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. In some species this diminution of surface is carried to the extreme, and the leaves have become very thick, assuming a cylindrical or almost globular form; and such leaves are capable of absorbing and retaining large supplies of water that serve to maintain the plant during those periods in which the roots have no moisture within their reach. We also find that many of the plants in question are further protected from a dangerous loss of moisture by the peculiar arrangement of their leaves, which are often so closely applied to the stem, or so closely overlapping one over another, that the total area of exposed surface is considerably reduced; and it frequently happens that the stem of the plant becomes thick and succulent, as well as the leaves, thus adding to the store of moisture kept in reserve for the rainless days. While some plants are almost invariably found in dry, stony places, others are very diverse in their habitats, sometimes growing in moist and shady places, and sometimes on cliffs or other rocky situations. In the latter we often find considerable modifications of size, form and structure, the same species being more or less luxuriant and thin-leaved when in damp soils, while in rocky places it becomes more or less stunted, with a tendency to produce thick and succulent leaves. A few of the plants that we include in the present chapter are to be found only on wet rocks, and are therefore of a nature very different from that of the species growing in dry places. They are always well supplied with moisture; and, being usually surrounded by a damp atmosphere, they lose but little water by evaporation, and thus require no reserves within their leaves or stems.