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may not be definite. Examine shoots that grow on the under side of
dense tree tops or in other partially lighted positions.
Suggestions.—55. The pupil should match leaves to determine whether any
two are alike. Why? Compare leaves from the same plant in size, shape, colour,
form of margin, length of petiole, venation, texture (as to thickness or thinness),
stage of maturity, smoothness or hairiness. 56. Let the pupil take an average leaf
from each of the first ten different kinds of plants that he meets and compare them
as to the above points (in Exercise 55), and also name the shapes. Determine how
the various leaves resemble and differ. 57. Describe the stipules of rose, apple, fig,
willow, violet, pea, or others. 58. In what part of the world are parallel-veined
leaves the more common? 59. Do you know of parallel-veined leaves that have
lobed or dentate margins? 60. What becomes of dead leaves? 61. Why is there no
grass or other undergrowth under pine and spruce trees? 62. Name several leaves
that are useful for decorations. Why are they useful? 63. What trees in your vicinity
are most esteemed as shade trees? What is the character of their foliage? 64.
Why are the internodes so long in water-sprouts and suckers? 65. How do foliage
characters in corn or sorghum differ when the plants are grown in rows or
broadcast? Why? 66. Why may removal of half the plants increase the yield of
cotton or sugar-beets or lettuce? 67. How do leaves curl when they wither? Do
different leaves behave differently in this respect? 68. What kinds of leaves do you
know to be eaten by insects? By cattle? By horses? What kinds are used for
human food? 69. How would you describe the shape of leaf of peach? apple?
elm? hackberry? maple? sweet-gum? corn? wheat? cotton? hickory? cowpea?
strawberry? chrysanthemum? rose? carnation? 70. Are any of the fore-going
leaves compound? How do you describe the shape of a compound leaf? 71. How
many sizes of leaves do you find on the bush or tree nearest the schoolroom
door? 72. How many colours or shades? 73. How many lengths of petioles? 74.
Bring in all the shapes of leaves that you can find.
Fig. 112.—Cowpea. Describe the leaves. For
what is the plant used?
 CHAPTER XII
LEAVES—STRUCTURE OR ANATOMY

Besides the framework, or system of veins found in blades of all

leaves, there is a soft cellular tissue called mesophyll, or leaf
parenchyma, and an epidermis or skin that covers the entire
outside part.
Mesophyll.—The
mesophyll is not all alike or
homogeneous. The upper
layer is composed of
elongated cells placed
perpendicular to the
surface of the leaf. These
are called palisade cells.
These cells are usually
Fig. 113.—Section of a Leaf, showing the air- filled with green bodies
spaces.
Breathing-pore or stoma at a. The palisade cells which chiefly called chlorophyll grains.
contain the chlorophyll are at b. Epidermal cells at c. The grain contains a great
number of chlorophyll
drops imbedded in the protoplasm. Below the palisade cells is the
spongy parenchyma, composed of cells more or less spherical in
shape, irregularly arranged, and provided with many intercellular air
cavities (Fig. 113). In leaves of some plants exposed to strong light
there may be more than one layer of palisade cells, as in the India-
rubber plant and the oleander. Ivy when grown in bright light will
develop two such layers of cells, but in shaded places it may be
found with only one. Such plants as iris and compass plant, which
have both surfaces of the leaf equally exposed to sunlight, usually
have a palisade layer beneath each epidermis.
 Epidermis.—The outer or epidermal cells of leaves do not bear
chlorophyll, but are usually so transparent that the green mesophyll
can be seen through them. They often become very thick-walled,
and are in most plants devoid of all protoplasm except a thin layer
lining the walls, the cavities being filled with cell sap. This sap is
sometimes coloured, as in the under epidermis of begonia leaves. It
is not common to find more than one layer of epidermal cells forming
each surface of a leaf. The epidermis serves to retain moisture in the
leaf and as a general protective covering. In desert plants the
epidermis, as a rule, is very thick and has a dense cuticle, thereby
preventing loss of water.
