BS Anatomy Final course

Anatomical Features of Hydrophytes

Plants that grow in water or very wet places are known as hydrophytes. They can be submerged
or partly submerged, floating or amphibious. Their structural adaptations are chiefly due to the
high water content and the deficient supply of oxygen.

The various adaptations are as follows:

(i) The reduction of protective tissue (epidermis here is meant for absorption and not for
protection).

(ii) The reduction of supporting or mechanical tissue (i.e., absence of sclerenchyma).

(iii) The reduction of conducting tissue (i.e., minimum evolution of vascular tissue).

(iv) The reduction of absorbing tissue (roots chiefly act as anchors, and root hairs are lacking).

(v) There is special evolution of air-chambers (aerenchyma) for aeration of internal tissues.

Epidermis:

In aquatic plants, the epidermis is not protective but absorbs gases and nutrients directly from the
water. The epidermis in typical hydrophyte has an extremely thin cuticle, and the thin cellulose
walls permit ready absorption from the surrounding water. Generally the chloroplasts are found in
epidermal cells of leaves, especially when the leaves are very thin; these chloroplasts utilize the
weak light under water for photosynthesis.

In submerged plants, stomata are not present, and exchange of gases takes place directly by the
cell walls. The floating leaves of aquatic plants have abundant stomata on the upper surface.

Lack of Sclerenchyma:

Submerged plants generally have few or no sclerenchymatous tissues and cells. The water itself
gives support to the plant, and protects it to some extent from injury. The thick walls of tissues,
their density and the presence of collenchyma in certain plants give some rigidity. The strands of
sclerenchyma occasionally exist, especially along the leaf margins, and increases tensile strength.
A few star-shaped idioblasts or sclereids are present, which give mechanical support to the body
of aquatic plant.

Minimum Development of Vascular Tissue:

In the vascular tissues, the xylem visibles greatest reduction and in many aquatic plants consists
of only a few elements, even in the stele and main vascular bundles. In certain aquatic plants in
the stele and large bundles, and frequently in the small bundles, xylem elements are lacking.

In these plants, there is well evolved xylem lacuna in the position of xylem. These lacunae
resemble typical air-chambers (air-spaces). In several aquatic plants, the phloem is fairly well
developed as compared with the xylem. The endodermis is generally present around the stele, but
it is weakly developed.

Reduction of Absorbing Tissue:

The root-system in hydrophytes is feebly evolved and root hairs and root cap are absent. In some
floating plants such as Utricularia, Ceratophyllum, etc., no roots are evolved, and in submerged
plants such as Vallisneria, Hydrilla, etc., water dissolved mineral salts and gases are absorbed by
their whole surface.

In plants like Pistia, Eichhorma, etc., no root cap evolves, but root pocket is formed instead. An
aquatic plant is, in reality, submerged in or floating up on a nutrient solution. In hydrophytes the
root system is functioning mainly as holdfasts or anchors, and a large apart of the absorption takes
place through the leaves and stems.

Development of Air-Chambers:

Chambers and passages filled with gases are usually found in the leaves and stems of hydrophytes.
The air-chambers are large, generally regular, intercellular spaces extending through the leaf and
often for long distances through the stem (e.g., Potamogeton, Pontederia).

The spaces are generally separated by partitions of photosynthetic tissue only one or two cells
thick. The chambers prepare and internal atmosphere for the plant. These air-chambers on the one
hand give buoyancy to the plant for the floating and on the other they serve to store up air (oxygen
and carbon dioxide).

The carbon dioxide that is given off in respiration is stored in these cavities for photosynthesis,
and again the oxygen it is given off in photosynthesis during the daytime is similarly stored in
them for respiration. The cross partitions of air passages, called diaphragms prevent flooding.

The diaphragms are provided with minute perforations through which gases but not water can pass.
Another specialized tissue frequently found in aquatic plants that gives buoyancy to the plant part
on which it occurs is aerenchyma. Here, very thin partitions enclose air spaces and the entire
structure consists of very feeble tissue. Aerenchyma in phellem is formed by a typical phellogen
of epidermal or cortical origin. At regular intervals individual cells of each layer of phellem
elongate greatly in the radial direction which the other cells of such layer remain small. However,
the term aerenchyma is applied to any tissue with several large intercellular spaces.
Anatomical Features of Xerophytes
They grow in deserts or in very dry places; they may withstand a prolonged period of drought
uninjured, for this purpose they have certain peculiar adaptations. The xerophytic plants have to
guard against excessive evaporation of water; this they do by reducing evaporating surface. Plants
form a long tap root which goes deep into the sub-soil in search of moisture. To retain the water
absorbed by the roots; the leaves and stems of certain plants become very thick and fleshy (viz.,
Aloe, Agave). Water tissue develops in them for storing up water; this is further facilitated by the
abundance of mucilage contained in them. Multiple epidermis sometimes evolves in the leaf (viz.,
Nerium). Modification of the stem into the phylloclade for storing water and food and at the same
time performing functions of leaves is characteristic of many desert plants (viz. Opuntia and other
cacti).

In xerophytes certain structural features are also common. Leaves are thick and leathery, well
evolved cuticle and abundant hairs. Well differentiated mesophyll is also present, and there is often
more than one layer of palisade tissue (viz. Nerium, Hakea).

The walls of epidermal and sub-epidermal cells are frequently lignified, and distinct hypodermis
may be present. They have a well-developed vascular system and often an abundance of
sclerenchyma, either in the form of sclereids of fibres (Hakea, Ammophila). The leaf is sometimes
cylindrical or rolled.

This organization is to protect the stomata, which can show peripheral photosynthetic tissue and
central water storage issue (diagrammatic); B, a portion (detailed) showing thick cuticle,
thickenings on the radial and outer walls of the epidermal cells and sunken stomata exist in
furrows. Some fleshy leaves (viz., Sedum), contain abundant thin-walled cells, the water storage
tissue.

