VII. PLANT GROWTH AND NUTRITION--PLANTS MAKE FOOD
_Problem.--Where, when, and how do green plants make food?_ _(a) How and why is moisture given off from leaves?_ _(b) What is the reaction of leaves to light?_ _(c) What is made in green leaves in the sunlight?_ _(d) What by-products are given off in the above process?_ _(e) Other functions of leaves._
LABORATORY SUGGESTIONS
_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.