72 PLANT LIFE 
is impossible completely to wash the soil away 
from this portion. Still further behind, the 
soil ceases to adhere to the surface. Other 
roots or rootlets are seen to be growing out, 
and still further away from the tip the 
diameter of the young root begins evidently 
to increase. Thus we distinguish four regions : 
(1) the tip and clean surface; (2) the hairy 
zone ; (3) the region from which young rootlets 
are springing; (4) The older parts which are 
getting thicker. 
The only part of the root which is actively 
absorbing water from the soil is the hairy 
zone, and the hairs themselves—outgrowths 
from the superficial cell layer—are the essential 
structures which perform this task. The apex 
is chiefly concerned with boring on through 
the soil, and it is covered with a characteristic 
covering of cells called the root-cap, the outer 
cells of which are continually being worn away 
by attrition in the soil, and as constantly being 
replaced by the formation of fresh layers from 
within. The superficial cells behind the 
region of the root-cap do not begin to elongate 
at once to form hairs. This does not happen 
till the part of the root from which they 
spring has ceased to elongate. The meaning 
of this at once becomes clear when we reflect 
that these delicate protuberances, the root- 
hairs, are in very intimate contact with the 
particles of soil—and if the part of the root 
which bears them were to continue to grow 
in length, they would be torn away from their 