The characteristic anatomical features of the xerophytes are as follows:

i. Epidermis and Thick Cuticle:

Heavy cuticularization and extreme cutinization of the epidermis and even of sub-epidermal cells
are common in xerophytes. The thickness of the cuticle shows different gradations. In certain cases
the thickness of cuticle is only slightly greater than normal, like that of plants of semi-xerophytic
habitats. In extreme xerophytes the cuticle may be as thick as, thicker than, the diameter of the
epidermal cell.

In addition to the presence of thick cuticle, the walls of epiderml cells become cutinized and
sometimes also those of underlying cell. Along with well-evolved cutinized layers the epidermal
and subepidermal cells also become lignified. In some cases the covering of wax is formed on the
epidermis (viz., Calotropis). The epidermal cells are usually radially elongated. In the leaves of
Nerium and Ficus, the epidermis becomes multilayered.

In many xerophytes in addition to a cutinized epidermis, single to multi-layered hypodermis is also

present. In most plants, the hypodermis of leaves is morphologically mesophyll and can be in the
form of a sheet of fibrous tissue or a layer of sclereids. The hypodermis of the stems seems to be
a part of the cortex. The hypodermis of stems and leaves can be cutinized to lignified. In many
plants, the mucilage, gums and tanning are commonly found in hypodermis.

Hairs:

In several xerophytic plants, especially those of alpine regions exposed to strong winds, a covering
of matted epidermal hairs on the underside of the leaves prevent water loss. Hairs can also be
abundant over the entire aerial part of the plant. The thick matting of hairs also prevents rapid
evaporation through stomata. The xerophytes that contain abundant hairs, on their leaves and
stems, are commonly called trichophyllous.

ii. Structure of Stomata:

The stomata are very minute opening produced in the epidermal layer in green aerial parts of the
plants. The stomata are essential for intake of carbon dioxide and oxygen and or the passage inward
and outward of other gases. The evaporation of the surplus water takes place by the stomata. When
the stomata are open, water escapes even when water loss is harmful to the plant.

This way, the reduction of transpiration is of great importance in xerophytes. The xerophytes can
contain less stomata, either by reduction of leaf surface or of stomatal number per unit area. To
reduce excessive transpiration usually the stomata that remain sunken in pits are formed. Such
stomata are commonly called sunken stomata (e.g., Hakea, Agave, etc.). In certain cases the
stomata are found in groups and they remain confined to depressions found on leaf surface (e.g.
Nerium, Banksia, etc.). Generally the depressions attack of wind gusts.
iii. Sclerenchyma:

The xerophytes commonly have a large proportion of sclerenchyma in their leaf structure than is
observed normally in mesophytes. The sclerenchyma is either found in groups or in continuous
sheets.

iv. Rolling of Leaves:

The leaves of several xerophytic grasses roll tightly under dry conditions. In these grasses, the
stomata are confirmed to the ventral surface of the leaf, so that when the leaf edges roll inward,
the stomata are effectively shut away from the outside air. As the stomata are situated on the inner
surface of the leaf, the air enclosed by the rolled leaf soon becomes saturated with water arid the
outward water diffusion stops.

In Ammophila arenaria, there is tight upward folding of the leaf and also the sheltered situated of
the stomata in furrows, greatly reduce air movement over stomatal areas. Special motor cells
(hinge) on the upper surface of the leaf are responsible for the inward rolling of leaves. In the
xerophytic grasses, the motor cells are well evolved.

v. Reduced Leaf Surface:

In many xerophytes, reduction of the leaf surface partly checks water loss because the total exposed
surface of the plant body is relatively small as compared with that of normal mesophytes (viz.,
Casuarina, Asparagus, etc.). In such xerophytes the leaves are either scale-like or very small in
size. Generally they are not found in the mature plant, or they persist as small scales or bracts.

In some plants the photosynthesis takes place in the stem where assimilatory tissues are well-
developed. The reduction of leaf surface is usually accompanied by well-evolved sclerenchyma,
water storage tissue and sunken stomata. Xerophytes, with reduced leaves, are called micro-
phyllous.

vi. Water Storage Tissue:

Many fleshy xerophytes contain water storage tissue and mucilaginous substance in them. In
leaves such tissues are situated beneath the upper or the lower epidermis or upon both sides of the
leaf and sometimes in the centre too. The storage cells are visually large and often thin- walled, as
in Begonia. The storage tissue can actually serve as a source of reserve water during drought. The
xerophytes, that possess fleshy leaves or stems, are called malacophyllous.

vii. Abundant Palisade Parenchyma:

In the stems of several xerophytes, the palisade tissue is present (viz., Capparis decidua). In the
xerophytic leaves the palisade is abundant and completely arranged.

viii. Latex Tubes:

In many xerophytic stems and leaves the laticiferous canals are present (viz., Calotropis,
Euphorbia, Asclepias, etc.). Because of vicosity latex the transpiration is reduced to some extent.
Flower: Important Parts and its Anatomy
Introduction to a Flower:

The flower consists of an axis, also known as receptacle and lateral appendages. The appendages are
known as floral parts or floral organs. They are sterile and reproductive. The sepals and petals which
constitute the calyx and corolla respectively are the sterile parts. The stamens and the carpels are the
reproductive parts. The stamens compose the androecium, whereas the free or united carpels compose
the gynoecium.

The vegetative shoot shows unlimited growth, whereas the flower shows the limited growth. In flower,
the apical meristem ceases to be active after the formation of floral parts. In more specialized flowers
there is a shorter growth period and they produce a small and more definite number of floral parts than
the more primitive flowers.

In still more advance flowers there are specialized characters, such as, whorled arrangement of parts
instead of spiral, adnation of parts of two or more different whorls, cohesion of parts within a whorl,
zygomorphic instead of actinomorphic condition, and epigynous condition instead of hypogynous
condition.

Important Parts of a Flower:

Sepals:

The sepals resemble leaves in their anatomy. Each sepal consists of ground parenchyma, a branched
vascular system and an epidermis. The chloroplasts are found in the green sepals but usually there is no
differentiation in the palisade and spongy parenchyma. They may contain crystal—containing cells,
laticifers, tannin cells and other idioblasts. The epidermis of sepals may possess stomata and trichomes.

