The process just described (photosynthe sis) furnishes not only all the food, but prac tically all the fuel in the world. The leaf utilizes, that is, stores up, only about one half of one per cent of the energy it re ceives in the form of sunshine. It makes use of the red and orange rays almost exclu sively, and forms little or no starch in blue light. The rays that affect the photographic plate, therefore, have little part in photo synthesis, while the red and orange rays, so important in this connection, are the ones that also produce the greatest effect on the eye. A square meter of sunflower leaves is estimated to produce about 25 grams of starch in the 15 hours of sunlight of a summer day. This would use up the carbon dioxid contained in 50 cubic meters of air (a meter is nearly 40 inches); or, in other words, should the leaves take all their carbon from the air directly above them, they would in a day consume all of it to a vertical height of about 165 feet.
The sugar formed in the chlorophyll grains is transformed, in great part at least, into starch, which makes its appearance in the form of glis tening white bodies embedded within the substance of the grain. This starch mostly disappears during the night, being changed back into sugar, and conducted away into the stem and thence to the roots, flowers or other parts. Leaving the palisade cells of the leaf (Fig. 34 p.), it passes through the collecting cells (col.) into conveying cells (cony.), and on to the conducting sheath (sh.) of one of the veins, by which it passes through the leaf-stalk into the stem.
The evaporation of water is of great advantage to the plant, for it concentrates in the leaf the salts contained in the water. The leaf thus becomes the meeting place of air food and soil food. These two sorts of crude food combine to form elaborated food. The first step is probably the formation of sugar, which then, by combining with nitrogen, sulfur, phosphorus and other elements, forms pro teids. These move from place to place, principally in the bast, and so reach the regions where they are needed.
The energy needed to elaborate food comes from the sunlight. The leaves have various devices to absorb all the sunlight possible. Some " follow the sun " all day long, thus facing eastward in the morning and westward at evening. At mid-day they are horizontal, except when the sunlight is exces sive, in which case they assume the "profile posi tion" with the edges pointing upward, thus avoid ing injury due to too strong light. Many such leaves assume a "sleep position" at night by fold ing ; they diminish thereby the loss of heat and avoid the precipitation of dew on the protected surfaces.
Most leaves have the power of turning toward the light, and so move out of the shadow of other leaves. Thus arise the beautiful " leaf-mosaics," e. g., of English ivy or of maple, in which no leaf unduly shades another. The usual arrangement of leaves on the stem is in regular vertical rows. The arrangement is known as phyllotaxy.
The stem.—The stem bears the leaves and fur nishes them with a constant supply of water, which it conveys from the root. On placing a plant with its roots in diluted red ink or other colored solution, we can trace the colored solution up through the wood-cells in the root (Fig. 29), through the stem (Fig. 36), into the finest veins of the leaf. It is easily seen that the colored solution travels only in the wood-cells and not in the other cells of the stem. We usually find the wood-cells associated with bast-cells, forming together the fibro-vascular bun dles (Figs. 21, 37). In dicotyledonous plants (e. g., squash, sunflower) these bundles form a circle near the outside of the stem ; while in monocotyledonous plants (e. g., corn, lilies) they are scattered through the stem. [See page 9.] The largest passages in the wood are called ducts, and in them the water travels faster tl—n in the other cells. They are formed by the breaking down of partitions, thus converting a long row of cells into a single continuous passage that may be as much as forty feet in length.
In the tracheids— long, narrow, tapering cells— the water travels more slowly than in the ducts, being hindered by the frequent end walls. The markings seen on the walls of the wood-cells are pits or thin places in the walls, by means of which water passes more readily from one cell to another. The passage of air is prevented by a delicate mem brane stretched across the pit.
The question may be asked, What causes the sap to rise ? Various explanations have been advanced and proved unsatisfactory, such as capil larity, barometric pressure, action of air-bubbles and root-pressure (the action of the root in forcing water upward, as seen in the bleeding stumps of the grape-vine). The one at present most in favor is that the sap is drawn up by water-attracting substances in the leaves, just as the water is pulled away from the soil -particles by the root - hairs. This process is known as osmosis. Sugar is a sub stance that acts in this way. For example, the conversion of the stored starch of the maple into sugar, in the spring, causes a rapid rush of sap into the stem, even though no leaves are present. This theory is not satisfactory in all respects, especially when applied to the rise of sap in very tall trees.