_Demonstration._--Water given off by plant in sunlight. Loss
of weight due to transpiration measured.
_Laboratory exercise._--
(_a_) Gross structure of a leaf.
(_b_) Study of stoma and lower epidermis under microscope.
(_c_) Study of cross section to show cells and air spaces.
_Demonstration._--Reaction of leaves to light.
_Demonstration._--Light necessary to starch making.
_Demonstration._--Air necessary to starch making.
_Demonstration._--Oxygen a by-product of starch making.
What becomes of the Water taken in by the Roots?--We have seen that more
than pure water has been absorbed through the root hairs into the roots.
What becomes of this water and the other substances that have been
absorbed? This question may be partly answered by the following
experiments.
[Illustration: Apple twigs split to show the course of colored water up the
stem.]
Passage of Fluids up the Stem.--If any young growing shoots (young
seedlings of corn or pea, or the older stems of garden balsam,
touch-me-not, or sunflower) are placed in red ink (eosin), and left in the
sun for a few hours, the red ink will be found to have passed up the stem.
If such stems were examined carefully, it would be seen that the colored
fluid is confined to collections of woody tubes immediately under the inner
bark. Water evidently rises in that part of the stem we call the wood.
[Illustration: Experiment to prove that water is given off through the
leaves of a green plant.]
Water given off by Evaporation from Leaves.--Take some well-watered potted
green plant, as a geranium or hydrangea, cover the pot with sheet rubber,
fastening the rubber close to the stem of the plant. Next weigh the plant
with the pot. Then cover it with a tall bell jar and place the apparatus in
the sun. In a few minutes drops of moisture are seen to gather on the
_inside_ of the jar. If we now weigh the potted plant, we find it weighs
less than before. Obviously the loss comes from the water lost, and
evidently this water escapes as vapor from either the stem or leaves.
[Illustration: The skeleton of a leaf. _M.R._, the midrib; _P._, the
leafstalk; _V._, the veins.]
The Structure of a Leaf.--In the experiment with the red ink mentioned
above we will find that the fluid has gone out into the skeleton or
framework of the leaf. Let us now examine a leaf more carefully. It shows
usually (1) a flat, broad _blade_, which may take almost any conceivable
shape; (2) a _stem_ which spreads out in the blade (3) in a number of
_veins_.
[Illustration: Section through the blade of a leaf as seen under the
compound microscope. _S_, air spaces, which communicate with the outside
air; _V_, vein in cross section; _S.T._, breathing hole (stoma); _E_, outer
layer of cells; _P_, green cells.]
The Cell Structure of a Leaf.--The under surface of a leaf seen under the
microscope usually shows numbers of tiny oval openings. These are called
_stomata_ (singular _stoma_). Two cells, usually kidney-shaped, are found,
one on each side of the opening. These are the _guard cells_. By change in
shape of these cells the opening of the stoma is made larger or smaller.
Larger irregular cells form the _epidermis_, or outer covering of the leaf.
Study of the leaf in cross section shows that these stomata open directly
into air chambers which penetrate between and around the loosely arranged
cells composing the underpart of the leaf. The upper surface of leaves
sometimes contains stomata, but more often they are lacking. The under
surface of an oak leaf of ordinary size contains about 2,000,000 stomata.
Under the upper epidermis is a layer of green cells closely packed together
(called collectively the _palisade layer_). These cells are more or less
columnar in shape. Under these are several rows of rather loosely placed
cells just mentioned. These are called collectively the _spongy tissue_. If
we happen to have a section cut through a vein, we find this composed of a
number of tubes made up of, and strengthened by, thick-walled cells. The
veins are evidently a continuation of the tubes of the stem out into the
blade of the leaf.
Evaporation of Water.--During the day an enormous amount of water is taken
up by the roots and passed out through the leaves. So great is this excess
at times that a small grass plant on a summer's day evaporates more than
its own weight in water. This would make nearly half a ton of water
delivered to the air during twenty-four hours by a grass plot twenty-five
by one hundred feet, the size of the average city lot. According to Ward,
an oak tree may pass off two hundred and twenty-six times its own weight in
water during the season from June to October.
