member, a tracheary elemeflt, one of the cells of which a vessel is comprised.
wall, general, the gelatinous wall which at first surrounds the pollen'mother cell and
then later the developing tetrad.
waUlayer, terminal, see wall layer , tertiary.
wall layer , tertiary, according to 'some authors, a layer present on the inside of the inner
layer of the secondary wall.
wall, special, the first wall formed which separates each of the pollen grains in the tetrad.
\'II\\tt Su\\~t\\te, 'S.maU granules. ocCurring em the tnner 'S.urface of the s,ocondary wall of
tracheids, fibres and vessels.
wood, compression, the reaction wood of the Coniferae, formed on the lower side of bent
or leaning trpnks and. branches.
wood, diffuse-porous; the $ecoiidary xylem of a single growth ring in which,the vessels
are more or less uniformly distributed or in which the diameter of vessels alters only
slightly across the growth ring.
wood, reaction, the secondary xylem ""ith special structure, produced in those parts of
trunks and branches which leaJ1 or are bent; apparently tends to return these organs
to their original position. Tension wood in the Dicotyledonae; compression wood in the
Coniferae.
wood, ring-porous. secondary xylem of a single growth ring in which the vessels pro·
duced at the beginning of a growth season are significantly larger than those produced
at the end Qf the &eaSQn.
wood, tension, the reaction wood of the Dicotyledoneae formed on the upper side of
leaning or bent trunks and branches.
woody, an entire plant or a plant organ with well developed secondary xylem.
Haber, A. H. 479,479
Haberlandt, G. 2, 6, 80, 84, 87, 95, Kallen, F. 91,101
101,130,136,141,207,221,237,274, Kaufman, P. B. 67, 71, 227, 230, 232,
284 233,237
Hammer, K. C. 161 Kavaljian, L. G. 65,66,71
Handley, R. 258,270 Kemp, M. 52, 53, 71
Handley, W. R. C. 106, JI7, 303,323 Kerr, T. 22,25,30,34,41,42
Hanstein, J. 7,46,56,65,70,120,429, Kersten, H. 106,117
436 Kessler, G. 120,129
496 Author Index
Blinning, E. 150,151,159,160 Day, A. C. 86,100
Burkholder, P. R. 283 De Bary, A. 2,5, 13/, 135, 137-139,
Burmeister, J. 70 141,145,160,169,191
Buvat, R. 34,42,44,59,60,69 De Candolle, A. P. 360, 395, 435
Bystrom, B. G. 271,479 De Fraine, E. 205,236
Dembo, N. 145,148,160
Dermen, H. 47,60,70,225,237
Canright, J. E. 382,435 De Robertis, E. D. P. 10,42
Carlquist, S. 5 Dittmer, H. J. 158,160,260,269
Carlson, M. C. 256, 269 Djaparidze, L.1. 98,100
Caspary, R. 415, 435 Douglas, G. E. 376,378,379,435
Catesson, A. 275, 284 Douglass, A. E. 313,323
Chalk, L. 78,79, 109,117, 142, 149, Douliot, H. 169,192
153, 158, 161, 169, 192, 219, 238, Duchaigne, A. 81,83,84
262, 270, 313, 320,323 Duckworth, R. B. 83,84
Chaloner, W. G. 146, 160 Duval-Jouvc, 1. 204,237'
Chan, A. P. 57,61,71,361,43,
Chattaway, M. M. 109, 117, 287,
323, 335, 341, 348 Eames, A. J. 2,5,6,8,67,70,78, 139,
Cheadle, V. I. 108, 112-114, 117, 160,169,171,173,191,234,237,269,
119, 124-126, 129, 330, 356, 358, 312,313,315, 3J7, 323, 332, 335, 339,
358 342,344,346,348,358,367,370,371,
Chouard, P. 356, 358 373,374,376, 378, 380, 386, 395, 435,
Chute, H. M. 380, 435 464,497,468,- 469, 472, 477, 479, 479
Clarke, S. H. 320, 323 Epprecht, W. 129
Clements, F. E. 202; 203, 239 Erdtman, G. 389-391,394,435
Clements, H. 460, 464 Esau, K. 2, 4, 6, 34, 36, 42, 54, 70
Clowes, F. A. L. 47,64-68,69,247, 78, 80, 84, 86, 91, 101, 109, 110,
269, 354 358 117,119-121,124,125; 127,128,129,
.c Clowes, H. C. 236 135,160, J66, 169, 173, 184, 191, 233,
Cockerham, G. 282, 284 237,244, 255, 256, 258, 264, 268, 269,
Cormack. R. G. H. 159.160,243,269 311,323, 325, 328-330,330,358, 384,
Corner, E. J. -H. 363,- 435, 466, 469, 435,~464, 478
479 Evenari, M. 188,189,191,263,269
Cossmann, K. F. 243, 269 Evert, R. F. 275, 284, 328,330, 337,
Cote, W. A. 86,100 348
Coulter, J. M. 203,236
Crafts, A. S. 124, 129, 184, 191, 327
330 Facey, V. 235,236,237
Creighton, H. B. 283 Fahn, A. 52, 59-6t,' 70,74,78, 87, 92, ,,
Crooks, D. M. 265,267,269 101,108,116,117,·120,129,142,145,
Cross, G. L. 53,69,225,236 148,157,160,169,175,178,187,189,
CrUger, H. lO6,117 191,~193, 197, 204, 215, 237, 280-282,
Cull,J. M. 451,464 284,285,298,313,314,323,324,329,