The traces are similar in origin and number. From the evidence of vascular system, sepals are clearly, in
nearly every case morphologically bracts—that is, and they have been derived directly from leaves and
are not sterile sporophylls.

Petals:

The petals also resemble leaves in their internal structure. They contain ground parenchyma, a more or
less branched vascular system, and an epidermis. They may also contain crystal containing cells, tannin
cells, laticifers and certain other idioblasts. They contain pigments—containing chromoplasts.

Very often, the epidermal cells of the petals contain volatile oils which emit the characteristic fragrance
of the flowers. In certain flowers the anticlinal epidermal walls of the petals are wavy or internally ridged,
whereas the outer walls may be convex or papillate. The epidermis may also possess stomata and
trichomes.

Stamen:
Commonly the stamen consists of a two-lobed four-loculed anther. The anther is found to be situated on
a slender filament which bears single vascular bundle. In certain primitive dicotyledonous families the
stamens are leaf-like and possess three veins, whereas in advance types they are single-veined.

The structure of filament is simple. The vascular bundle is amphicribral and remains surrounded by
parenchyma. The epidermis is cutinized and bears trichomes. The stomata may also be present on the
epidermis of both anther and filament. The vascular bundle is found throughout the filament and
culminates blindly in the connective tissue situated in between the two anther- lobes.

The outermost wall of the anther is the epidermis. Just beneath the epidermis there is endothecium which
usually possesses strips or ridges of secondary wall material mainly on those walls which do not remain in
contact with the epidermis. The innermost layer is composed of multinucleate cells; this is nutritive in
function and known as tapetum.

The wall layers which are located in between the endothecium and tapetum are often destroyed during
the development of the pollen sacs. On the maturation of the pollen the tapetum disintegrates and the
outer wall of the pollen sac now consists of only the epidermis and endothecium. At the time of
dehiscence of the anthers the pollen are released out through stomium.

Gynoecium:

The unit of gynoecium is called the carpel. A flower may possess one carpel or more than one. If two or
more carpels are present they may be united or free from one another. When the carpels are united the
gynoecium is known as syncarpous; when they are free the gynoecium is said to be apocarpous. A
gynoecium with single carpel is also classified as apocarpous.

The apocarpous gynoecium is termed simple pistil, whereas the syncarpous gynoecium is termed
compound pistil. The carpel is commonly interpreted as foliar structure. The carpel of an apocarpous or
syncarpous gynoecium is being differentiated into the ovary and the style. The upper part of the style is
differentiated as a stigma. The stigma is sessile.

The ovary consists of the ovary wall, the locule or locules and in a multilocular ovary, the partitions. The
ovules are found to be situated on the inner or adaxial (ventral) side of the ovary wall. The ovule-bearing
region forms the placenta. According to Puri (1952) the position of the placentae is related to the method
of union of carpels.

In a carpel the placenta occurs close to the margin. Since there are two margins, the placenta is double in
nature. The two halves may be united or separate. The number of double placenta in compound ovaries
is equal to number of carpels. When the carpels are folded, the ovary is multilocular and the placentae
occur in the centre of the ovary where the margins of the carpels meet.

This is axile placentation. When the partitions of the ovary disappear, it becomes free-central
placentation. When the carpels are joined margin to margin and the placentae are found to be situated
on the ovary placentation is parietal.
Most commonly the carpels has three veins, one dorsal or median and two ventral or lateral, and the
vascular supply of the ovules has been derived from the ventral bundles. The vascular bundles of the
ovary, possessing axile placentation appear in the center of the ovary, with the phloem turned inward and
the xylem wall, the outward.

The ovary and style are composed of epidermis, ground tissue of parenchyma, and vascular bundles. The
outer epidermis is cuticularized and may have stomata. The ovule consists of a nucellus which encircles
the sporogenous tissue. There are two integuments of epidermal origin, and a stalk, funiculus. The ovule
consists of parenchyma and contains a more or less dominant vascular system.

Vascular Anatomy of Floral Parts of Flower:

The study of the vascular anatomy has helped in solving many intricate problems of floral morphology. It
has shown that many structures are not what they appear to be or what they are commonly taken to be.
The fundamental vascular plan remains more or less unaltered and can always be of some help (Puri,
1952).

Morphologically the flower is a determined shoot with appendages, and these appendages are
homologous with leaves. This commonly accepted view is sustained by the anatomy of the flowers.
Flowers, in their vascular skeletons, differ in no essential way from leaf stems.

They are often more complex than most stems. Taxonomy and comparative morphology have in large
measure determined the structural nature of the flower. Anatomy of the flower has aided in the solution
of certain puzzling conditions.

Pedicel:

The Pedicel and the receptacle have typical structure, with a normal vascular cylinder. The cylinder may
be unbroken or it may contain a ring of vascular bundles. In the region where floral organs are borne, the
pedicel expands into the receptacle.

The vascular cylinder also expands and the vascular bundles increase somewhat in number, and finally
traces begin to diverge. In the simplest cases vascular traces for different organs and whorls of organs
arise quite independently (e.g., in Aquilegia). In other cases various degrees of fusion may take place
between bundles situated more or less in the same sectors.

The appendage traces are derived from the receptacular stele exactly as leaf traces are derived in typical
stems. When the floral organs are numerous and closely placed the gap of traces break the receptacular
stele into a meshwork.
Sepals:

The sepals are with very few exceptions, anatomically like the leaves of the plant in question. A sepal
usually receives three traces derived from the same or different sources. As regards the morphological
nature of the sepals, they have often been considered as equivalent to bracts and foliage leaves.

Such a view is born out by a study of vascular anatomy which reveals practically the same vascular pattern
as exists in foliage leaves and bracts of the same plant.

Petals:

In their vascular supply the petals are sometimes leaf like, but much more often they are like stamens.
The petals may have one, three or several traces. Very commonly there is but one trace. The petals appear
to be sometimes modified leaves, like the sepals, but in the great majority of families they are sterile
stamens.

