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, CO_{2}.
=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_ (C_{6}H_{10}O_{5})_{_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 CO_{2} + 6
parts H_{2}O = C_{6}H_{12}O_{6} + 6 O_{2}). 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.
[Illustration: FIG. 118.—TRUNK GIRDLED BY A WIRE. See Fig. 85.]
=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 digested food cannot readily pass down and 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.
[Illustration: FIG. 119.—THE COMMON PITCHER PLANT (_Sarracenia
purpurea_) showing the tubular leaves and the odd, long-stalked
flowers.]
=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
“leaves” of the florist’s smilax are cladophylla. Where are the
leaves on this plant? In most of the cacti, the entire plant body
performs the functions of leaves until 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.
[Illustration: FIG. 120.—EXCLUDING LIGHT AND CO_{2} FROM PART OF A
LEAF.]
[Illustration: FIG. 121.—THE RESULT.]
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. =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 if one of the waterweeds Cabomba (water-lily family) 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_ (CO_{2}).
[Illustration: FIG. 122.—TO SHOW THE ESCAPE OF OXYGEN.]
[Illustration: FIG. 123.—TO ILLUSTRATE A PRODUCT OF RESPIRATION.]
[Illustration: FIG. 124.—RESPIRATION OF THICK ROOTS.]
[Illustration: FIG. 125.—TO ILLUSTRATE TRANSPIRATION.]
=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).
[Illustration: FIG. 126.—TO ILLUSTRATE TRANSPIRATION.]
[Illustration: FIG. 127.—LOSS OF WATER.]
[Illustration: FIG. 128.—TO SHOW SAP PRESSURE.]
=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 different plants and under different
conditions. To illustrate: cut off a strong-growing small plant
near the ground. By means of a bit of rubber tube attach a glass
tube with a bore of approximately the diameter of the stem. Pour in
a little water. Observe the 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? =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 being transplanted. How would you
prune a tree or bush that is being transplanted? Fig. 130 may be
suggestive.
[Illustration: FIG. 130.—AN APPLE TREE, with suggestions as to
pruning when it is set in the orchard. At _a_ is shown a pruned top.]
[Illustration: FIG. 129.—BEFORE AND AFTER PRUNING.]