Currier, H. B. 119,129 330, 356, 358, 361, 413-415, 422,
Cusick, F. 362, 4JS 436, 442, 444, 446, 449, 464, 465
Cutter, E. G. 52,69,162,191 Fahnenbrock, M. 38,42
First, T. 70.436
Fisk, E. L. 71
Dadswell, H. E. 86,101,317-319,322, Flint, L. H. 148,160
324 Floresta, P. la 342,348
Daumann, E. 413,435 Florin, R. 147,148,160
Davey, A. J. 260, 261, 269 Foard, D. E. 479,479
Davies. G. W. 41. 43. 317. 324 Ford, E. S. 458,464
Author Index 499
MacArthur, M. 460,464
MacDaniels, L. H. 2,6,8,67,70,78, Niigeli, C. W. 28,47,71, 118
139,160,169,171,173,191,234,237, Nast, C. G. 173, 192, 216, 236, 382,
269,312-315,317,323,332,335,339, 391,434
342,344,346,348,359,367,370,373, Negbi, M. 479,479
374,376,378,380,435,437,458,460, Nelson, P. E. 344,348
461, 464, 467, 468, 469, 472, 479 Netolitzky, F. 467-469,474,479
Maclachlan, G. A. 269 Newman,1. V. 274,285
MacMasters, M. M. 451,464 Nightingale, O. T. 458,464
Author Index
,
" R. B. 202,204,215,217,218, Ziegenspeck, H. 151,161
Zimmermann, J. G. 413,438
Zimmermann, M. 422,438
Zodda, G. 356,359
polsky, C. 422,438 Zohary, M. 186, 192~63, 271, 442.
19,J.O. 460,465 444,446,449,464,465
SUBJECT INDEX
Numbers in italics indicate that emphasis is placed- on the subject. Numbers in bold
, type indicate that the subject is cited in a figure or in a legend to a figure
508 Subject In
Cell wall (cont.) Chimonanthus, 174
primary. 22, 23, 25, 27, 28-30, 29, Chitin, 29
34,36,75,77,287 Chloranthaceae. 112, 113, 178
secondary, 23-26,24,26,27,28,29, Chlorenchyma, 77, 165
3D, 31, 32, 36, 37, 37, 40, 85, 95, Chlorophyll, 13-15,189,244
103,287 Chloroplast, to, 11, 12, 12-15, 17
in secondary xylem, 314 80,142,186,189,197,199,200,
in sieve elements, 124 213,215,218,224,244,332,333
structure of cellulose, 28,29 Chromatin, 10
tertiary, . 24 Chromonemata, 10
Cellular endosperm, 425,426 Chromopiast, 8, to, 11,13, 15, 45E
Cellulose, 23, 25, 26, 28, 29, 33, 82, 83, Chromosome, 10,47
140, 141, 144, 166, 245, '246, 314, Chrysanthemum
319,333,389,469,474 anethi/oNum, 62, 176
a-cellulose, 86 indicum, 139
crystals, 31-33,444,446 leucanthemum, 376
microfibrils, 26,29;33,34,'318 mori/olium, 59
orientation, 30-33 Cicer, 432
structure, 28,29 ariefinum, 433
Celtis~ 248 Cichorium, 132,163,449
Centaurea, 197,394 pumilum, 446
Central'cylinder (see 'also Vascular cy- Circumscissile dehiscence of fruit,
linder), 263 Cistaceae, 309
Central meristem (in shoot apex) (see Cistus, 414
also Rib meristem), 53 . villosus, 417,419
Central mother cells (in shoot apex), Citrus, 76,148, 2 to, 219, 220, 243,
53,54,55,55, 56,57,58, '58,59-61 414, 419, 434, 441, 458, 458.
Centranthus, 414 limon, 418,419
macrosiphon, 424 Clematis, 341
Centrospermae, 406 Climbing root, 240
Ceratonia, 220; 280, 298, 301, 307, 322, Closing layer (in lenticel), 345,
414,422,474 Closing membrane (see also Pit n
siliqua, 280,467 brane), 36
Ceratophyllum, 205 Cluster type of vessel arrangement,
submersum, 211 Cochlearia armoracia, 81
/
Cerddiumfloridum, ...,.,.469 Cocos, 441
Cerds, 321,322 Coenocyte, 9
siliquastr~m, 229,297,308,414 Coffea, -"'77,441
Chalaza, 370,403,405,425 - " . . . . . - Cohesion-(fusion in flower}, 360,
Chalazal . ,.'/ _....,/ 376,376
'chamber, 426 Cohesion mechanism (in organ m
haustorium, 426 ment), 449
pole; 410,412 Cohesion tissue, 447,448,449
region, 424 Colchicine, 47,60, 225
Chamaerops hurnilis, 147 /
Colchicum, 145,414
Chelidonium rnajus, 466,473 rilchii, 418,418
Chenopodiaceae, 67,92,118,138,153, steveni, 261
169, 177,178, 186, 187: 220,260, ,Coleoplile, 251,268,431,477,478
274, 321, 329, 351, '352, 354 'Coleorrhiza, 251, 431, 477, 478,
Chenopodial type of embryo develop- Coleus, 48,81,106,235,256
ment, 429 , b/umei, 50,58
Chimera (see also Cytochimera) Collenchyma, 3, 45, 73, 80-84, 8:
- polyploid, 47 333
variegate-d,
, 47 angular, 81,82,82
Subject Index 509
annular, 81 Corpus,' '-.!7, 225
in fruit; 442,455, 458, 46~ Cortex (cortical tissue), 3,4, 5, 45, 46,
lacunar, 81 169, 172'-175, 181, 182, 241, 256,
lamellar, 81,82 286;339
in leaf, 81, 128, 209, 212, :218, 234 collenchyma, 46
in root, 244 in desert plants, 189
in stem, 81,164,165 fleshy, 187; 188
Colleter, 156 ~or~gin, 59 .
