CHAPTER FIFTEEN 



ROOTS AND THEIR ENVIRONMENT 



Roots commonly develop in soil. Floating plants, however, 

 develop roots in water ; and many plants in the moist tropics 



develop roots in the air. Some 

 of our own climbing plants have 

 aerial roots. Corresponding 

 with the differences in their en- 

 vironments, there are marked 

 differences in the structures 

 and processes of roots. 



The response of roots to 

 gravity. The downward 

 growth of a primary root is 

 a response to gravity. The 

 primary root is positively gee- 

 tropic; it turns tow^ard the 

 earth. We must clearly un- 

 derstand, however, that gravity 

 does not pull the root into the 

 soil. The soil particles are 

 heavier than the roots, and 

 this means that gravity pulls 

 more on them than it does on 

 the roots. A root will even 



means of tendrils, the older stems by downward intO merCUry, 



means of adventitious aerial roots. . . -^ 



which is 1 5 times as heavy as 

 the root. Gravity merely determines the direction of 

 growth. The penetration of the soil is due to growth pres- 

 sure ; the cells of the root multiply and enlarge, forcing the 

 tip downward. Secondary roots also respond to the pull of 



182 



Fig. 104. Young and old stems of Bos- 

 ton ivy. The young stems are hold by 



