The water which ascends from the roots of a growing plant into its leaves holds in solution about as much mineral matter as is contained in ordinary well-water. The warmth of the summer air causes some of this water to evaporate from the foliage. More comes up to supply the loss, and this also dries out slowly through the sunlit hours. As the water evaporates the mineral substances (salts) which have come up with it from the soil remain behind in the leaf cells. These will enter into the composition of the living protoplasm which is filling the new cells of shoot and root.
Though we know how plants make starch, comparatively little has been learned about the more vital process of protoplasm-making. But it is believed that in green plants this work, too, can go on only in the presence of light.
As the water from the roots is to go directly to the leaf-laboratories, Nature has taken care that the precious fluid shall not be wasted on the way. So the trunks and branches of trees and shrubs are wrapped in a skin of cork, which prevents the ascending nutritive water from evaporating. But once in the leaves it is desirable, in most cases, that the water may evaporate and give up its chemical and mineral treasures. So the leaf, broad and thin, exposes the largest possible proportion of surface to the light and air.
Over the whole leaf - veins, cells, and all - there is stretched a transparent skin. A powerful microscope shows this skin to be itself a sheet of cells, often very irregular in form and generally destitute of chlorophyll.
In tropical plants it is comparatively thick; for were it not so, the ardent sun would soon parch all the juices out of the foliage. The oleander, in its native soil, has to endure long droughts, and its leaves are provided with skins four times as thick as those of some leaves which grow in moist climates. But, thick or thin, the leaf-skin must not keep the air away from the green cells, or the little chlorophyll-grains would get no carbonic acid to digest, and the luckless vegetable would die of starvation. Neither must it totally check the evaporation of the water which has ascended from the roots. So the leaf-skin is full of pores, through which air and vapor can pass freely. To these openings botanists give the name of "stomata" or "mouths" (Fig. 20). They open into passages which are channelled out, as it were, in the fleshy part of the leaf, and their office is best described by the term transpiration. They enable the leaves to breathe out any moisture which may be contained in them over and above the plant's immediate needs. Thus the "transpiration" of a plant-body is comparable to the perspiration of an animal body.
During rainy or misty weather, when leaves naturally contain more fluid than they need, these stomata gape open; and during times of drought, when it is desirable that the plant's fluids shall be saved, they close. This timely opening and shutting is effected by a mechanism extremely simple, yet perfect in its working.
Each breathing pore is like a double door, whose leaves to left and right are cells. And these cells, like their neighbors in leaf-skin and tissue, become distended in moist weather, and shrink in time of drought. As soon as they become flaccid they collapse, like an empty pair of bellows, and their sides bulge like the bellows-leather. This bulging brings the walls of the two stoma cells into contact, so that the double door is shut (Fig. 21).
Fig. 20. - A cell of the leaf-skin and one stoma of a fern (Pteris creticd).
But when damp weather causes the cells to swell again, they stand erect and their sides are drawn apart. Then the double door is open, and the superabundant moisture in the leaf can pass out freely.
Fig. 21. - Closed stoma of a Cycas. (From the Vegetable World).
Each stoma opens into one of the spaces in the leaf-tissue.
In general these little holes are irregularly placed, but on grass-blades and lily-leaves they are ranged in long, straight rows. The number of them in a square inch of leaf-surface varies from two hundred in the foliage of the mistletoe to two hundred thousand in that of the lilac. In the white lily they are unusually large, and easily seen by a simple microscope of moderate power, and some one has had the patience to compute that on the lower surface of the leaf there are sixty thousand of these little breathing-pores to every square inch. In land-plants they are most numerous on the lower or shadowed side of the leaf, where moisture can not be drawn through them too fast by the ardent rays of the sun. But the floating leaves of water-plants have all their sto-mata on their upper surfaces, which alone come into contact with the air, and leaves which grow under water have no stomata at all.
Beach and desert plants must live between glaring skies and parching sands. So, whatever their more favored relatives do, these plants develop succulent leaves. Such foliage is born by the South African groundsel (Fig. 22 (1)), which has so adapted itself to circumstances that it is singularly unlike the too-familiar groundsel invading our gardens.
Fig. 22. - Four natives of South Africa.
1, a groundsel (Senechio (Kleinia) Haworthii); 2, a typical cactus (Eichinocactus corynodes); 3, a spurge (Euphorbia globosa); 4, a milkweed (Stapelia cacti.
The moisture which fate vouchsafes such plants must be treasured for times of need, not drawn speedily away by high winds or scorching sun. So the stomata in their leaves are very few, and the leaf-skin is thick and tough, so that vapor may not exude through it.