Colour substances (in wood), 288,< 289 In root, 45, 74, 243-245, 245, 247,
Calpa (in pollen grain), 394' 252, 255, 255, 258, 259, 260, 261,
Columella, 63,64,65,242 261
Commelina communis, 150 sclerenchyma, 46, 243
Commelinaceae, 151,243 in stem 44,45,74,95,187,243
Common bundle, 178,179 Cortical bundle, 169
Companion cell, 118,121,126-128,327 Cortical root, . . . 241
Complementary cell, 34S~ 346, 347, 347 Corypha talieri, 147
Compositae, 76, 82, 118, 130; 132, 135, Cotoneaster, 414
154, 167, 219, 243, 361,371, 394, dammeri, 256
398, 414, 440, 449, 453: 455, 468 Cotyledon, I, I, 34, 162, 177, 193,
Conduplicate carpel, 395 264-266, 266, 269, 424, 429, 432,
Coniferae (see also Conifers), 76 474, 475, 476,477 ______
Coniferales, 38, 105, 120, 124, 148, Cotyledonary bundle, 268
219,225,252,290 Cotyledonary node, 265, 267, 268
Conifers (Coniferae), 115, 142, 203, Cotyledonary trace, 265-267
221,247,275,277,278,282,290 Crambe, 440
apices, 52 Crassula, '40,290, 298
resin ducts, 292 Crassulaceae, 220,221,'309; 376
wood (secondary xylem), 290, 317 Crataegus, 124
Connecting strands (in sieve areas), azarolus, 300
1/9,120,123,125,126 Cremocarp, 441,453,452-454
Connective, 361,382 Crepis, 427
Contlactile root, 260-262,261 Cross-cell, 451,452
Convallaria majalis, 168 Cross field, 291
Convolvulaceae, 118, 132, 150, 167 Cross zone, 366
Convolvulus, 132 Crucifer type of embryo development,
Corchorus, 86,92 427,429
capsularis, 93, 94 Cruciferae, 39,78, 144, 15:1,219,221,
Cordaitales, 177 244,397,414,440,468
Cordyline, 168,252,343 Crypsis schoenoides, 143
Cork, 189, 331, 333, 335, 337, 338, Cryptomeriajaponica, 56
340,343,348 Cryptostegia, 135
cambium, 273,331 grandi/loya, 131
cell in Gramineae, 141, 144, 159- Crystals, 17-19, 18, 143, 188, 189,
cell in phellem, 332,341 214, 215, 221, 287, 327,328, 358,
commercial, 342, 348 419
in fruit, 460 substances and types, 11,18
interxylary, 351 Crystal chamber, 287,328
in root, 337 Crystal sand, 18
storied, 338,343 Cucumis, 441
tissue, 137, 189, 331, 335, 339, 348 Cucurbita, 81,86, lOS, 119, 122, 123,
wound. 343 ' 123,183,399,401
Comus, 331, 380 pepo, 394
Corolia, 360, 380 Cucurbitaceae, 118,144,149,167-169.
sympetalous, 371 256,380,414
510 Subject Index
Cupressaceae, 252,291,327 Darbya, 379
Cupressjnociadus, 146 Datura, 399,401
Cupressus, 293,321,341 Daucus, 248, 261, 264, 265
sempervirens, 293,326 carota, 256
Curcuma, 343 Degeneria, 382,395,397
Cuticle, 139, 140, 141, 142, 144, 147, vitiensis, 384
148, 156,159,334 Degeneriaceae, 395, 398
in fruit, 140,452,456,460 Dehiscence (in fruit), 439,440,442-449
layer, 140,141 in capsule, 446
in leaf. 140, 196, 203, 205, 206, 218, in fo11icle, 442
221 in legume, 444
in nectary. 416,417,419 Delphinium, 439,442,444
in ovule: inner, 406; middle, 406; Dermatogen, 56,65
\)\)\'eT, ~% DelYfIiX',1na:rn Mpir.tftfl1n, 1.....,
in petal, 381 Dextrins, 125
proper, 140 Diacytic (caryophyllaceous) type of sto-
in root, 140 ma, 149,'151
in seed, 467,469,471 Dianthus, 142,149,414
in stamen, 382 . caryophyllus, 176,197
in stem, 140, 189,334, 342 Diaphragm, 207,210
in style, 399,403 Dichotomous branching, see Branching
Cuticularization, 140 Dicotyledons,S, 19, 83, 86, 92, 93,95
Cutin, 17,23,25,29,140-142,144,389, bundle sheath, 215
468 cambium, 273
Cutinization, 140,203,242,471 cell wall, 38, 41
Cycadaceae, 142 embryo, 427-431
Cycas, 52, 53, 54, 85, 221 epidermis, 139,159
revo/uta, 52-54, 222 leaf, 193-195, 209, 212, 213, 215,
Cyclanthaceae, 151 216, 218, 224, 225, 231, 232, 234,
Cydonia, 414 235
oblonga, 95,98,461 periderm, 335
Cyperaceae, 87, 142, 144, 145, 147, phellogen in roots, 335
243,248,254 root, 240, 243, 247, 249, 254, 261
Cyperales, 151 secondary xylem, 296-311,313,317
Cyperus papyrus, 75 shoot apex, 48
Cypsela, 380,440,449-451, 450,453, sieve-t~be members, 124,127