There are various outgrowths of the epidermis. Hairs are the chief
of these. They may be (1) simple, as on primula, geranium, nægelia;
(2) once branched, as on wall-flower; (3) compound, as on
verbascum or mullein; (4) disk-like, as on shepherdia; (5) stellate,
or star-shaped, as in certain crucifers. In some cases the hairs are
glandular, as in Chinese primrose of the greenhouses (Primula
Sinensis) and certain hairs of pumpkin flowers. The hairs often
protect the breathing-pores, or stomates, from dust and water.
Stomates (sometimes called breathing-pores) are small
openings or pores in the epidermis of leaves and soft stems that
allow the passage of air and other gases and vapours (stomate or
stoma, singular; stomates or stomata, plural). They are placed near
the large intercellular spaces of the mesophyll, usually in positions
least affected by direct sunlight. Fig. 114 shows the structure. There
are two guard-cells at the mouth of each stomate, which may in
most cases open or close the passage as the conditions of the
atmosphere may require. The guard-cells contain chlorophyll. In Fig.
115 is shown a case in which there are compound guard-cells, that
of ivy. On the margins of certain leaves, as of fuchsia, impatiens,
cabbage, are openings known as water-pores.
Stomates are very numerous, as will be seen from the numbers
showing the pores to each square inch of leaf surface:
Fig. 114.—Diagram of Stomate of Fig. 115.—Stomate of Ivy, showing
Iris (Osterhout). compound guard-cells.

Lower Upper
surface surface
Peony 13,790 None
Holly 63,600 None
Lilac 160,000 None
Mistletoe 200 200
Tradescantia 2,000 2,000
Garden Flag
11,572 11,572
(iris)
The arrangement of stomates on the leaf
differs with each kind of plant. Fig. 116 shows
stomates and also the outlines of contiguous
epidermal cells.
The function or work of the stomates is to
regulate the passage of gases into and out of
the plant. The directly active organs or parts are
Fig. 116.—Stomates ofguard-cells, on either side the opening. One
Geranium Leaf. method of opening is as follows: The thicker
walls of the guard-cells (Fig. 114) absorb water
from adjacent cells, these thick walls buckle or bend and part from
one another at their middles on either side the opening, causing the
stomate to open, when the air gases may be taken in and the leaf
gases may pass out. When moisture is reduced in the leaf tissue, the
guard-cells part with some of their contents, the thick walls
straighten, and the faces of the two opposite ones come together,
thus closing the stomate and preventing any water vapour
from passing out. When a leaf is actively at work making
new organic compounds, the stomates are usually open;
when unfavourable conditions arise, they are usually
closed. They also commonly close at night, when growth
(or the utilizing of the new materials) is most likely to be
active. It is sometimes safer to fumigate greenhouses and
window gardens at night, for the noxious vapours are less
likely to enter the leaf. Dust may clog or cover the
stomates. Rains benefit plants by washing the leaves as
well as by providing moisture to the roots.
Lenticels.—On the young woody twigs of many plants
(marked in osiers, cherry, birch) there are small corky
spots or elevations known as lenticels (Fig. 117). They
mark the location of some loose cork cells that function as
stomates, for green shoots, as well as leaves, take in and
discharge gases; that is, soft green twigs function as
leaves. Under some of these twig stomates, corky material
may form and the opening is torn and enlarged: the
lenticels are successors to the stomates. The stomates lie
in the epidermis, but as the twig ages the epidermis
perishes and the bark becomes the external layer. Gases
continue to pass in and out through the lenticels, until the
branch becomes heavily covered with thick, corky bark.
With the growth of the twig, the lenticel scars enlargeFig. 117.—
lengthwise or crosswise or assume other shapes, often Lenticel
s on
becoming characteristic markings. Young
Fibro-vascular Bundles.—We have studied the fibro- Shoot
of Red
vascular bundles of stems (Chap. X). These stem bundles Osier
continue into the leaves, ramifying into the veins, carrying (Cornus)
the soil water inwards and bringing, by diffusion, the .
elaborated food out through the sieve-cells. Cut across a
petiole and notice the hard spots or areas in it; strip these parts
lengthwise of the petiole. What are they?