However, since stamens are the homologous of the leaves, it is not always possible to determine from
anatomical evidence along whether one trace petals in certain families are modified stamens or whether
they have come more directly from leaf-like structures.

Stamens:

A stamen generally receives a single trace which remains almost un-branched throughout its course in the
filament. In the anther region it may undergo some branching. In a few Ranalian families and rarely
elsewhere as in some members of the Lauraceae and Musaceae, three traces are present in each stamen.
In Ravenala (Musaceae) each filament is traversed by 25 to 28 small vascular bundles.

Most of these disappear as the anther is approached, and the system of central bundles consisting of
three or four bundles, continues into the connective. From other evidence the above mentioned families
appear to be fairly primitive, it seems highly probable that the single trace condition is one of reduction
from three.

In the simple flower of Aquilegia the stamens traces pass off, one to each organ in several whorls. Above
the supermost whorl of stamens the vascular cylinder becomes complete again.

Carpels:

The carpel is commonly looked upon as a leaf-like organ folded upward, i.e., ventrally with its margins
more or less completely fused and bearing the ovules. This conception has been supported by the
anatomy. The details of origin, number and course of the bundles forming the vascular supply are exactly
like those of leaves the carpel has one, three, five or several traces.

The three trace carpel is most common. The five-trace carpel is nearly as common as the three traces, and
carpels with seven, nine and more traces are increasingly less and less common. The evidence that the
one-trace carpel (nearly always an achene) has been derived by reduction from the three-trace type.

The median trace which leaves the stele below the other carpel traces is known as the dorsal trace
because it becomes the dorsal (midrib) bundle of the folded organ. The outermost traces are known as
ventral or marginal traces because they become the bundles that run along the ventral edge of the carpel,
i.e., along or near the margins of the organ if it were unfolded.

The upward and inward folding of the sides of the carpel brings about the inversion of these ventral
bundles. The phloem remains on the ventral side in the carpel, whereas it is on the dorsal side in the
midrib (dorsal) bundle. This important condition may be easily understood when it is remembered that
the carpel is leaf-like, with its margins folded upward. The ovule traces are derived from the ventral
bundles.

When floral parts are fused, the vascular bundles of these parts may also be fused. If carpels are united,
the lateral bundles, either those of the same carpel or those of two adjacent carpel, may be fused in pairs.

The fusion in the vascular tissue of a carpel may be present in the ventral bundles from an origin as one
trace throughout their length, or may exist only in part of the carpel; where the ventral bundles arise as
separate traces, they may unite at any point in their course.

In syncarpy there are fusion changes similar to those in free carpels. The lines that separate the carpels
and their margins have been disorganised. The inverted ventral bundles form a ring of bundles in the
centre. These bundles usually lie in pairs. Here each pair consists of the ventral bundles, of the same
carpel, or more often of bundles from each of two adjacent carpels.

In the centre of a three carpellary syncarpous ovary there may be a ring of six or three ventral bundles. If
the ring consists of three bundles, each bundle is morphologically double and represents either the two
ventral traces of one carpel or one from one carpel and one from the adjacent carpel.

Several workers proposed that the evolutionary changes in the structure of the gynoecium of the flower
of angiosperms involve various manners of union of carpels of the same flower.

In such angiospermous flower the carpel may become joined by their margins to the receptacle (Fig. 44.6
B), or they may grow together laterally in a closed folded condition (Fig. 44.6 C), or they may become
laterally united in an open folded condition (Fig. 44.6. A).

The junction of carpels in an open condition may result in a unilocular ovary showing parietal placentation
as shown in fig. 44.6 A. Folding combined with union of carpels with each other may form an ovary with
as many locules as there are carpels. In such cases the ovules are borne on the central column of tissue
where the carpels come together showing, axile placentation (Fig. 44.6 B, C).
The Inferior Ovary:

The inferior ovary is formed by the adnation of the sepals, petals and stamens to the carpels or by the
sinking of the gynoecium in a hollowed receptacle with fusion of the receptacle walls about the carpels.
The vascular system is thought to show this structure in that the bundles found in the appendages of
different whorls are variously fused but all show the usual orientation of xylem and phloem.

In certain flowers with inferior ovary (e.g., Calycanthaceae, Santalaceae and Juglandaceae) there is
evidence that the ovary is partially enclosed in hollowed receptacle. Here the vascular bundles are
prolonged from the axis to the level below the insertion of floral parts, other than the carpels, where
traces to the parts diverge.

The main bundles continue farther from the periphery in a downward direction with a corresponding
inversely oriented position of the xylem and the phloem. These bundles at lower levels give branches to
the carpels. This type of orientation of the vascular system is thought to be the result of the invagination
of the receptacular axis.

The structure, anatomy and morphology of mature seeds: an

overview
More general seed structural features:

• Seeds are the dispersal and propagation units of the Spermatophyta (seed plants):
Gymnosperms (conifers and related clades) and Angiosperms (flowering plants). A comparison
of these to major groups is presented on the "Seed evolution" webpage. Several seed-related
issues differ between gymnosperms (750 species) and angiosperms (250000 species). If not
otherwise stated, the information given in this website refers to typical Angiosperm seeds.
 • Seeds are mature, fertilized ovules. Ovules are structures of seed plants containing the female
gametophyte with the egg cell, all being surrounded by the nucellus and 1-2 integuments. In
angiosperms the double fertilization results in formation of the diploid embryo and the triploid
endosperm.
• Embryo: Young sporophyt, dipoid (2n), result of fertilization. The mature embryo consists of
cotyledons (seed leaves), hypocotyl (stem-like embryonic axis below the cotyledons), radicle
(embryonic root).
• Seed and embryo types were defined by Martin (1946). These and the resulting evolutionary
trends are found on the "seed evolution" webpage.
• Endosperm: Food storage tissue, triploid (3n), result of double fertilization, 2/3 of the genome is
of maternal origin.
• Testa (seed coat): Outer protective layer of the seed, developed from the integuments of the
ovule, diploid maternal tissue.
• Fruits are mature, ripened ovaries containing seeds. The pericarp ("fruit coat") is diploid
maternal tissue.
• Perisperm: Diploid maternal food storage tissue originates from the nucellus. Only in some
species, e.g. Beta vulgaris, Piper nigrum, Coffea arabica, many Caryophyllales.
• Endospermic seeds: The endosperm is present in the mature seed and serves as food storage
organ. Testa and endosperm are the two covering layers of the embryo. The amount of
endosperm in mature seeds is highly species-dependent and varies from an abundant
endosperm layer (Nicotiana tabaccum) to a single layer (Arabidopsis thaliana).
• Non-endospermic seeds: The cotyledons serve as sole food storage organs as in the case of pea
(Pisum sativum). During embryo development the cotyledons absorb the food reserves from the
endosperm. The endosperm is almost degraded in the mature seed and the embryo is enclosed
by the testa.