454,455 stem, 167,173
Cystolith, 19,41,119,138,221 tension ~ood, 318,
Cytochimera, 225 ~/ type of node, 177,178
Cytoplasm, 7,8,9, 10, 13-15, 17-)9, vascular cylinder, 169,175,249
30, 34, 77, 204, 211, 224, 234, 258 ? vessel, 107,111
components and structure, 9, 10 'Dictyostele, 170,173
in egg cell, 427 Differentiation, 44,184
in embryo sac, 412,425 in cambial cells, 278
in pollen grain, 387 in floral organs, 362
in sieve elements, 124,125 in embryo, .~29, 431, 432
in sporogenous cell, 406 in laticifers, 132
in connection with thickenings of tra- in leaf, 234
cheary elements, 106 in meristematic zone, 46
Cytoplasmic strapd, 8 in metaxylem, 267
" in phloem, 248,253,275
in primary xylem, 251
Dactyloctenium robecchi, 143 in procambium,' 102,273
Dahlia, 16 ' .... in protophloem,\ 184
Subject Index 511
soft, 92, 9~
",
dehiscent, 439,440,442-449
substitute, '87,98 dry, 439-441,449,455
textile, 94 false, 440
wood, 36,313,314 fleshy, 378,441
xylary, 85,87,92 fleshy false, 460-461
Fibre-like cell, 222 .indehiscent dry, 440,449-455
Fibre-sclereid, 328" limticels, 344
Flbre-tracheid, _26, 39, 40, 85-87, 89, schizocarpic, 441
91,91,92,100,108,112, 116, 290,~ syncarpous, 439
296,315 Fruit dispersal, 449,455
Ficus, 131,195 Fumaria officinalis, 455
carica, 59 Fungi, 9
elastica, 138,138,145 fungar hyphae (in a vessel), 315
sycomorus, 315, 3~1 symbiotic, 240·
Filament, 361,382;419 Funiculus, 400, 403, 404~ 405, 408,
Filicinae" 52, 173,250 458,466
FlageUariaceae, 151 Furcraeagigantea, 93,94
Flankmeristem, in apex, 53,54,54,55, . Fusiform cell, 21
56-59, 58 ~ / -Fusiform· initial, see Initial
Flavones, 14,15
Floral organs, see Floral parts
Floral parts (organs), 80,360 Gagea lutea, 466
developing, 362, 363, 363, Galanthus niva/is, 354,466
free, 360 Gamete
fusion, 360,378,380 female (see also Egg cell), 410
traces, 367 male, 1,409,424,43-"'.
types, 360 Gametophyte, 175
Floral receptacle, see Receptac1e female· (see also Embryo sac), 410-
Flower, 2,61,360-434 413,424
-apetalous, 380 male 407-409,409
axis, 360,361 Garidella, 414
bisexual, 422 unguicularis, 416,417
epigynous, 361 Generative apospory, 432
female, 422 Generative cclI, of male gametophyte,
fusion of several flowers, 380 407,409,409
hypogynous, 361 Gentiana, 414.
male, 422 Geraniaceae, 149
ontogeny. 361-367,363,365 Germinat.ion, 2,241,476,477
perigynous, 361 epigeal, 2,474
sympetalous, 371,405 / hypogeal, 2,268,269,475
unisexual, 380,422 pollen grain, 400
vascularization, 367-381 Germination tube (see also Pollen tube),
zygomorphic, 364 395
Follicle, 380,439,441,442,444 Geropogon, 213,448,449
Fragaria, 439 Geum urbanum, .....429
Fraximls, 106, 110, 124,276, 289, 298, Gibberellins, 282
321,328,440 Ginkgo, 38,52,54,55,177,213,290,346
syriaca, 301 hi/oba, 56
Fritillaria, 411,412,414 Ginkgoales, 105
imperialis, 421 Gladiolus segetum, 261
Fructose, 422 Gland, 155,219
Fruit, 2, 80, 95, 439-463 chalk (calcium-secreting), IS5, 157,
aggregate, 441 158
classification, 439 epidermal, 155,157,219
514 Subject Index
peltate, 153,155
"
Hydathode, 160,220,221,421
in seed, 467 types, 221 "-
shaggy, 154, 154 Hydrilla verticillata, 407
squamiform, 153 'Hydrophytes, 74,142; 205,207
stellate, 153, 154,216 leaf, 74,205, 207
in stigma, 398,399,400,403 root, 207
types, 153,154,154,155 ' stem, 74~207
unicellular, 153, 400, 416, 460, 463 Hygrochastic movements, 144
uniseriate, 153 Hy_menocallis caribaea, 108
vesiculate, 153,154 Hy';;e-'lOcarpos circinnatlls, 444
Halophyte, 187,204 Hyoscyamus~ 440,447'
Haloxylon, 145,148,187,332,337,344 Hypanthium, 378, 379, <457, 460, 461
articulatum, '148,352 Hyparrhenia hirra, ' 217'
Hamelid patens, 376 Hypericaceae. 344,398,468
Hancornia, 135 /.,...Hypericum, 255,440
Haplostele, 170;'170 ;,--- Hypoblast, 431
