ABSORPTION TISSUE 



125 



unicorn are so abundant and so large that they form dense 

 mats, which are readily seen without magnification. These 

 hairs are, therefore, macroscopically as well as microscopically 

 diagnostic. The root hairs of false unicorn (Plate 39, Fig. 2) 

 have white, wavy, often decidedly indented walls. The terminal, 

 or end wall, is rounded and much thicker than the side walls. 



In sarsaparilla (Plate 39, Fig. 1) the root hairs are curved 

 and twisted. The end wall is thicker than the side walls. In 

 some hairs the walls are as thick as the walls of the thin-walled 

 bast fibres. This accounts for the fact that the root hairs 

 are persistent on even the older portions of sarsaparilla root, 

 and it serves also to explain why these root hairs remain on 

 the root even after being pulled from the firmly packed earth 

 in which the root grows. 



WATER ABSORPTION BY LEAVES 



In many xerophytic terrestrial plants, the trichomes occurring 

 on leaves act as a water-absorbing tissue. In such plants the 

 walls of the hairs are composed largely of cellulose. It is ob- 

 vious that these hairs absorb the water of condensation caused 

 by dew and light rains — water which could not reach the plant 

 except by such means. 



There is no special tissue set aside for the absorption of 

 gases from the air. Carbon dioxide, which contributes the 

 element carbon to the starch formed by photosynthesis, enters 

 the leaf by way of the stoma and lenticels. The structure and 

 the chief functions of these will be considered under aerating 

 tissue. 



