Most plants obtain the greater part of their food, first, from the soil in the form of a watery solution, and, secondly, from the air in the form of a diffusible gas. In a few cases all food material is obtained from the air, as in the case of epiphytic plants. In such plants, the aerial roots have a modified outer layer--velamen--which functions as a water-absorbing and gas-condensing tissue. Many xerophytic plants absorb water through the trichomes of the leaf. Such absorption tissue enables the plant to absorb any moisture that may condense upon the leaf and that would not otherwise be available to the plant. The water-absorbing tissue of roots is restricted to the root hairs, which are found, with few exceptions, only on young developing roots. ROOT HAIRS =Root hairs= usually occur a short distance back of the root cap. There is, in fact, a definite zone of the epidermis on which the root hairs develop. This zone is progressive. As the root elongates the root hairs continue to develop, the zone of hairs always remaining at about the same distance from the root cap. With the development of new zones of growth the hairs on the older zone die off and finally become replaced by an epidermis, or a periderm, except in the case of sarsaparilla root, and possibly other roots that have persistent root hairs. Each root hair is an outgrowth from an epidermal cell (Plate 38, Fig. 3). The length of the hair and its form depend upon the nature of the soil, whether loose or compact, and upon the amount of water present.