Haostorium of embfyo, 429,431,432, Hypocotyl, 1,1-3,162,166,182,241,
476,477 251, 258, 260, 264, 265, 266, 267,
Haustorium of endosperm, 425 y 267-269,429,431,432,474,475,476
branched, 426 Hypocotyl - radicle axis, - 265
chalaza!, 426 Hypocotyl - root axis, 2,251
secondary, 425 Hypodermis, 138,152,189, 197, 199,
Heartwood (Duramen), 289,291,296, 201,221,222,461
314,315 biseriate, 22l
Hedera, 376,441 uniseriate, 221
helix, 76~ 378 Hypophysis, 429
Hedysarum pallens, 444,446,446 Hypoxis setosa, 261
Helianthus, 65,414,474 Hypsophyll, 193
annuus, 64
Helobiae, 144
Helxine so/eirolii, 140 Idioblast, 5, 18, 19, 85, 95, 159, 164,
Hemicellulose, 23,25,26,29,77,83,474 221
Hemp (see also Cannabis sativa), 94 Idioblas-tic parenchyma cells, 78
Hepatica fri/aba, 455 . Impatiens, 219, 413, 422, 425, 461,
Heracleum, 83 461,463
Herftlera, 262 balS(lmina, 19
Hesperidium, 441 roylei, 426
Heteropteris anomala, 349 Indehiscent fruit (see also Fruit), 449-
Hevea, 130,132,135 , 455,468 '
brasiliensis, 132,135 Infi'orescence, 361,439,449
Hibiscus, 156,414 Initials, 46,47.52,68
cannabinus (Kenaf), 94 apical, 46
syriacus, 48 apical, in root, 6~, 64, 65, 241, 242
Hilum a'picat, in shoot, 51, 52, 55. 57, 59,
in starch grain, 17 60
in seed, 466 cambial, 92, 274, 275, 278, 279, 287,
Himantandra, 382 356
Histogen, 65 fibre. 87,92
Histogen theory; 46,54 fusiform, 87,272,274,275,276,277,
Hordeum, 15, 153, 181, 251, 253 277, 278, 279, 287, 309, 310, 313
bulbosum, 49 hypodermal, 382
spontaneum, 393 laticifer, 131,134
Hoya carnosa, 91 marginal, 227,229,230,311
Hudsonia 309 parenchyma, 175
516 Subject Index
Initials (cont.) ·Ixiolirion montanum, 261
pennanent, ~3, 64, 64, 65, 68 Juglans, 328,335,376,380,441
phellogen, 332,333 nigra, 378
ray, 274,275,276,279,309 Juice vesicle, 458,458
root cap (calyptrogen), 65 Juncaceae, 142,254,342
sclereid, 98,99 Juncales, 151
storied meristem of protective tissue, Juncus, 74
343 . maritimus, 234
submarginal, 227,228,229,230 Juniperus chinensis, 147
temporary, 64 ..' 64,65 J ussiaea peruviana, 209
types of cambial; 275 Jute (see also Corchorus capsularis) ,
vascular, 175' 94
Initiation
integument, 405
lateral root, 246,254 KalancllOe, 256
leaf, 224-225,232 fedtschenkoi, 158
Integument, 403, 405, 408, 449, 450, Kapok fibres, 92
466,468,470 Karyolymph, 10
inner, 404,405,451,467,468,470, Kenaf, see Hibiscus cannabinus
472 Kingia, 104,168,356,358
outer, 404,405,406,451,467,468) australis, 169
.,_,_ Korper-Kappe theory, 65-67
472
Integumental tapetum, 406 Krugiodendron, 315
InterCellular spaces, 23,98,145
in collenchyma, ' 82 ' Labiatae, 156,380,414,441
in epidermis, 139,381 Lactic acid, 133, 174
in fruit, 452,455-458,460,463 ,Lactuca, 132,468
in leaf, 201,202,204,221 sativa, 449,450
in lenticel, 344 Lamina (of leaf), 194, 195, 196, 197,
in nectariferous tissue, 419 206, 209, 213, 225,226, 228, 228-
in parenchyma, 74-76 234
in petal, 381 histogenesis, 228-231
in root, 243,262 origin, 227
in seed, 469 Landolphia, 135
in style, 401 Larix, 26
Intercellular substance, 23, 109::0- Latania,." 252
Internal surface area (of leaf), 201- Lateral meristems, 45,273,332
202,204 Latex, 15, 130, 135 ~
.<1
Orchis, 252,467
Organic acids, 130,422
\ Panicoideae, 218
Panicum, 218
Organismal theory, 9' .......... Papaine, 130
Ornithogalum, 425 Papaver, 398,440
Ornjthopus compress«s, 446 rhoeas, 400
Orobanchaceae, ) 69 ,somni/e'rum, 130
OrobuYlc-he, \45 , "
Ya'l>1l.'V'elaC'ta't, 1>2, \~.. , ~,,~
Orobus, 432 Papilionaceae, 151; 252, 256, 414, 432,
angustijolius, 433 433
Oryza, 15,181,233,23), 234,427 Para rubber tree, 135'
sativa, 232 Paracytic (rubiaceous) type of stoma,
Osteosciereid, 95,96,469 149,150
Ovary, 361, 394, 408, 414, 421, 461 Para,sites (plants), 248
inferior, 361, 366, 376-380, 378, Parenchyma, 3, 38, 61, 73-78, 80, 81,
/ ...