More special seed structural features:

• Hilum and funiculus: Funicular scar on seed coat that marks the point at which the seed was
attached via the funiculus to the ovary tissue.
• Micropyle: The Micropyle is a canal or hole in the coverings (seed coat) of the nucellus through
which the pollen tube usually passes during fertilization. Later, when the seed matures and
starts to germinate, the micropyle serves as a minute pore through which water enters. The
micropylar seed end has been demonstrated to be the major entry point for water during
tobacco seed imbibition and germination. During germination the tobacco testa ruptures at the
micropylar end and the radicle protrudes through the micropylar endosperm.
• Chalaza: Non-micropylar end of the seed. The base of an ovule, bearing an embryo sac
surrounded by integuments.
• Raphe: Ridge on seed coat formed from adnate funiculus.
• Arillate: General term for an outgrowth from the funiculus, seed coat or chalaza; or a fleshy
seed coat.
• Aril: Outgrowth of funiculus, raphe, or integuments; or fleshy integuments or seed coat, a
sarcotesta. Arils probably often aid seed dispersal, by drawing attention to the seed after the
fruit has dehisced, and by providing food as an attractant reward to the disperser. The aril of the
nutmeg produces the spice mace and the seed itself is the nutmeg.
• Strophiole: Outgrowth of the hilum region which restricts water movement into and out of
some seeds. In some hard-coated legume seeds, e.g. Melilotus alba and Trigonella arabica, a
 plug covering a special opening - the strophiolar cleft - must be loosened or removed before
water can enter, and then only through this region.
• Operculum: A little seed lid. It refers to a dehiscent cap of a seed or a fruit that opens during
germination.
• Carunculate: Seed with an excrescent outgrowth from integuments near the hilum, as in
Euphorbia.
• Fibrous: Seed with stringy or cord-like seed coat, as mace in Myristica.
• Funicular: Seed with a persistent elongate funiculus attached to seed coat, as in Magnolia.
• Strophiolate: Seed with elongate aril or strophiole in the hilum region.
• Fruit: Strictly, the ripened ovary of a plant and its contents. More loosely, the term is extended
to the ripened ovary and seeds together with any structure with they are combined, e. g. the
apple (a pome) in which the fruit (core) is surrounded by flesh derived from the floral
receptacle.
• Achene: A small, usually single-seeded, dry indehiscent fruit, e.g. lettuce.
• Caryopsis: A dry, nut-like fruit typical of grasses, e. g. a cereal grain. It is an achene with the
ovary wall united with the seed coat.
• Elaiosomes: A specialty in the dispersal through animals is that through ants (myrmecochory).
Such seeds or fruits bear attachments, the elaiosomes that contain lures and nutriments.
Myrmecochory is common with plants that live at the forest soil like violets (Viola).
• Caruncle: A reduced aril, in the form of a fleshy, often waxy or oily, outgrowth near the hilum of
some seeds. Usually it is brightly colored. It acts as an aid to dispersal. Viola seeds have an oily
caruncle and are sought and dispersed by ants.
• Mucilage: A layer of polysaccharide slime produced by some seeds upon imbibition. Serves in
water uptake during imbibition and germination.

Secondary Growth
Some Important Definitions:

Primary tissues: Tissues generated from the growth of an apical meristem.

Cambium: A lateral meristem constituting a sheet of cells. Growth of these cells increases the
girdth of the plant organ involved.

Secondary tissues: Tissues generated from the growth of a cambium.

Vascular Cambium: A cambium that gives rise to secondary xylem to the inside, and to secondary
phloem to the outside.

Periderm: A structure that consists of a cork cambium (phellogen), with cork tissue (phellem) to
the outside, and in some cases a layer of cells derived from and to the inside of the cork cambium
called phelloderm. Functions to limit dehydration and block pathogens after the epidermis is
disrupted by the onset of secondary growth: Link to view of a periderm of Tilia.

Cork: (phellem) you need know only the term "cork": Tissue dead at maturity generated from a
cork cambium. The cell walls of the tissue are impregnated with suberin. This water-proofs the
tissue. The cork used to seal wine bottles is "cork" tissue harvested from a species of oak.The cell
theory was first proposed by Robert Hooke in 1665 after microscopic exaination a slice of cork.

Cork Cambium: A cambial layer that functions to produce cork, and in some cases, phelloderm.
In roots is derived initially from pericyle. In stems from the cortex. Unlike the vascuar cambium
these cambial layers do not persist for the duration of the life of the plant organ. Over time one
cork cambium will be supplanted by another generated from parenchyma cells further inside:

Phelloderm: In some periderms a layer of living secondary tissue is generated by the cork cambium
to the inside. We will not consider thie phelloderm in the following exercise.

Secondary Growth in Stems

IIa. Cross Section of Tilia (basswood) Stem at the End of Primary Growth

This stem differs somewhat from that of Medicago or Coleus. The obvious difference is in the
organization of the vascular tissue. The pith rays are only one cell layer wide and the primary
vascular tissue appears as a continuous ring. As in the stems studied earlier, the ground tissue
inside the vascular tissue is called the pith and that outside the cortex. Dermal tissue consists of an
epidermis.

