Roots of forest trees frequently make use of decaying matter by means of fungi, which grow in close contact with them.
The leaf—The seed-leaves are commonly gorged with food, con sisting of proteids (nitrogenous substances, like white of egg), fats, oils, sugar and starch. This food is mostly manufactured in the foliage leaves.
When starch is heated it sepa rates into water and carbon di oxid Evidently it may be formed by causing these two substances to unite. This is just what the foliage leaf brings about. It is supplied with water by the activity of root and stem, and it absorbs carbon dioxid from the air. By utilizing the energy of the sunlight the leaf is able to break the bond of union between the carbon and the oxygen of the carbon dioxid, thus leaving the carbon free to combine with water and so to pro duce starch, and the oxygen free to escape into the air. The energy used in this process is set free again if the starch be burned, either by ordinary combustion or by the slower combustion that takes place in plant or animal cells. All elaborated foods, such as proteids, fats, oils and sugars, yield up their stored energy in the same way.
In order to make as much starch as possible, the leaf must expose the greatest possible surface to the sunlight and air, but in so doing it runs the risk of losing too much water by evaporation. To meet this difficulty, it has devices that enable it to increase or diminish evaporation (transpiration) according to its needs. Its surface is made water proof by waxes, varnishes and resins, so that water can escape only at the pores or stomata that are thickly scattered (Fig. 30) over one or both of its surfaces,—as many as 3,500 per square inch in some instances. A section through a stomate is shown in Fig. 31, and a diagram of a stomate in Fig. 32. In the guard-cells, which surround the stomata, the plant possesses automatic devices of wonderful efficiency for regulating transpiration. When the water-supply is abundant, especially in the presence of sunlight, the guard-cells absorb water and ex pand. The pressure causes the walls that•bound the pore or stomate to curve away from each other, thus causing the stomate to open. This is due to the fact that these inner walls are thicker than the outer walls. The effect is the same as would be produced on a rubber tube by thickening one side by cement ing an extra strip of rubber on it. If such a tube be closed at one end while air or water is pumped in at the other, it will bend so that the thickened side becomes concave.
The absorption of the water by the guard-cells is aided in sunlight by the action of the chloro phyll grains which they contain ; these produce sugar, which aids the cell in taking up water from the other cells of the epidermis that have no chloro phyll grains.
When, therefore, the water-supply is sufficient, and especially when sunlight, temperature and other conditions are favorable for leaf activity, the stomata open and permit the leaf to absorb carbon dioxid. On the other hand, lack of water and unfavorable conditions cause them to close.
The somata are usually closed at night; hence it is t' n p•ssIble to fumigate plants with poisonous s that would kill them if applied through the Closing at night prevents the stomata clog ging with dew. Water-proof materials, as well as hairy coverings of the leaf, protect the stomata from dew and rain. Leaves so protected appear silvery under water and do not become wet for a long time. If such protection is found on the lower side only, the stomata will be found on that side only. House plants should have the leaves washed occasionally to prevent the clogging of the stomata with dust. The devices by which desert plants check evaporation will be discussed later.
The carbon dioxid, passing through the stomata, comes directly into contact with the leaf-cells, which are sufficiently separated from each other to allow it to pass freely between them (Figs. 33, 34). The great absorptive surface which they expose is kept continually moist and is thus able to absorb with great rapidity, much as the moist lung sur faces absorb oxygen. The absorbed carbon dioxid passes into the cells and comes into contact with the green chlorophyll grains. The chlorophyll (leaf green) in these bodies is divided into very minute drops (Fig. 35), thus giving it an enormous ab sorptive surface. At the same time that it takes up carbon dioxid it absorbs sunlight, and with the energy thus received decomposes the carbon dioxid and causes the carbon to unite with the water, thus forming sugar. This may be illustrated by the equation : 6H20 = 0sH12Oe + 602 Carbon dioxid Water Grape engar Oxygen This eluation, however, states merely the begin ning and end of what is probably a long and com plicated process. Oxygen is given off and may be seen arising in bubbles from water plants. Air that has been "vitiated" by animals may have its oxygen restored by green plants in sunlight. Aquaria are often maintained for long periods when a proper balance is struck between plant and animal life.