2Q4}4 HOW CROPS GROW. 
smooth and unbranched throughout their entire length. 
Other agricultural plants have roots 
which are not only visibly branched, 
but whose finest fibers are more or 
less thickly covered with minute 
hairs, scarcely perceptible to the un- 
assisted eye. These root-hairs consist 
always of tubular elongations of the 
external root-cells, and through them 
the actual root-surtace exposed to the 
soil becomes something almost incal- 
culable. The accompanying figures 
illustrate the appearance of root-hairs. 
Fig 38 represents a young, seed- 
ling, mustard-plant. A is the plant, 
as carefully lifted from the sand in 
which it grew, and B the same plant, 
freed from adhering soil by agitating 
in water. The entire root, save the 
tip, is thickly beset with hairs. In 
fig. 39 a minute portion of a barley- 
root is shown highly magnified. The 
hairs are seen to be slender tubes that 
proceed from, and form part of, the 
outer cells of the root. 
The older roots lose their 
hairs, and suffer a thickening of 
the outermost layer of cells by 
the deposition of cork. These 
dense-walled and nearly imper- 
vious cells cohere together and 
constitute a rind, which is not 
found in the young and active 
roots. 
As to the development of 
the root-hairs, they are more 
Fig. 38. 
