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 aérating 
tissue. 