IIb. Cross Sections of Tilia (basswood) Stem: 1, 2 and 3 Years Old:

Section at the end of the first year: By the end of the first year, the primary structure of the stem
has been transformed by the growth of the vascular and cork cambiums. The pith in the midde is
intact as is the primary xylem. The secondary xylem is continuous with the primary xylem and
extends out to the vascular cambium. The boundaries of the secondary xylem can be determined
by where the rays begin in the cylinder of xylem as rays are a characteristic of secondary vascular
tissue. Beyond the vascular cambium is secondary phloem followed by primary phloem. The
obvious fibers visible are in the primary phloem and have differentiated since the end of primary
growth. Beyond the phloem is cortex bounded by a periderm. The cork cambium is the last living
tissue layer in the stem. Note the epidermis being sloughed off.

Section at the end of three years growth: The obvious changes visible here are the growth rings
present in the secondary xylem, and the growth of certain rays in the phloem forming wedge-
shaped regions in that tissue.

The sequence of tissues outlined before are the same from the center outward: pith, primary xylem,
secondary xylem, vascular cambium, secondary phloem, primary phloem, cortex, and periderm.

Using higher magnification it can be seen that the growth increments are areas where smaller thick-
walled vessel members border larger thin-walled vessel members. The smaller cells make up late
summer's growth and the larger cells early spring growth. By observing this boundary you should
be able to tell in which direction is the pith - think about it.
The rays in the xylem are continuous with those in the phloem. The enlargement of some of the
phloem rays relieves the tension on the phloem created by the expanding cylinder of xylem. This
stress tends to create longitudinal rips in the phloem which would destroy its integrity. The
expansion of these rays (they are called dialated rays) prevents these tears. The phloem outside of
this ray tissue consists of bands of fibers alternating with areas containing sieve-tube members and
companion cells.

Tangential (face) view of vascular cambium: This is a view of a longitudinal section made just
inside the secondary phloem perpendicular to the rays. It provides us with a face-view of the sheet
of vascular cambium. In it we can clearly see the two types of cells that make of the tissue: ray
initials and fusiform initials. The ray initials give rise to the rays in both the phloem and xylem.
The fusiform initials have their long axes arranged vertically. These cells give rise to tracheary
elements in the xylem as well as to sieve-tube members and companion cells in the phloem.

IIc. Gross structure of woody stems:

Woody stems are mostly seconday xylem (wood) surrounded by bark. The xylem may include
heart-wood and sap-wood. Heart-wood is dead and non-functional. The sap wood is functional and
has living parenchyma cells. The boundary between the bark and wood is the vascular cambium.
The bark is divided into two regions by the cork cambium: the living area inside the cork cambium
is the inner bark, and the dead tissue outside is the outer bark. Evidence of earlier cork cambiums
can be easily discerned in some woody stems.

Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing
season, cells of the secondary xylem have a large internal diameter; their primary cell walls are
not extensively thickened. This is known as early wood, or spring wood. During the fall season,
the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is
denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease
in the number of vessel elements and a seasonal increase in the number of tracheids. It results in
the formation of an annual ring, which can be seen as a circular ring in the cross section of the
stem. An examination of the number of annual rings and their nature (such as their size and cell
wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each
season.

Secretory Structures in Plants

The group of cells concerned in secretion of cutin, wax, suberin etc. is generally termed as
secretory structures. The term secretion includes excretion and recretion also.

The term secretion implies the act of separation of by-products of metabolism from protoplast.
These substances may be stored in insoluble forms within the cell or exuded from it, and have
either a special physiological function or no use to the plant and regarded as waste. The removal
of waste products of metabolism is defined as excretion.

The process, which eliminates the salt from a cell and thus regulates the ion content of it, is defined
as recretion. In the strict sense, the term secretion refers to those end products that take part in the
metabolism process (ex. hormones and enzymes). The process of cell wall formation, cutinization,
cuticularization, wax deposition, suberization etc. are also the examples of secretion.

Examples of excretion are terpenes, saponins, rubber, tannins, crystals etc. Later findings reveal
that the borderline between secretion and excretion is ill defined.

As for example, the terpenes are regarded as excretions, though they play important roles
in:

(i) Attracting pollinators,

(ii) Repelling animal foes,

(iii) Blocking wounds in plant organs and

(iv) Having antiseptic properties etc.

Therefore, the present day authors use the term secretion to denote excretion and recretion as well.
Many of the end products have enormous commercial value, e.g. rubber, opium, gutta-percha etc.

The secretory structures vary greatly in structure and position. They may be either simple glandular
trichomes or multicellular glands with vascular tissues. They may be external when originate from
epidermis or deep seated or internal such as laticifers and resin ducts. The various structures, both
external and internal, involved in secretion are discussed below.

External Secretory Structure:

These include glandular trichomes, nectary, osmophores, hydathodes and salt glands, of which
hydathodes and salt glands.

Glandular Trichomes:

These trichomes consist of a stalk with a head above. The stalk may be unicellular or multicellular
and in the latter case the cells may be arranged in several rows. The head is the secretory part and
may be composed of single cell (ex. Pelargonium) or many cells (e.g. Callitriche). The head is
covered with a cuticle. The secretion is accumulated beneath the cuticle.

This type is exampled by the volatile oils like camphors, balsams, peppermint oil, resins etc.
Trichomes may also secrete nectar (ex. stipules of Vicia sepium) and water (ex. leaves of
Hygrophila). Several of the secretions of the glandular trichomes have role in plant defense. Some
are repellent to insects. The glandular hairs of tomato and wild potato (Solatium berthaultii)
provide resistance to aphids.

These trichomes secrete sticky exudates that trap the aphids. Plant breeders are now trying to raise
a hybrid of cultivated potato that possesses these useful aphid-trapping glandular hairs.

The glandular trichomes of insectivorous plants have specialized functions:

(i) They secrete mucilage that is effective in trapping insects;

(ii) They contain proteolytic enzymes;

(iii) They have absorptive functions, i.e. the products of digestion move into the leaf through them.

