The green plant secures its carbon from the air. In other words, much of the solid matter of the plant comes from one of the gases of the air. By volume, carbon dioxide forms only a small fraction of 1 per cent. of the air. It would be very disastrous to animal life, however, if this percentage were much increased, for it excludes the life-giving oxygen. Carbon dioxide is often called “foul gas.” It may accumulate in old wells, and an experienced person will not descend into such wells until they have been tested with a torch. If the air in the well will not support combustion,—that is, if the torch is extinguished,—it usually means that carbon dioxide has drained into the place. The air of a closed schoolroom often contains far too much of this gas, along with little solid particles of waste matters. Carbon dioxide is often known as carbonic acid gas.
Appropriation of the Carbon.—The carbon dioxide of the air readily diffuses itself into the leaves and other green parts of the plant. The leaf is delicate in texture, and when very young the air can diffuse directly into the tissues. The stomates, however, are the special inlets adapted for the admission of gases into the leaves and other green parts. Through these stomates, or diffusion-pores, the outside air enters into the air-spaces of the plant, and is finally absorbed by the little cells containing the living matter.
Chlorophyll (“leaf green”) is the agent that secures the energy by means of which carbon dioxide is utilized. This material is contained in the leaf cells in the form of grains (p. [86]); the grains themselves are protoplasm, only the colouring matter being chlorophyll. The chlorophyll bodies or grains are often most abundant near the upper surface of the leaf, where they can secure the greatest amount of light. Without this green colouring matter, there would be no reason for the large flat surfaces which the leaves possess, and no reason for the fact that the leaves are borne most abundantly at the ends of branches, where the light is most available. Plants with coloured leaves as coleus, have chlorophyll, but it is masked by other colouring matter. This other colouring matter is usually soluble in hot water: boil a coleus leaf and notice that it becomes green and the water becomes coloured.
Plants grown in darkness are yellow and slender, and do not reach maturity. Compare the potato sprouts that have grown from a tuber lying in a dark cellar with those that have grown normally in the bright light. The shoots have become slender, and are devoid of chlorophyll; and when the food that is stored in the tuber is exhausted these shoots will have lived useless lives. A plant that has been grown in darkness from the seed will soon die, although for a time the little seedling will grow very tall and slender. Why? Light favours the production of chlorophyll, and the chlorophyll is the agent in the making of the organic carbon compounds. Sometimes chlorophyll is found in buds and seeds, but in most cases these places are not perfectly dark. Notice how potato tubers develop chlorophyll, or become green, when exposed to light.
Photosynthesis.—Carbon dioxide diffuses into the leaf; during sunlight it is used, and oxygen is given off. How the carbon dioxide which is thus absorbed may be used in making an organic food is a complex question, and need not be studied here; but it may be stated that carbon dioxide and water are the constituents. Complex compounds are built up out of simpler ones.
Chlorophyll absorbs certain light rays, and the energy thus directly or indirectly obtained is used by the living matter in uniting the carbon dioxide absorbed from the air with some of the water brought up from the roots. The ultimate result usually is starch. The process is obscure, but sugar is generally one step; and our first definite knowledge of the product begins when starch is deposited in the leaves. The process of using the carbon dioxide of the air has been known as carbon assimilation, but the term now most used is photosynthesis (from two Greek words meaning light and placing together.)
Starch and Sugar.—All starch is composed of carbon, hydrogen, and oxygen (C6H10O5)n. The sugars and the substance of cell walls are very similar to it in composition. All these substances are called carbohydrates. In making fruit sugar from the carbon and oxygen of carbon dioxide and from the hydrogen and oxygen of the water, there is a surplus of oxygen (6 parts CO2 + 6 parts H2O = C6H12O6 + 6 O2). It is this oxygen that is given off into the air during sunlight.
Digestion.—Starch is in the form of insoluble granules. When such food material is carried from one part of the plant to another for purposes of growth or storage, it is made soluble before it can be transported. When this starchy material is transferred from place to place, it is usually changed into sugar by the action of a diastase. This is a process of digestion. It is much like the change of starchy foodstuffs to sugary foods effected by the saliva.
Fig. 118.—Trunk Girdled by a Wire. See Fig. [85].