To obtain the most striking instances of protection, however, one must examine plants which belong to permanently dry regions, such as may be found in the United States along the Mexican border, or in the regions of tropical deserts. In the first place, it will be noticed that the plants in general produce smaller leaves than in other regions. That this holds a direct relation to the dry conditions is evident from the fact that the same plant often produces smaller leaves in dry conditions than in moist. One of the most striking features of an arid country is the absence of large leaves. These reduced leaves are of various forms, such as the needle leaves of pines, or the thread-like leaves of certain sedges and grasses, or the narrow leaves with inrolled margins such as is common in many heath plants.

The extreme of leaf reduction has, been reached by the Cactus plants, whose leaves, so far as foliage is concerned, have disappeared entirely, and the leaf work is done by the surface of the globular, cylindrical, or flattened stems.

A covering of hairs is an effective sun screen, and it is very common to find plants of dry regions characteristically hairy. In such regions it is to be observed also that dwarf growths prevail, so that the plant, as a whole, does not present such an exposure to the drought as in regions of greater moisture. One of the most prominent measures of protection in dry regions is the organization of what are known as water reservoirs. Nearly all plants of such regions have leaves which are known as fleshy, that is, they are thick and juicy, being reservoirs of stored tip moisture which is doled out cautiously according to the needs of the plant, without any wastefulness.

The whole subject of plant protection is an immense one, and the illustrations given above are merely intended to suggest that there is such a subject, and to lead to some observation of the various schemes of protection which are to be seen plainly on every hand.