244 



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-surface 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. 39. 



Fig. 38. 



