ABSORPTION 



15 



plant is growing. As a root elongates, new root hairs are constantly 

 being formed just back of the root tip and at the same time the 

 oldest hairs farther back are dying of^". There is thus an area of 

 root hairs, rather definite in extent, which is constantly moving 

 forward as the root elongates and so is coming into contact with 

 fresh supplies of food materials in the 

 soil (Fig. 2). 



In the case of some plants that grow 

 in very dry places such as deserts, the 

 root hairs become thick-walled and 

 persist for several months or even years. 

 This is true also of a few plants that 

 grow where the water supply is abun- 

 dant, such as the honey locust {Gledit- 

 sia triacanthos) (Fig. 3), the Kentucky 

 coffee tree {Gymnocladus dioica), the 

 red bud {Cercis canadensis), and a 

 number of species of the composite fam- 

 ily. It is probable that in all of these 

 cases the habit of producing thick- 

 walled root hairs was formed at a time 

 when the plants concerned grew only 

 in very dry situations. 



The rate of absorption from the soil, 

 especially of water, depends not only 

 upon the structural characteristics of 

 the root system but also upon a num- 

 ber of external factors. One of these, 

 of course, is the actual amount of water 

 present in the soil. If there is a deficient 

 amount of water present a state of 

 physical dryness (actual absence of 

 sufficient water) exists and the rate 

 of absorption is reduced. A second 

 factor concerned here is temperature. 

 A high temperature, up to a certain 

 limit, increases the rate of absorption 



while a low temperature decreases it. A third factor affecting the rate 

 of absorption is the amount of salts, that is, of soluble materials, in 

 the soil. Either a low temperature or a high concentration of salts 

 in the soil brings about a condition that is spoken of as physiological 



Growing 



point 



Region, of 

 elongation 



Root cap 



Fig. 2.— Root tip, .showing 

 root cap, growing region, and 

 root hairs. (From Transeau's 

 General Botany. Copyright 

 1923 by World Book Com- 

 pany, Yonkers-on-Hudson, 

 Xew York.) 