 Fall of the Leaf.—In most common deciduous plants, when the
season’s work for the leaf is ended, the nutritious matter may be
withdrawn, and a layer of corky cells is completed over the surface of
the stem where the leaf is attached. The leaf soon falls. It often falls
even before it is killed by frost. Deciduous leaves begin to show the
surface line of articulation in the early growing season. This
articulation may be observed at any time during the summer. The
area of the twig once covered by the petioles is called the leaf-scar
after the leaf has fallen. In Chap. XV are shown a number of leaf-
scars. In the plane tree (sycamore or buttonwood), the leaf-scar is in
the form of a ring surrounding the bud, for the bud is covered by the
hollowed end of the petiole; the leaf of sumac is similar. Examine
with a hand lens leaf-scars of several woody plants. Note the
number of bundle-scars in each leaf-scar. Sections may be cut
through a leaf-scar and examined with the microscope. Note the
character of cells that cover the leaf-scar surface.
Suggestions.—To study epidermal hairs: 75. For this study, use the leaves of
any hairy or woolly plant. A good hand lens will reveal the identity of many of the
coarser hairs. A dissecting microscope will show them still better. For the study of
the cell structure, a compound microscope is necessary. Cross-sections may be
made so as to bring hairs on the edge of the sections; or in some cases the hairs
may be peeled or scraped from the epidermis and placed in water on a slide. Make
sketches of the different kinds of hairs. 76. It is good practice for the pupil to
describe leaves in respect to their covering: Are they smooth on both surfaces? Or
hairy? Woolly? Thickly or thinly hairy? Hairs long or short? Standing straight out or
lying close to the surface of the leaf? Simple or branched? Attached to the veins or
to the plane surface? Colour? Most abundant on young leaves or old? 77. Place a
hairy or woolly leaf under water. Does the hairy surface appear silvery? Why?
Other questions: 78. Why is it good practice to wash the leaves of house plants?
79. Describe the leaf-scars on six kinds of plants: size, shape, colour, position with
reference to the bud, bundle-scars. 80. Do you find leaf-scars on
monocotyledonous plants—corn, cereal grains, lilies, canna, banana, palm,
bamboo, green brier? 81. Note the table on page 88. Can you suggest a reason
why there are equal numbers of stomates on both surfaces of leaves of
tradescantia and flag, and none on upper surface of other leaves? Suppose you
pick a leaf of lilac (or some larger leaf), seal the petiole with wax and then rub the
under surface with vaseline; on another leaf apply the vaseline to the upper
surface; which leaf withers first, and why? Make a similar experiment with iris or
blue flag. 82. Why do leaves and shoots of house plants turn towards the light?
What happens when the plants are turned around? 83. Note position of leaves of
beans, clover, oxalis, alfalfa, locust, at night.
 CHAPTER XIII
LEAVES—FUNCTION OR WORK

We have discussed (in Chap. VIII) the work or function of roots

and also (in Chap. X) the function of stems. We are now ready to
complete the view of the main vital activities of plants by considering
the function of the green parts (leaves and young shoots).
Sources of Food.—The ordinary green plant has but two sources
from which to secure food,—the air and the soil. When a plant is
thoroughly dried in an oven, the water passes off; this water came
from the soil. The remaining part is called the dry substance or dry
matter. If the dry matter is burned in an ordinary fire, only the ash
remains; this ash came from the soil. The part that passed off as gas
in the burning contained the elements that came from the air; it also
contained some of those that came from the soil—all those (as
nitrogen, hydrogen, chlorine) that are transformed into gases by the
heat of a common fire. The part that comes from the soil (the ash) is
small in amount, being considerably less than 10 per cent and
sometimes less than 1 per cent. Water is the most abundant single
constituent or substance of plants. In a corn plant of the roasting-ear
stage, about 80 per cent of the substance is water. A fresh turnip is
over 90 per cent water. Fresh wood of the apple tree contains about
45 per cent of water.