379,421,440,460,4 61 /' 95,98,105,183,208,235,286
intermediate or pseudo-bferior, 361 aliform, 308, 321
multiloculate, 441 apotracheal, 302,305,305-307, J22
superior, 361 axial;-,- 287, 288, 290, 291, 304-308
syncarpous, 376 banded or metatracheal, 307
types of development, 367 confluent, 308,321'
unilocular, 397,398 cortical, 164, 165, 283
vascularization of inferior, 378, 379 diffuse, 302,306
wall, 361, 367, 376, 378, 398, 414, in fruit, 441,455'
440,455,461 ground, 183
Ovule, 2,361,380,395,397,398,402, initial, 298,307
403-407, 404, 406, 408, 424, 426, interfascicular, 173,273
434,466 'palisade (see also Palisade paren~
amphitropous, 404,405 chyma), 197
anatropous, 403,404,405,466 paratracheill, 305, 308, 321, 322
atropous. 403,404 phloem, 118, 128, 189, 260, 261,
campylotropous, 404.405,408 283, 325, 326
d.ldno\l\)})O\\~, 4\\4,4.0$ 'i.'Q."'j,' 281
hemianatropolls, 404.405 root, 242,258,260
orthotropous, 403 scanty paratrachdll, 308
types, 403, 404, 405 secondary, 260
Oxalis, 261 spongy (see also Spongy parenchyma) ..
hifta. 261 197 '
Oxidases, 34,247 stellate, 74,75
terminal, 307
unilaterally paratrache:al. 308
Paeonia, 114, 363 vasicentric, 308, 321, 322
olbi/lora, 413 water-storing, 187, 189, 189, 209
Palisade wood, 286,287,298, 322
cells, 189,190,197,200,201,213 wound, 283
parenchyma, 196, 197, 198. 200, xylem, 85,87,102,286,287
208,209,221 Parietal cell (primary)
tissue, 190, 197, 200, 202-204, 215 in anther, 382,'383.384
Palmae (,see also Palms), 49, 128, 142, in ovule, 406,407
148,151,243,248,250,252,273,354 Parthenium argentaturn, 135, 235
Palms (see also Palmae), 343, 354-356 Parthenocarpy, 439
Panama rubber, 135 Parthenogenesis
Pandanaceae, 151,250 diploid, 432
Pandanus, 31,252 haploid, 432
haerbachii, 150 Pa~sage cell, 245,247
522 Subject Index
,
structure, 286 247,; 325, 329
\
Subject Index 529
"-,,_
phylogeny, 126 Spongy parenchyma",(spongy tissue),
S\e'te })\?ttc, 119, 120, 112, lll, l23, 196, 197, l<}S, 200, 200, 202, 204,
329 221,231,381 --
compound, 121,123,123,127
'- Spore, 1, 21,40
simple, 123, 127 Sporobolus spicaws, 203
Sieve tube, 123,121,215,328,329 Sporogenous cell (s'ee also Primary spo-
Sieve-tube member (element), 121, J 23, rOgerlOusceII) 405,412 .
124,127,128,327;328 Sporophyte, 1,102,175,466
Silene, 414 ,/ Sporophytic budding, 432
Silica (silicon salts), 17, 23, 142, 144, Sporopollenin, 389
156,218 Stamen, 360, '364; 381-394, 384, 395,
bodies, 144 414,417,422,460
cell, 141, 143, 143, 144,159 bundle, 372,380 "..
Siliqua, 439,446,447 / epipetalous, 376
Simarubaceae, 78,219"" /' phylogeny, 381
Simultaneous type of pollen grain devel- structure and tissue differentiation,
opment, 387 382-386
Sinapis alba, 255,467
Siparuna bifida, If.
