STRUCTURE AND GROWTH OF ROOTS 



135 



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. 



D. Root-hairs. Just behind the root- FlG - 4S- Seedlings of 



. , . /<- T-i- mustard. The one at 



cap the root-hairs appear. (See Figures the left shows ^ 



sticking to the root- 

 hairs. The one at 

 the right has grown 

 in moist air. It shows 

 root-hairs free from 

 soil. 



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 