Carbon.—Carbon enters abundantly into the composition of all
plants. Note what happens when a plant is burned without free
access of air, or smothered, as in a charcoal pit. A mass of charcoal
remains, almost as large as the body of the plant. Charcoal is almost
pure carbon, the ash present being so small in proportion to the
large amount of carbon that we look on the ash as an impurity.
Nearly half of the dry substance of a tree is carbon. Carbon goes off
as a gas when the plant is burned in air. It does not go off alone, but
in combination with oxygen in the form of carbon dioxide gas, CO2.
The green plant secures its carbon from the air. In other words,
much of the solid matter of the plant comes from one of the gases of
the air. By volume, carbon dioxide forms only a small fraction of 1
per cent. of the air. It would be very disastrous to animal life,
however, if this percentage were much increased, for it excludes the
life-giving oxygen. Carbon dioxide is often called “foul gas.” It may
accumulate in old wells, and an experienced person will not descend
into such wells until they have been tested with a torch. If the air in
the well will not support combustion,—that is, if the torch is
extinguished,—it usually means that carbon dioxide has drained into
the place. The air of a closed schoolroom often contains far too
much of this gas, along with little solid particles of waste matters.
Carbon dioxide is often known as carbonic acid gas.
Appropriation of the Carbon.—The carbon dioxide of the air
readily diffuses itself into the leaves and other green parts of the
plant. The leaf is delicate in texture, and when very young the air can
diffuse directly into the tissues. The stomates, however, are the
special inlets adapted for the admission of gases into the leaves and
other green parts. Through these stomates, or diffusion-pores, the
outside air enters into the air-spaces of the plant, and is finally
absorbed by the little cells containing the living matter.
Chlorophyll (“leaf green”) is the agent that secures the energy by
means of which carbon dioxide is utilized. This material is contained
in the leaf cells in the form of grains (p. 86); the grains themselves
are protoplasm, only the colouring matter being chlorophyll. The
chlorophyll bodies or grains are often most abundant near the upper
surface of the leaf, where they can secure the greatest amount of
light. Without this green colouring matter, there would be no reason
for the large flat surfaces which the leaves possess, and no reason
for the fact that the leaves are borne most abundantly at the ends of
branches, where the light is most available. Plants with coloured
leaves as coleus, have chlorophyll, but it is masked by other
colouring matter. This other colouring matter is usually soluble in hot
water: boil a coleus leaf and notice that it becomes green and the
water becomes coloured.
Plants grown in darkness are yellow and slender, and do not reach
maturity. Compare the potato sprouts that have grown from a tuber
lying in a dark cellar with those that have grown normally in the
bright light. The shoots have become slender, and are devoid of
chlorophyll; and when the food that is stored in the tuber is
exhausted these shoots will have lived useless lives. A plant that has
been grown in darkness from the seed will soon die, although for a
time the little seedling will grow very tall and slender. Why? Light
favours the production of chlorophyll, and the chlorophyll is the agent
in the making of the organic carbon compounds. Sometimes
chlorophyll is found in buds and seeds, but in most cases these
places are not perfectly dark. Notice how potato tubers develop
chlorophyll, or become green, when exposed to light.
Photosynthesis.—Carbon dioxide diffuses into the leaf; during
sunlight it is used, and oxygen is given off. How the carbon dioxide
which is thus absorbed may be used in making an organic food is a
complex question, and need not be studied here; but it may be
stated that carbon dioxide and water are the constituents. Complex
compounds are built up out of simpler ones.