Siphonostele, 170,170,171. 172, 174,
174
trace, 367,369,373,380
StaTch, 1, S, 18, B1, 1M, 224, 242, 25S,
262,287, 311, 387, 461,471
assimilation, 17
_-
amphiphloic, 170.17/-17] compound gfain, 15, 16~ 17
ciadosiphonic, 114 grain, 7, 8, 12, 15-17, 16, 78, 130,
ectophloic, 170,17/,173 167,234,474
phyllosiphonic, 174 half-compound grain, 16
types, 171-174 simple grain, 16
Slime, 124,125 storage. 12, 17. 128, 164,218,327"
body, 128 329,333
plug, 122-123,124 Starch sheath, 166,167,218
Smilax, 213,243,248 Stelar theory, 169
Solanaceae, 118, 167, 398 Stele, 169
SoIanad type (of embryo development), in floral receptacle, 367,370,373,378
427,430 in hypocotyl, 265, 267
Solanum, 150, 385 polycyclic, 173
dulcamara, 334,335 in root, 263,265
tuberosum. 81 in stem, 166, 169
villosum, 385 types, 169-175
Solenostele, } 72, 173 Stem, 2-5, 80; 162-190, 165, 176, 178,
Somatic apospory, 432 179, 182, 183, 185, 188, 190, 241,
Sonchlls, 132 329, 335, 349, 350, 351, 354, 356,
oleracells, 133,392 357, 358
Sorb:Js sorbi/olia, 378 leaning, 318
Sorghum, 131 lenticels, '344
vulgare, 94 periderm, 331
Sparaum, 307 succulent, 188,189,189,190
junceum, 298,456 tension wood, 318,319
Special wall, 387 vascular system, 167-186264, 265,
Spermatophyta (see also Spermatophy- 268
tes), 64,102,193 xeromorphic, 188, 188
Spermatophytes (see also Spermato- Stenolobium, 328
phyta), 1-2, 5, 46, 47, 53, 56, 6J', StercuIiaceae, 219
225,240,247 Stereome, 80
Sphenopsida, 173 Stigma, 361,395,397,398,399,400,
Spiraea vanhauttei, 119,378 402,424
530 Subject Index
Stilt-roots, 262 Suberin, 17,23,25,29,106,166,167,
Slipule, 48, 194, 194-195, 211. 219, 245,247,257,332,337,344
413 Suberin lamella, 167, 245-247, 258,
Stock, 283 332,342
Stomata, ]40. 141, 143, 145-153, Suberization~ 236,351
146-150,157,160,419,421 Suberized cells, 235, 245, 283, 337,
caryophyllaceous or diacytic type, 342-343,346,449
151 Subsidiary cell, 141, 145, 146, 147,
in cataphyll, 232 149,151,222
in conifer leaf, 147, 148, 221, 222 Substomatal chamber, 145, 147, 148,
cruciferous or anisocytic type, 150 419
in flower, 145,381,403 Successive type of pollen grain develop.
in fruit, 458,460 ment, 387
in grasses, 143,217 Succulent
in leaf, 141, 143, 145, 149, 195,206, features, 203
217,222,223 leaves, 202,209,
and lenticel, 344, 346 p~ants, 78, 203
modified, 157,221,417,418 roots, 240
ontogeny, 150,151,'152 stems, 188,189,190
ranunculaceous or anornocytic type, Sucrose, 422
149 Sudan IV, 140,452
in rhizomes, 145 Sugar beet (see also Beta), 260, 261
'fubiaceous or paracytic type, 150' Sugars, 78, 130, 260, 421, 422, 463
in seeds', 145 Sui generis concept, 395
in stem, 145, 188,344, 346 ," Sulcus (in pollen grain), 391
sunken, 206,221,222,421 Supporting tissue, 80, 83
types, 147,149-151 in leaf, 209,218,230
types in dicotyledons, 149,149 Surface meristem, of apex, 53, 54, 55
types in monocotyledons, 150, 151 Suspensor, 429,431
in xeromorphic leaves, 203 function, 432
Stomatal aperture. 147, 148, 150, 160 types, 433
Stomium;, 383,384,385 variation in structure, 432
"• \
Subject Inde>; 531
\
143, 204, 220, 224, 287, 288, 314, cork, 137"'\~
315,327,332, 333 endosperm, 424; 473
cells containing, 78,219,220 expansion, 463
Tapetal periplasmodium, 386 glandular, 398,401
Tapetum, 21,383,434 ground, 137,227,455
amoeboid, 386,386 ,,- integumental, 468
glandular or secretory" 372,386 lignified, in fruit'l 454
integumental, 406 mature or permanent, 44,45,56,67,
Taraxacum, 132,261 73
kok-saghyz, 132, 135 nectariferous, ,413, 415, 415-417,
'ta.r..aceae, 152, l~ 1 4li,42~,42\)-422
Taxodiaceae, 252,291,327 nucelIar, 452,466
Taxodium distichum, 225 nutritious, 406: ,
Taxus, 291 resistence, 447,448
baccala, 56 sporoge'nous: i~; anther, 382, 386
Tegillum, ".390 sterile, in anther, 382
Tegophylls, "395 storage, 344,424,473
Telome theory, 382 supporting, 80, 83, 209, 218, 230
Telomes, fertile, 394 -transmitting, 398-403,
Tepal, 360,421,439 water-storing, 139, 189, 202, 204
Terminal cell, 427,429 Tofte/dia palustris, 414
Tertiary layer (of cell wall), 41 Tomato (see also Lycopersicon esculen-
Tertiary waJ1 (layer), 24 tum), 441
'"C~">.t_~(~ee<I.(s() Se.edGcat}. 95',466-473, TQn.Q\)\a~t, '1~,\4, 125
467,470,472,475,476,477 Torus, 24;'37,38,288,290,321
formation, 466-468 Trabeculae, 10,41,290
histological structure, 468-473 Trace, see Branch trace, Carpel trace,
Tetracentraceae, 112, 113 Leaf trace" Petal trace, Sepal trace
Tetrad, see Pollen grain and Stamen trace
Teucrium polium, 91 Trachea (see also Vessel), 103,106
