Page:Popular Science Monthly Volume 56.djvu/735

Rh growth of the potato tubers or the plumping out of the grains of corn, wheat, or rice, the same fact remains that storehouses are being filled with the same organic compound. There must be many preliminary steps before this process of storing is complete, and for these we need to seek elsewhere in the growing plant.

Even the most careless observer can not but be at home with the fact that the whole port and bearing of ordinary plants is for sun exposure. They rise from the ground as closely placed stems of grass or less neighborly orchard or forest trees, and hang out their leaves to catch the sun. The economy of substance is so well studied that there is a very large exposure at a minimum of expenditure of tissue. In short, the leaves are the organs for association with the sunlight. They reach toward the sun where light is scanty, as in the window, and even turn their faces to the orb of day, shifting the position hour by hour from sunrise to nightfall. The rapidity with which we come to the fundamental fact that leaves are for the sun almost surprises one. The purpose is as easily inferred, but the steps in the process are not so quickly taken. The facts that leaves are par excellence the starch factories and the sunlight the inobtrusive chemist are granted, and it remains only to show something of the steps of proof that science may have discovered.

We need, therefore, to consider starch from the standpoint of its composition, and upon this the chemists are fairly well agreed. It consists of three elements, with their atoms so arranged that the molecule of starch has the composition of six parts of carbon, ten of hydrogen, and five of oxygen, or, to express the formula in terse chemical terms, it stands C₆10₁₀O₅. If we can account for the bringing of these atoms together in the production of a single molecule of starch the laboratory has been explored and the secret is ours, even if we can not put it to practical use in our so-called "starch factories."

The independent plant, beyond serious question, gets its food from outside itself. There are two sources for these substances—namely, the soil-water bathing the absorbing roots, and the atmosphere, with which the aërial branches and their leaves are constantly surrounded. From the soil come the water and all the salts, ash constituents, and the like that may be dissolved therein, while the gases of the atmosphere bring, among its chief contributions, a constant and, in an always exceedingly diluted form, the carbon dioxide, or, sometimes called, the carbonic-acid gas. This compound, familiar to us as a product of combustion, fermentation, and decay, is composed of carbon and oxygen, and has the symbol CO₂ associated with it by chemists.