Chlorophyll absorbs certain light rays, and the energy thus directly
or indirectly obtained is used by the living matter in uniting the
carbon dioxide absorbed from the air with some of the water brought
up from the roots. The ultimate result usually is starch. The process
is obscure, but sugar is generally one step; and our first definite
knowledge of the product begins when starch is deposited in the
leaves. The process of using the carbon dioxide of the air has been
known as carbon assimilation, but the term now most used is
photosynthesis (from two Greek words meaning light and placing
together.)
Starch and Sugar.—All starch is composed of carbon, hydrogen,
and oxygen (C6H10O5)n. The sugars and the substance of cell walls
are very similar to it in composition. All these substances are called
carbohydrates. In making fruit sugar from the carbon and oxygen of
carbon dioxide and from the hydrogen and oxygen of the water,
there is a surplus of oxygen (6 parts CO2 + 6 parts H2O = C6H12O6 +
6 O2). It is this oxygen that is given off into the air during sunlight.
Digestion.—Starch is in the form of insoluble granules. When
such food material is carried from one part of the plant to another for
purposes of growth or storage, it is made soluble before it can be
transported. When this starchy material is transferred from place to
place, it is usually changed into sugar by the action of a diastase.
This is a process of digestion. It is much like the change of starchy
foodstuffs to sugary foods effected by the saliva.
Distribution of the Digested Food.—
After being changed to the soluble form, this
material is ready to be used in growth, either
in the leaf, in the stem, or in the roots. With
other more complex products it is then
distributed throughout all the growing parts
of the plant; and when passing down to the
root, it seems to pass more readily through
the inner bark, in plants which have a
definite bark. This gradual downward
diffusion through the inner bark of materials
suitable for growth is the process referred to
when the “descent of sap” is mentioned.
Starch and other products are often stored in
one growing season to be used in the next
season. If a tree is constricted or strangled
by a wire around its trunk (Fig. 118), the
Fig. 118.—Trunk Girdled digested food cannot readily pass down and
by a Wire. See Fig. 85. it is stored above the girdle, causing an
enlargement.
Assimilation.—The food from the air and that from the soil unite
in the living tissues. The “sap” that passes upwards from the roots in
the growing season is made up largely of the soil water and the salts
which have been absorbed in the diluted solutions (p. 67). This
upward-moving water is conducted largely through certain tubular
canals of the young wood. These cells are never continuous tubes
from root to leaf; but the water passes readily from one cell or canal
to another in its upward course.
The upward-moving water gradually passes to the growing parts,
and everywhere in the living tissues, it is, of course, in the most
intimate contact with the soluble carbohydrates and products of
photosynthesis. In the building up or reconstructive and other
processes it is therefore available. We may properly conceive of
certain of the simpler organic molecules as passing through a series
of changes, gradually increasing in complexity. There will be formed
substances containing nitrogen in addition to carbon, hydrogen, and
oxygen. Others will contain also sulphur and phosphorus, and the
various processes may be thought of as culminating in protoplasm.
Protoplasm is the living matter in plants. It is in the cells, and is
usually semi-fluid. Starch is not living matter. The complex process
of building up the protoplasm is called assimilation.
Respiration.—Plants need oxygen for respiration, as animals do.
We have seen that plants need the carbon dioxide of the air. To most
plants the nitrogen of the air is inert, and serves only to dilute the
other elements; but the oxygen is necessary for all life. We know that
all animals need this oxygen in order to breathe or respire. In fact,
they have become accustomed to it in just the proportions found in
the air; and this is now best for them. When animals breathe the air
once, they make it foul, because they use some of the oxygen and
give off carbon dioxide. Likewise, all living parts of the plant must
have a constant supply of oxygen. Roots also need it, for they
respire. Air goes in and out of the soil by diffusion, and as the soil is
heated and cooled, causing the air to expand and contract.
The oxygen passes into the air-spaces and is absorbed by the
moist cell membranes. In the living cells it makes possible the
formation of simpler compounds by which energy is released. This
energy enables the plant to work and grow, and the final products of
this action are carbon dioxide and water. As a result of the use of
this oxygen by night and by day, plants give off carbon dioxide.
