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. In short, for the formation of our starch the water (H₂O) from the soil and the CO₂ of the atmosphere, when brought together, may be made to combine with the formation of starch. A single diagram, while not perhaps an absolute statement of fact, may serve to represent the final result: {6CO₂ { = C₆H₁₀O₅ + 12O. {5H₂O In other words, the six molecules of carbon dioxide and five of water combine with the formation of one molecule of starch and the liberation of twelve atoms of oxygen. [Illustration: FIG. 5.--DIAGRAMMATIC VIEW OF THE PROCESS OF PHOTO-SYNTHESIS.] This driving off of such a large amount of oxygen, entirely against the whole tendency of that element, it is assumed, is at the expenditure of much force. The only one adequate to this work is solar energy, and this is abundantly at hand. That we need not seek further for this power is proved by many and conclusive tests. Vegetable physiologists to-day are able not only to locate the sun as the chemist, that effects the changes necessary for the production of starch, but can show in what cells and portions of those cells the forces effect the synthesis. The chlorophyll granules in the living cell are the microscopic laboratories in which a silent chemist, powerful beyond all measurements, builds out of inorganic materials the food substance of the whole world of animals and plants. [Illustration: FIG. 6.--PORTION OF LEAF MAGNIFIED, ILLUSTRATING PHOTO-SYNTHESIS.]