From which Surface of the Leaf is Water Lost?--In order to find out whether
water is passed out from any particular part of the leaf, we may remove two
leaves of the same size and weight from some large-leaved plant[14]--a
mullein was used for the illustrations given below--and cover the upper
surface of one leaf and the lower surface of the other with vaseline. The
leaf stalks of each should be covered with wax or vaseline, and the two
leaves exactly balanced on the pans of a balance which has previously been
placed in a warm and sunny place. Within an hour the leaf which has the
upper surface covered with vaseline will show a loss of weight. Examination
of the surface of a mullein leaf shows us that the _lower surface of the
leaf is provided with stomata_. It is through these organs, then, that
water is passed out from the tissues of the leaf.
Footnote 14: The "rubber plant" leaf is an easily obtainable
and excellent demonstration.
[Illustration: Experiment to show through which surface of a leaf water
passes off.]
Factors in Transpiration.--The amount of water lost from a plant varies
greatly under different conditions. The humidity of the air, its
temperature, and the temperature of the plant all affect the rate of
transpiration. The stomata also tend to close under some conditions, thus
helping to prevent evaporation. But there seems to be no certain regulation
of this water loss. Consequently plants droop or wilt on hot dry days
because they cannot obtain water rapidly enough from the soil to make up
for the loss through the leaves.
[Illustration: Diagrams of a stoma. _a_, surface view of a closed stoma;
_b_, the same stoma opened. (After Hanson.) _c_, diagrams of a transverse
section through a stoma, dotted lines indicate the closed position of the
guard cells, the heavy lines the open condition. (After Schwendener.)]
Green Plants Food Makers.--We have previously stated that green plants are
the great food makers for themselves and for animals. We are now ready to
attack the problem of how green plants _make_ food.
The Sun a Source of Energy.--We all know the sun is a source of most of the
energy that is released on this earth in the form of heat or light. Every
boy knows the power of a "burning glass." Solar engines have not come into
any great use as yet, because fuel is cheaper, but some day we undoubtedly
will directly harness the energy of the sun in everyday work. Actual
experiments have shown that vast amounts of energy are given to the earth.
When the sun is highest in the sky, energy equivalent to one hundred horse
power is received by a plot of land twenty-five by one hundred feet, the
size of a city lot. Plants receive and use much of this energy by means of
their leaves.
Effect of Light on Plants.--In young plants which have been grown in total
darkness, no green color is found in either stems or leaves, the latter
often being reduced to mere scales. The stems are long and more or less
reclining. We can explain the changed condition of the seedling grown in
the dark only by assuming that light has some effect on the protoplasm of
the seedling and induces the growth of the green part of the plant. If
seedlings have been growing on a window sill, or where the light comes in
from one side, you have doubtless noticed that the stem and leaves of the
seedlings incline in the direction from which the light comes. The
experiment pictured shows this effect of light very plainly. A hole was cut
in one end of a cigar box and barriers were erected in the interior of the
box so that the seeds planted in the sawdust received their light by an
indirect course. The young seedling in this case responded to the influence
of the stimulus of light so as to grow out finally through the hole in the
box into the open air. This growth of the stem to the light is of very
great importance to a growing plant, because, as we shall see later, food
making depends largely on the amount of sunlight the leaves receive.
[Illustration: Two stages in an experiment to show that green plants grow
toward the light.]
Effect of Light on Leaf Arrangement.--It is a matter of common knowledge
that green leaves turn toward the light. Place growing pea seedlings,
oxalis, or any other plants of rapid growth near a window which receives
full sunlight. Within a short time the leaves are found to be in positions
to receive the most sunlight possible. Careful observation of any plant
growing outdoors shows us that in almost every case the leaves are so
disposed as to get much sunlight. The ivy climbing up the wall, the
morning-glory, the dandelion, and the burdock all show different
arrangements of leaves, each presenting a large surface to the light.
Leaves are often definitely arranged, fitting in between one another so as
to present their upper surface to the sun. Such an arrangement is known as
a _leaf mosaic_. In the case of the dandelion, a _rosette_ or whorled
cluster of leaves is found. In the horse-chestnut, where the leaves come
out opposite each other, the older leaves have longer petioles than the
young ones. In the mullein the entire plant forms a cone. The old leaves
near the bottom have long stalks, and the little ones near the apex come
out close to the main stalk. In every case each leaf receives a large
amount of light. Other modifications of these forms may easily be found on
any field trip.
