i68 



Science of Plant Life 



of some plants Hke the cactuses are killed, and when the next 

 wet season comes, many adventitious roots develop from the 

 parts of the root system still aHve. 

 In desert plants these new adven- 

 titious roots do most of the absorb- 

 ing work during the moist season. 



Root hairs. The young roots of 

 land plants generally bear root hairs. 

 These are delicate elongations of the 

 epidermal cells of the root. They 

 are especially concerned 



with the absorption of 

 water and mineral salts, 

 and their presence in- 

 creases the absorbing 

 surface of the root from 

 two to fifty times. Since ^ c. ■ .i. li. t ■ j 



•^ Fig. 97. Stages in the growth of an onion seed- 



the rate of absorption ling, showing the lifting and shedding of the seed 



depends in part upon '^"^'^ ^""^ ""^ development of the primary and 

 ^ . secondary roots. 



the surface area m con- 

 tact with the soil water, the advantage in root hairs is evident. 

 Root hairs are usually short-lived structures, their duration 

 being best measured in days. They begin to develop at a 

 short distance from the tip of the root. Farther back they 

 have attained full length, and beyond this they are in a dy- 

 ing or dead condition. Thus from day to day the zone of 

 root hairs moves forward with the growth in length of the 

 root. This brings the root hairs continually in contact with 

 new supphes of water and food materials in the soil. As a 

 plant enlarges, its root system becomes more complex through 

 repeated branching and the elongation of the branches. Most 



