A Civic Biology, Presented in Problems

by George W. Hunter

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.

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.

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.

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.