[Illustration: A lily, showing long narrow leaves.]
[Illustration: The dandelion, showing a whorled arrangement of long
irregular leaves.]
Starch made by a Green Leaf.--If we examine the palisade layer of the leaf,
we find cells which are almost cylindrical in form. In the protoplasm of
such cells are found a number of little green-colored bodies, which are
known as _chloroplasts_ or _chlorophyll bodies_. If we place the leaf in
wood alcohol, we find that the bodies still remain, but that the color is
extracted, going into the alcohol and giving to it a beautiful green color.
The chloroplasts are, indeed, simply part of the protoplasm of the cell
colored green. These bodies are of the greatest importance directly to
plants and indirectly to animals. _The chloroplasts, by means of the energy
received from the sun, manufacture starch out of certain raw materials._
These raw materials are soil water, which is passed up through the bundles
of tubes into the veins of the leaf from the roots, and carbon dioxide,
which is taken in through the stomata or pores, which dot the under surface
of the leaf. A plant with variegated leaves, as the coleus, makes starch
only in the green part of the leaf, even though these raw materials reach
all parts of the leaf.
[Illustration: An experiment to show the effect of excluding light (but not
air) from the leaves of a green plant. The result of this experiment is
seen in the next picture. (Experiment performed by C. Dobbins and A.
Schwartz.)]
[Illustration: Starchless area in a leaf caused by excluding sunlight by
means of a strip of black cloth.]
Light and Air necessary for Starch Making.--If we pin strips of black
cloth, such as alpaca, over some of the leaves of a growing hydrangea which
has previously been placed in a dark room for a few hours, and then put the
plant in direct sunlight for an hour or two, we are ready to test for
starch. We then remove some of the covered leaves and extract the
chlorophyll with wood alcohol (because the green color of the chlorophyll
interferes with the blue color of the starch test). A test then shows that
starch is present only in the portions of the leaves exposed to sunlight.
From this experiment we infer that the sun has something to do with starch
making in a leaf. The necessity of a part of the air (carbon dioxide) for
starch making may also easily be proved, for the parts of leaves covered
with vaseline will be found to contain no starch, while parts of the leaf
without vaseline, but exposed to the sun and air, do contain starch.
[Illustration: Diagram to show starch making. Read the text carefully and
then explain this diagram.]
Air is necessary for the process of starch making in a leaf, not only
because carbon dioxide gas is absorbed (there are from three to four parts
in ten thousand present in the atmosphere), but also because the leaf is
alive and must have oxygen in order to do work. This oxygen it takes from
the air around it.
[Illustration: Diagram to illustrate the formation of starch in a leaf.]
Comparison of Starch Making and Milling.--The manufacture of starch by the
green leaf is not easily understood. The process has been compared to the
milling of grain. In this case the mill is the green part of the leaf. The
sun furnishes the motive power, the chloroplasts constitute the machinery,
and soil water and carbon dioxide are the raw products taken into the mill.
The manufactured product is starch, and a certain by-product (corresponding
to the waste in a mill) is also given out. This by-product is oxygen. To
understand the process fully, we must refer to a small portion of the leaf
shown below. Here we find that the cells of the green layer of the leaf,
under the upper epidermis, perform most of the work. The carbon dioxide is
taken in through the stomata and reaches the green cells by way of the
intercellular spaces and by osmosis from cell to cell. Water reaches the
green cells through the veins. It then passes into the cells by osmosis,
and there becomes part of the cell sap. The light of the sun easily
penetrates to the cells of the palisade layer, giving the energy needed to
make the starch. This whole process is a very delicate one, and will take
place only when external conditions are favorable. For example, too much
heat or too little heat stops starch making in the leaf. This building up
of food and the release of oxygen by the plant in the presence of sunlight
is called _photosynthesis_.
[Illustration: Diagram (after Stevens) to illustrate the processes of
breathing and food making in the cells of a green leaf in the sunlight.]