Plants respire; but since they are stationary, and more or less
inactive, they do not need so much oxygen as animals do, and they
do not give off so much carbon dioxide. A few plants in a sleeping
room need not disturb one more than a family of mice. It should be
noted, however, that germinating seeds respire vigorously, hence
they consume much oxygen; and opening buds and flowers are
likewise active.
Transpiration.—Much more water is absorbed by the roots than is
used in growth, and this surplus water passes from the leaves into
the atmosphere by an evaporation process known as transpiration.
Transpiration takes place more abundantly from the under surfaces
of leaves, and through the pores or stomates. A sunflower plant of
the height of a man, during an active period of growth, gives off a
quart of water per day. A large oak tree may transpire 150 gallons
per day during the summer. For every ounce of dry matter produced,
it is estimated that 15 to 25 pounds of water usually passes through
the plant.
When the roots fail to supply to the plant sufficient water to
equalize that transpired by the leaves, the plant wilts. Transpiration
from the leaves and delicate shoots is increased by all the conditions
which increase evaporation, such as higher temperature, dry air, or
wind. The stomata open and close, tending to regulate transpiration
as the varying conditions of the atmosphere affect the moisture
content of the plant. However, in periods of drought or of very hot
weather, and especially during a hot wind, the closing of these
stomates cannot sufficiently prevent evaporation. The roots may be
very active and yet fail to absorb sufficient moisture to equalize that
given off by the leaves. The plant shows the effect (how?). On a hot
dry day, note how the leaves of corn “roll” towards afternoon. Note
how fresh and vigorous the same leaves appear early the following
morning. Any injury to the roots, such as a bruise, or exposure to
heat, drought, or cold may cause the plant to wilt.
 Water is forced up by root pressure or sap pressure. (Exercise
99.) Some of the dew on the grass in the morning may be the water
forced up by the roots; some of it is the condensed vapour of the air.
The wilting of a plant is due to the loss of water from the cells. The
cell walls are soft, and collapse. A toy balloon will not stand alone
until it is inflated with air or liquid. In the woody parts of the plant the
cell walls may be stiff enough to support themselves, even though
the cell is empty. Measure the contraction due to wilting and drying
by tracing a fresh leaf on page of notebook, and then tracing the
same leaf after it has been dried between papers. The softer the
leaf, the greater will be the contraction.
Storage.—We have said that starch may be stored in twigs to be
used the following year. The very early flowers on fruit trees,
especially those that come before the leaves, and those that come
from bulbs, as crocuses and tulips, are supported by the starch or
other food that was organized the year before. Some plants have
very special storage reservoirs, as the potato, in this case being a
thickened stem although growing underground. (Why a thickened
stem? p. 84.) It is well to make the starch test on winter twigs and on
all kinds of thickened parts, as tubers and bulbs.
Carnivorous Plants.—Certain plants capture insects and other
very small animals and utilize them to some extent as food. Such are
the sundew, which has on the leaves sticky hairs that close over the
insect; the Venus’s fly-trap of the Southern States, in which the
halves of the leaves close over the prey like the jaws of a steel trap;
and the various kinds of pitcher plants that collect insects and other
organic matter in deep, water-filled, flask-like leaf pouches (Fig. 119).
The sundew and the Venus’s fly-trap are sensitive to contact.
Other plants are sensitive to the touch without being insectivorous.
The common cultivated sensitive plant is an example. This is readily
grown from seeds (sold by seedsmen) in a warm place. Related wild
plants in the south are sensitive. The utility of this sensitiveness is
not understood.
 Parts that Simulate Leaves.—We have
learned that leaves are endlessly modified to
suit the conditions in which the plant is
placed. The most marked modifications are
in adaptation to light. On the other hand,
other organs often perform the functions of
leaves. Green shoots function as leaves.
