CHAPTER? IV 
ABSORPTION TISSUE 
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 the 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 epi- 
dermis, 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. 
A root hair is formed by the extension of the peripheral wall 
of an epidermal cell. At first this wall is only slightly papillate, 
but gradually the end wall is extended farther and farther from 
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