Manufacture of Fats.--Inasmuch as tiny droplets of oil are found _inside_
the chlorophyll bodies in the leaf, we believe that fats, too, are made
there, probably by a transformation of the starch already manufactured.
Protein Making and its Relation to the Making of Living Matter.--Protein
material is a food which is necessary to form protoplasm. Protein food is
present in the leaf, and is found in the stem or root as well. Proteins can
apparently be manufactured in any of the cells of green plants, the
presence of light not seeming to be a necessary factor. How it is
manufactured is a matter of conjecture. The minerals brought up in the soil
water form part of its composition, and starch or grape sugar give three
elements (C, H, and O). The element nitrogen is taken up by the roots as a
nitrate (nitrogen in combination with lime or potash). Proteins are
probably not made directly into protoplasm in the leaf, but are stored by
the cells of the plant and used when needed, either to form new cells in
growth or to repair waste. While plants and animals obtain their food in
different ways, they probably make it into living substance (_assimilate_
it) in exactly the same manner.
[Illustration: An example of how a tree may exert energy. This rock has
been split by the growing tree.]
Foods serve exactly the same purposes in plants and in animals; they either
build living matter or they are burned (oxidized) to furnish energy (power
to do work). If you doubt that a plant exerts energy, note how the roots of
a tree bore their way through the hardest soil, and how stems or roots of
trees often split open the hardest rocks, as illustrated in the figure
above.
Starch-Making and its Relation to Human Welfare.--Leaves which have been in
darkness show starch to be present soon after exposure to light. A corn
plant sends 10 to 15 grams of reserve material into the ears in a single
day. The formation of fruit, and especially the growth of the grain fields,
show the economic importance of this fact. Not only do plants make their
own food and store it away, but they make food for animals as well. And the
food is stored in such a stable form that it may be sent to all parts of
the world in the form of grain or other fruits. Animals, herbivorous and
flesh-eating, man himself, all are dependent upon the starch-making
processes of the green plant for the ultimate source of their food. When we
remember that in 1913 in the United States the total value of all farm
crops was over $6,000,000,000, and when we realize that these products came
from the air and soil through the energy of the sun, we may begin to
realize why as city boys and girls the study of plant biology is of
importance to us.
[Illustration: Experiment to show that oxygen is given off by green plants
in the sunlight.]
Green Plants give off Oxygen in Sunlight.--In still another way green
plants are of direct use to us in the city. During this process of
starch-making oxygen is given off as a by-product. This may easily be
proven by the following experiment.[15] Place any green water plant in a
battery jar partly filled with water, cover the plants with a glass funnel
and mount a test tube full of water over the mouth of the funnel. Then
place the apparatus in a warm sunny window. Bubbles of gas are seen to rise
from the plant. After two or three hours of hot sun, enough of the gas can
be obtained by displacement of the water to make the oxygen test.
Footnote 15: Immediate success with this experiment will be
obtained if the water has been previously charged with
carbon dioxide.
That oxygen is given off as a by-product by green plants is a fact of
far-reaching importance. City parks are true "breathing spaces." The green
covering of the earth is giving to animals an element that they must have,
while the animals in their turn are supplying to the plants carbon dioxide,
a compound used in food-making. Thus a widespread relation of mutual
helpfulness exists between plants and animals.
Respiration by Leaves.--All living things require oxygen. It is by means of
the oxidation of food materials within the plant's body that the energy
used in growth and movement is released. A plant takes in oxygen largely
through the stomata of the leaves, to a less extent through the _lenticels_
or breathing holes in the stem, and through the roots. Thus rapidly growing
tissues receive the oxygen necessary for them to perform their work. The
products of oxidation in the form of carbon dioxide are also passed off
through these same organs. It can be shown by experiment that a plant uses
up oxygen in the darkness; in the light the amount of oxygen given off as a
by-product in the process of starch-making is, of course, much greater than
the amount used by the plant.
Summary.--From the above paragraphs it is seen that a leaf performs the
following functions: (1) breathing, or the taking in of oxygen and passing
off of carbon dioxide; (2) starch-making, with the incidental passing out
of oxygen; (3) formation of proteins, with their digestion and assimilation
to form new tissues; and (4) the transpiration of water.