Nectary:

Nectary can be defined as a gland or part of a flower that secretes nectar to the exterior of plants.
They are divided into floral and extrafloral nectaries. The former is situated within the flower and
is directly involved in pollination; the latter occurs on the vegetative organs and is not directly
associated with pollination. The nectaries are present on the epidermis.

They may be deeply sunken, or raised above over an outgrowth, or at the level of the organ that
bears them. The tissues that compose the nectary are commonly known as nectariferous tissues.
These tissues may not form any anatomically differentiated structure – termed non-structural
nectaries (e.g. on leaves of Dracaena reflexa, in bracts of Sansevieria zeylanica, on tepals of
Cattleya percivaliana etc.).

In some cases it is difficult to distinguish the non-structural nectaries from the surrounding tissues.
The histochemical test reveals that the parenchyma cells composing the nonstructural nectaries
have a high acid phosphatase activity like the anatomically differentiated ones. The nectariferous
tissues may form anatomically differentiated structure termed structural nectaries, which are
macroscopically recognizable also.
It consists of stalk cells and secretory cells with cuticle. The cells of the nectary are specialized
parenchyma cells that are small, thin walled and contain dense cytoplasm, dictyosomes,
endoplasmic reticulum, small vacuoles and large nuclei (Fig. 11.1). The nectaries may be provided
with special vascular tissues. The nectar is supplied by phloem and it accumulates between the
cuticle and secretory cells.

The nectar of non-structural nectaries is exuded through stomata. Nectar, in structural nectaries, is
secreted by epidermal cells or trichomes directly to the outside. The nectar contains glucose,
sucrose and fructose as major components. The followings are also recorded in the nectar of
different plant species: maltose, melobiose, mucilage, proteins, phosphates, mineral ions, organic
acids, oxidases, sucrose, vitamins etc.

The non-enzymatic proteins are detected in the floral nectary of Erica, Bergenia etc. The lipids are
reported from Jacaranda, Trichocereus etc. The nectar also contains all the essential aminoacids,
which with their smell attract some of the anthophilous insects. It is suggested that nectar provides
the aminoacid requirements for the insects. The cells of nectaries may absorb the sugary fluid. So
they possess both secretory and absorptive properties.

The extra floral nectaries are very common in dicotyledonous and much less in monocotyledonous
plants. It rarely occurs in Poaceae (e.g. Andropogon and Eragrostis). In dicots it is present on all
organs, namely —in the cotyledons (e.g. Ricinns communis), on leaf margins (e.g. Rosa, Populus
etc.), on phyllodes (e.g. Acacia longifolia), on pulvinus (e.g. Thunbergia grandiflora), on stipules
(ex. Vicia fava) etc.

Osmophores:

Osmophores can be defined as certain special areas on floral organs, which differ in structure from
the neighbouring cells and have the fragrance producing properties to attract pollinators.
The fragrance of a flower is due to volatile low molecular weight terpenes. ‘The fragrant materials
are not exuded by the glandular trichome. This substance occurs as droplets in the cytoplasm of
epidermal cells. The oil droplets of terpenes, at an appropriate temperature diffuse out of the cell
in gaseous form through the cell wall and cuticles.

As a result the fragrance is produced. In many plants certain localized cells are only involved in
producing the fragrant materials. The cells differ markedly from the other normal cells situated
nearby and are termed as osmophores. The osmophores secrete terpenes as the main fragrant
materials. In some species belonging to Araceae the fragrant substance may contain amines and
ammonia in addition to terpenes.

The osmophores appear as flaps, cilia or brushes; they can be stained with neutral red and thus can
be identified from the neighbouring cells. Osmophores are present in Asclepiadaceae, Araceae,
Orchidaceae, Aristolochiaceae and Liliaceae.

The osmophores of Ceropegia (Fig. 11.2) consists of an epidermal layer and two rows of
isodiametric cells situated below the epidermis. The epidermis contains dense cytoplasm and the
isodiametric cells are filled with starch grains. The starch disappears after the emission of fragrant
materials.

Internal Secretory Structure:

The internal secretory structures may be composed of a single cell or groups of cells. They may
occur throughout a tissue (e.g. oils or enzymes) or may be localized in distribution.

Example: castor oil obtained from the endosperm of Ricinus; ground nut oil extracted from the
cotyledons of Arachis; the source of palm oil is the mesocarp of the fruit of Elaeis guineensis; the
seed of Carthamus tinctorius yields safflower oil etc. Sometimes resin or oil secreting idioblasts
are formed, e.g. oleo-resin cells are present in the ground tissue of the rhizome of Zingiber
officinale, the oil cells secreting the aromatic oil occur in the phloem of Cinnamomum zeylanicum.

Glands and Ducts:

They comprise a group of cells or sometimes a single cell that is readily distinguishable from the
neighbouring cells and secretes a specific substance. These cells are thin walled with dense
protoplasm and sometimes occur as layer surrounding a cavity, known as secretory cavity.

The secretion is discharged and accumulates within the internal cavity. These cavities may be more
or less spherical or much elongated like tubes and respectively termed as glands or ducts.

The internal cavities originate by three ways and accordingly the following three types of
glands are recognized:

(i) Schizogenous glands:

These are formed by the dissolution of middle lamella, thus separating apart the cells to form
cavity. Example: oil glands of Eucalyptus, the secretory ducts of Rhus glabra, resin duct of Pinus
etc. This cavity remains surrounded by a ring of intact parenchyma cells, termed epithelium, which
forms a well-defined boundary of the gland.

(ii) Lysigenous glands:

These glands originate by lysis of a few cells thus forming the cavity (ex. glands present in the
leaves and fruits of Citrus sp., that are also formed schizogenously).

(iii) Schizolysigenous glands:

These glands arise by both phenomenon schizogeny and lysis, i.e. the middle lamella and some of
the adjoining cells disintegrate to form cavity. Example: Eugenia caryophyllata where the glands
are present in the floral parts that are the source of oil of clove. In contrast to schizogenous gland
the lysigenous — and schizolysigenous gland have no clear-cut boundary.