These shoots may look like leaves, in which
case they are called cladophylla. The foliage
of common asparagus is made up of fine
branches: the real morphological leaves are
the minute dry functionless scales at the
bases of these branchlets. (What reason is
there for calling them leaves?) The broad
Fig. 119.—The Common “leaves” of the florist’s smilax are
Pitcher Plant cladophylla. Where are the leaves on this
(Sarracenia purpurea)
showing the tubular plant? In most of the cacti, the entire plant
leaves and the odd, long- body performs the functions of leaves until
stalked flowers. the parts become cork-bound.
Leaves are sometimes modified to
perform other functions than the vital processes: they may be
tendrils, as the terminal leaflets of pea and sweet pea; or spines, as
in barberry. Not all spines and thorns, however, represent modified
leaves: some of them (as of hawthorns, osage orange, honey locust)
are branches.
Suggestions.—To test for chlorophyll. 84. Purchase about a gill of wood
alcohol. Secure a leaf of geranium, clover, or other plant that has been exposed to
sunlight for a few hours, and, after dipping it for a minute in boiling water, put it in a
white cup with sufficient alcohol to cover. Place the cup in a shallow pan of hot
water on the stove where it is not hot enough for the alcohol to take fire. After a
time the chlorophyll is dissolved by the alcohol which has become an intense
green. Save this leaf for the starch experiment (Exercise 85). Without chlorophyll,
the plant cannot appropriate the carbon dioxide of the air. Starch and
photosynthesis. 85. Starch is present in the green leaves which have been
exposed to sunlight; but in the dark no starch can be formed from carbon dioxide.
Apply iodine to the leaf from which the chlorophyll was dissolved in the previous
experiment. Note that the leaf is coloured purplish-brown throughout. The leaf
contains starch.86. Secure a leaf from a plant which has been in the dark for about
two days. Dissolve the chlorophyll as before, and attempt to stain this leaf with
iodine. No purplish-brown colour is produced. This shows that the starch
manufactured in the leaf may be entirely removed during darkness.

Fig. 120.—Excluding Light and

Fig. 121.—The Result.
CO2 from Part of a Leaf.

87. Secure a plant which has been kept in darkness for twenty-four hours or
more. Split a small cork and pin the two halves on opposite sides of one of the
leaves, as shown in Fig. 120. Place the plant in the sunlight again. After a morning
of bright sunshine dissolve the chlorophyll in this leaf with alcohol; then stain the
leaf with the iodine. Notice that the leaf is stained deeply except where the cork
was; there sunlight and carbon dioxide were excluded, Fig. 121. There is no starch
in the covered area. 88. Plants or parts of plants that have developed no
chlorophyll can form no starch. Secure a variegated leaf of coleus, ribbon grass,
geranium, or of any plant showing both white and green areas. On a day of bright
sunshine, test one of these leaves by the alcohol and iodine method for the
presence of starch. Observe that the parts devoid of green colour have formed no
starch. However, after starch has once been formed in the leaves, it may be
changed into soluble substances and removed, to be again converted into starch
in certain other parts of the living tissues. To test the giving off of oxygen by day.
 89. Make the experiment illustrated in Fig. 122.
Under a funnel in a deep glass jar containing fresh
spring or stream water place fresh pieces of the
common waterweed elodea (or anacharis). Have the
funnel considerably smaller than the vessel, and
support the funnel well up from the bottom so that the
plant can more readily get all the carbon dioxide
available in the water. Why would boiled water be
undesirable in this experiment? For a home-made
glass funnel, crack the bottom off a narrow-necked
bottle by pressing a red-hot poker or iron rod against
it and leading the crack around the bottle. Invert a
test-tube over the stem of the funnel. In sunlight
bubbles of oxygen will arise and collect in the test-
tube. If a sufficient quantity of oxygen has collected, a
lighted taper inserted in the tube will glow with a
brighter flame, showing the presence of oxygen in
greater quantity than in the air. Shade the vessel. Are
bubbles given off? For many reasons it is
impracticable to continue this experiment longer than
a few hours. 90. A simpler experiment may be made
Fig. 122.—To show the if one of the waterweeds Cabomba (water-lily family)
Escape of Oxygen. is available. Tie a number of branches together so
that the basal ends shall make a small bundle. Place
these in a large vessel of spring water, and insert a
test-tube of water as before over the bundle. The bubbles will arise from the cut
surfaces. Observe the bubbles on pond scum and waterweeds on a bright day. To
illustrate the results of respiration (CO2).
