STRUCTURE AND GROWTH OF ROOTS 



r 35 



plant which is common in conservatories, shows the root- 

 cap very well. This plant has prop roots which develop 

 prominent root-caps even while they are growing through 

 the air. They fit over the end of the root as a glove 

 fits over the end of your finger. Their development 

 while the root is in the air indicates that such struc- 

 tures may develop even when there is no need for them. 

 Evidently there are forces within the 

 plant as well as forces without which 

 affect its growth. The forces within are 

 called the forces of heredity ; the plant 

 inherits them. The forces without are 

 called the forces of environment; these 

 the plant does not inherit. All the con- 

 ditions which surround a plant form its 

 environment. 



the left shows soil 

 sticking to the root- 

 hairs. The one at 

 the right has grown 

 in moist air. It shows 

 root-hairs free from 

 soil. 



D. Root-hairs. — Just behind the root- FlG - 45- — Seedlings of 



, i , i /ri t-i ■ mustard. The one at 



cap the root-hairs appear, (bee Figures 



45 and 46) These hairs are really cells 



of the epidermis whose outer walls are 



enormously extended. They grow nearly 



as thickly together as the little hairs 



which compose the surface of velvet. 



Suppose there were no root-hairs, as is the case with water 



roots. Absorption could still go on, but not nearly so 



rapidly, for the absorbing surface would be very much 



reduced. The hairs greatly increase the " coast line of 



absorption." A surface through which liquids can pass 



is called a permeable surface. A root with hairs has five 



or ten times as much permeable surface as the same root 



would have if it had no hairs. Water roots make up for 