Laticifers

Laticifers can be defined as a specialized cell or a row of such cells that secrete the milky fluid
termed latex. The word laticifer is used as a general term to denote the various latex-secreting
structures — latex cell, latex vessel, latex duct, latex tube and laticiferous duct. The laticiferous
duct is a cavity into which latex is secreted.

The latex cell may be simple or branched and is derived from the enlargement of a single cell. The
latex ducts are elongated, branched and aseptate. The latex vessel is simple or branched tube that
usually anastomoses with similar tubes; it is formed as a result of enlargement and union of chain
of cells. The latex tube usually means either a latex cell or the latex vessel.

The borderline between the different terms of latex secreting structures (i.e. latex cell, -duct, -
vessel, -tube etc.) is ill defined and so the word laticifer is introduced as a general term. The
laticifers may be simple or compound on the basis of origin. The simple laticifer is derived from a
single cell whereas the compound laticifer originates from a longitudinal file of cells.

Latex is fluid produced in the latex vessels or cells. It is usually white and milky (ex. Euphorbia,
Asclepias, Lactuca etc.), yellow and brown (e.g. Cannabis), orange and sometimes colourless and
clear (e.g. Morus, Nerium etc.). It contains a number of substances including sugars, proteins,
alkaloids, oils, mineral salts, organic acids, terpenes, resins, rubber etc.

The latex of Euphorbia milii contains starch grains that are dumb-bell shaped. The proteolytic
enzyme papain is present in the latex of Carica papaya. The latex of Asclepias syriaca contains the
enzyme pectinase. The latex of some Euphorbia species is rich in vitamin B1.

The cell wall of laticifers is thick and may be thicker than the adjacent cells. They are not lignified.
The growth of cell wall occurs through apposition process. The tip of latex cell is thin walled. The
walls are composed of cellulose, hemicellulose and pectin.

The laticifers may occur throughout the plant body or their distribution is restricted to certain
tissues. Laticifers are grouped into two: non-articulate and articulate. The former is derived from
the enlargement of a single cell. This cell has the potentiality of unlimited and rapid growth, and
elongates to form long latex tubes. The tubes may remain unbranched termed non-articulate
unbranched laticifer (e.g. Vinca, Cannabis, Urtica etc.).

In some plants (e.g. Euphorbia, Nerium etc.) the tubes may branch called non-articulate branched
laticifers that seldom anastomose with similar types. The non-articulate laticifers are coenocytic
and multinucleated, and also termed as laticiferous cell.

There is continuity of laticifers between the shoots and branches. Laticifers grow through the
intercellular spaces and the enzyme pectinase helps in the process. Pectinase is secreted by the
growing tip of laticifers and dissolves the middle lamella.

The articulate laticifers, also termed laticiferous vessel, consist of longitudinal files of cells. The
transverse end walls of the individual cell either remain intact or break down partly or wholly to
form a continuous tube —the latex vessel. So the articulated laticifers are always compound in
origin. They occur in primary or secondary phloem and may be present in cortical parenchyma.

They may remain either as a single chain of cells without any anastomosis termed articulated non-
anastomosing laticifer (e.g. Convolvulus, Allium, Musaetc.), or they may anastomose with similar
file of cells to form a complex anastomose system called articulated anastomosing laticifer (e.g.
Lactuca, Papaver, Caricn papaya etc.).

The enzyme cellulase is found in the latex of articulated laticifers and it is absent from non-
articulated laticifers. So it is suggested that cellulase is involved in the lysis of common transverse
walls during the formation of articulated laticifers.

Latex occurs in 900 genera distributed in 20 families of mostly in dicotyledons ( e.g. Apocynaceae,
Asclepiadaceae, Compositae, Euphorbiaceae, Papaveraceae etc.) and in a few families of
monocotyledons (e.g. Araceae, Musaceae and Liliaceae).

They are very much economically important, a few of which are mentioned below: The opium, a
medicinally important alkaloid is obtained from Papaver somniferum. The most important latex is
rubber whose principal source is Hevea brasiliensis. The species of Palaquium gutta yields gutta-
percha. The latex of Achras sapota yields chicle, from which chewing gum is made.

Function of laticifers:

Regarding the function of laticifers different views were expressed since 1877, a brief account
of which is mentioned below:

(1) It is a vital sap vessel and similar to the blood vessels of animals.

(2) It takes part in translocation of assimilates as it is associated with phloem.

(3) It stores food materials.

(4) It is now considered as secretory tissues where the secretory substances do not re-enter the
plant metabolism.

(5) Sen and Chawan (1972) suggested that laticifiers regulate the water balance in plants.

(6) It has role in the transport of oxygen.

(7) It has role in healing up of wounds.

(8) It acts as a defense against herbivores and microorganisms. [The human ocular tissues are
damaged by the latex of Calotropis procera].

(9) McCay and Mahlberg (1973) reported the absence of bacterial activity from laticifers of
Asclepias in vivo.

Myrosin Cell:

It is a special type of secretory cell where the enzyme is produced and stored. Myrosin cells are
idioblasts, which contain the enzyme myrosinase (β-thioglucosidase). They are reported from
Cruciferae, Capparidaceae, Moringaceae, Tropaeolaceae etc.

They are more or less similar in shape to the neighbouring cells and may be elongated or assume
various shapes. They occur in the integument of seeds, in the exocarp and endocarp of fruits, in
the cotyledons, in the cortex of stem, in the leaf lamina, spongy parenchyma etc.

The enzyme myrosinase hydrolyses thioglucosides to produce mustard oil and other substances.
The enzyme and substrate occur in different cells, i.e. the enzyme is produced in myrosin cells
and thioglucosides are present in the other parenchyma cells.

The reaction between the enzyme and substrate is brought about only when the tissues are damaged
and thus mustard oil is released. When insects feed on such plants mustard oil is released in the
insect gut. It is demonstrated that mustard oil is toxic to a number of insects.