Thalamus, see Receptacle Tracheary element, 39, 40, 68, 100,
ThaNetrum, 395 102~1l6, .104,· 175, 184, 207, 215,
Thesium, 425 248;252,253,260,286
Thrincia, 163 development, 106
Thuja, 335 in dicotyledons" 113, 114
orientalis, 142 length, 112,l1:i
Thunbergia, 351 in monocotyledons, 114,115
grandiflora, 354 perforation plate, 113
mysorensis, 354 phylogenesis, 111-115
Thymelaea, 197 secondary, 105
hirsuta, 198,281,306 secondary walI, 103
Thymelaeaceae, 142 thickness of wall, 113
Thymus capitatus, ISS type of pitting, 113
Typhaceae, 342 types, 106
Tilia, 120,298,314,315,329,440 Tracheid, 24,25',26,31,31,32,33,37,
Tiliaceae, 363,468 38, 86, 89, 92, ]03, 104, 106, 112,
Tillandsia, 92,241 175,208,215,221,224,286
usneoides, 93,94 development of fibres from, 92
Tissue length, 31~
abscission, 234,447,447,463 in secondary xylem, 288, 290, 296~
absorbing, 139 318
cohesion, 447,448,449 Tracheid-like cell, 204,205
complementary, 346,348 Tracheoid idioblasts, 205
conjunctive, 354, 356 Tracheophyta, 102,115,169
532 Subject Index
_-
218
irificum, 65, 153, 181,244; .250, 251, arrangement in stem, 175-184
268,451,451,478 bicollateral, 168, 173, 209, 240
durum, 16 in carpel, 367-370,373,395
vulgare, 66 collateral, 167, 173, 181, 209, 210,
Trochodendraceae, It3 240 .-
Troc/lOdendron, 95,112 concentric, 209
ara'Iioidcs, 96,310 in floral organs, 367,371,372,373,
Tropaeolaceae, 2i9 374, 376, 376, 378, 378, 380, 420
Tropaeolum, 156,414,415 ~/ in frui( 442, 447, 452, 453, 454,
majus, 8,256 460 /---
Tube cells, 451,452 • in leaf, 168,204,209,212,218,219,
Tulipa, 224,354,412 222, 225, 228, 230, 230, 232, 234
Tunica, 54,57,58, 61, 224, 225, 232 in" monocotyledons (prim'ary or se-
Tunica~corpus theory, 57,66 condary), 185,355,356,357,358
Tyloses, 287,287,294,314,315 in nectariferous tissue, 413
Tylosoids, 292 in ovules, "370,395,397
in petiole, 209,212
in transitron region, 264-268
Ulmus, 110,234,328,335,412,440 types, 167-169
americana, 341 vestigial, 380
Umbelliferae, 76, '195,414,421,431, " Vascular cambium, 4, 5, 45, 68, 102,
441,468, 247, 272, 273-283, 274, 332, 335,
,
Unifacial, leaf, . . . . 234 339 \
Subject Index 533
cell division, 278~279, 279 Veratrum, 45~
development, 273'...275 album, 354
seasonal activity, 279-282 Verbascum, 15,153,154
structure, 273-279 ........ Vessel (see also Trachea), 106, 110,
Vascular cylinder (central cylinder) 112,215,260,262,287,294
meristem, 65 \,- annular, 106
in pedicel, 367 , arrangement' in secondary xylem,
primary, 173,175 298~304~314
in root, 245,247-251,-249,250,255, development, 109, 110
263 / length, 106
in stem, 166,167,169,189 pitted, 106
Vascular ray, 274,326 spiral, 106
Vascular system, 102, 251 Vessel member, 33,40, 103. 106, '107,
development in leaf, 231 108,110, lll c I13,26J,286,296
in flower, 370,378,380 / Vessel member'- tracheids, 108
in hydrophytes, 207 Vesse1-tracheids, '108
in leaf, 176,2H:c218 Vestigial
in monocotyledons, 181 petal traces, 380
ontogeny of primary, 184_,;.18.6 -Vascular bundles, 380
primary, 167-169,178,179,264 Viburnum
in root, 264 odoratissimum, 310
in scutellum, 251 linus, lOS
in stem, 167-186,176,188,264 Vicia, 219,248,'444,474
in xerophyte leaf, 202 faba, 249,250,413,476
Vascular tissue, 85,169,172,173,175, Villarsia reniforrnis, 424
184, 187, 205, 211, 213, 220, 221, Vinca, 48,131,132
235,236,252 major, SO
in floral organs, 367,370,371, 379, rosea, 382,384
primary, 264 Viola, 414
secondary (see also Secondary phloem tricolor, 467,'468
and Secondary xy1em), 2®, 273, Violaceae, 468 -
283 Viscin threads, 381,388
Vascu1arization, 174,177.182 Viscum, 140,331
in cotyledons, 177 album. 35. 83
in flower, 367-381,374 Vitis, 81, 87, 89. 119, 120, 121, 123,
Vasicentric tracheids, 308 128, 195, 210, 328~330, 335, 337,
Vegetative cell, of male gametophyte, 341,344
407,409,424 )ljnifera, 48, 194, 257, 287. 330, 336
Vegetative reproduction, 432
Vein (see also Venation and Vascular
bundle), 5,266,267 WaIl layers
development in leaf, 231,232 G-layer,86
endings, 213, 214, 215, 220, 232 central, 24
in flower, 371,381,384 inner, 24
lateral, 181,371 outer, 24
in leaf, 204,211,212,213,214, 215, secondary, 24,25
218,221,229 terminal, 24
Velamen, 139, 157,242 tertiary, 24
Venation (see also Vein), 211,212,213 Wall thickening' iri tracheary elements,
dichotomous, 213,232 103
parallel, 212,213,214,215 annular, 103,104,105,106,215,244
reticulate, 212,213 dense helical, 104
Ventral meristem (see also Adaxial me~ helical, 103, 104, 105, 105,
ristem), 227 reticulate, 103,104,105,244
Subject Index.