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 — velarnen — 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 



121 



