A few species of Willows and Sallows that grow on the banks of streams belong to the order Salicaceæ, and have the following features in common:—Their leaves are simple, stipulate, and deciduous. The flowers are imperfect, in erect catkins with small scales at the base, the male and the female flowers being produced on separate trees or shrubs. Each male flower consists only of a small scale and two or more stamens; and the female of a similar scale, and a conical ovary of one cell with a forked style. The fruit is a conical capsule of two valves, containing several seeds that are covered with white, silky hairs. The species referred to are the Almond-leaved and the Bay-leaved Willows, the Dark-leaved Sallow, and the Purple Osier, but we refrain from introducing descriptions since the identification of these trees is somewhat difficult for a beginner.
[XX]
ON WALLS, ROCKS, AND STONY PLACES
Several of our flowering plants are to be seen most frequently on walls and rocks, or in other situations where there is hardly a trace of soil of any kind. Some of these thrive in such dry spots, with often such free exposure to the rays of the spring or summer sun, that it is difficult to understand how they manage to survive the periods of drought through which they live until we become acquainted with certain peculiarities of their form and structure.
In the first place we must recall the fact that plants lose a considerable amount of moisture by evaporation from the transpirating surfaces of their leaves, and that this loss must necessarily be greatest when the air is warm and dry unless there is some means by which the transpiration is automatically regulated according to the requirements of the plant and to the varying conditions under which it has to exist.
The leaves of plants are covered with a thin skin or epidermis which consists of a single layer of cells, and which is practically impermeable to moisture. In this epidermis, however, on one or both sides of the leaf, are minute pores (stomata) through which water vapour is free to pass; and beneath the porous epidermis is a loose, cellular tissue, with air-spaces, from which the moisture can readily pass, in the form of vapour, to these stomata.
Each of the stomata is bordered by a pair of crescent-shaped guard-cells, placed with their concave sides towards each other, and joined at the ends. Further, the guard-cells are capable of changing their form, becoming straighter, and thus reducing or even closing the aperture between them; and becoming again curved, opening or enlarging the pore. The former change takes place during darkness, thus preserving the plant from the cooling effects of evaporation during the chilly nights; and also during dry weather when the plant is in danger of losing much more moisture than the roots can absorb.
So far, however, we have been dealing with a regulating process that is common to green plants in general; but we must look for some additional protection against loss of moisture in the plants which grow in such places that they have to live through longer or shorter periods during which the roots have little or no moisture within reach.
From what has been said concerning the structure of the leaf it will be understood that, as a rule, the larger the surface the greater will be the loss of water in a given time. But when we examine the leaves of the plants that grow on dry walls and rocks, we frequently find that they are more or less thick and fleshy—that the material of the leaf is disposed in such a manner as to reduce the area of the surface as compared with other leaves made up of the same amount of tissue.