91. In a jar of germinating seeds (Fig. 123) place carefully a small dish of
limewater and cover tightly. Put a similar dish in another jar of about the same air
space. After a few hours compare the cloudiness or precipitate in the two vessels
of limewater. 92. Or, place a growing plant in a deep covered jar away from the
light, and after a few hours insert a lighted candle or splinter. 93. Or, perform a
similar experiment with fresh roots of beets or turnips (Fig. 124) from which the
leaves are mostly removed. In this case, the jar need not be kept dark; why? To
test transpiration.
94. Cut a succulent shoot of any plant, thrust the end of it through a hole in a
cork, and stand it in a small bottle of water. Invert over this a fruit jar, and observe
that a mist soon accumulates on the inside of the glass. In time drops of water
form. 95. The experiment may be varied as shown in Fig. 125. 96. Or, invert the
fruit jar over an entire plant, as shown in Fig. 126, taking care to cover the soil with
oiled paper or rubber cloth to prevent evaporation from the soil.
 97. The test may also be made by
placing the pot, properly protected, on
balances, and the loss of weight will be
noticed (Fig. 127). 98. Cut a winter twig,
seal the severed end with wax, and allow
the twig to lie several days. It shrivels.
There must be some upward movement
of water even in winter, else plants would
shrivel and die. 99. To illustrate sap
pressure. The upward movement of sap
water often takes place under
considerable force. The cause of this
force, known as root pressure, is not well
understood. The pressure varies with
Fig. 123.—To
different plants and under different
illustrate a
conditions. To illustrate: cut off a strong-
Product of
growing small plant near the ground. By
Respiration.
means of a bit of rubber tube attach aFig. 124.—
glass tube with a bore of approximately Respiration of
the diameter of the stem. Pour in a little water. Observe the Thick Roots.
rise of the water due to the pressure from below (Fig 128).
Some plants yield a large amount of water under a pressure sufficient to raise a
column several feet; others force out little, but under considerable pressure (less
easily demonstrated). The vital processes (i.e., the life processes). 100. The pupil
having studied roots, stems, and leaves, should now be able to describe the main
vital functions of plants: what is the root function? stem function? leaf function?
101. What is meant by the “sap”? 102. Where and how does the plant secure its
water? oxygen? carbon? hydrogen? nitrogen? sulphur? potassium? calcium? iron?
phosphorus? 103. Where is all the starch in the world made? What does a starch-
factory establishment do? Where are the real starch factories? 104. In what part of
the twenty-four hours do plants grow most rapidly in length? When is food formed
and stored most rapidly? 105. Why does corn or cotton turn yellow in a long rainy
spell? 106. If stubble, corn stalks, or cotton stalks are burned in the field, is as
much plant-food returned to the soil as when they are ploughed under? 107. What
process of plants is roughly analogous to perspiration of animals? 108. What part
of the organic world uses raw mineral for food? 109. Why is earth banked over
celery to blanch it? 110. Is the amount of water transpired equal to the amount
absorbed?
 Fig. 125.—To illustrate Transpiration.

Fig. 129.—Before and after Pruning.

111. Give some reasons why plants very close to a house may not thrive or may
even die. 112. Why are fruit trees pruned or thinned out as in Fig. 129? Proper
balance between top and root. 113. We have learned that the leaf parts and the
root parts work together. They may be said to balance each other in activities, the
root supplying the top and the top supplying the root (how?). If half the roots were
cut from a tree, we should expect to reduce the top also, particularly if the tree is