The Structures and Processes of Roots 



167 



stem. On its sides are numerous secondary roots which 

 extend at right angles, or grow obliquely downwards. Un- 

 like stems, roots possess no 

 definite nodes from which 

 branches arise. A second- 

 ary root may originate at 

 any point on the primary 

 root. 



In many seedlings there 

 are also roots that develop 

 from the first node of the 

 stem. All roots arising 

 from stems and leaves are 

 called adventitious roots. 

 The "prop roots" that 

 develop from the lower 

 nodes of corn stems and 

 the roots that grow from 

 " cuttings " are familiar 

 examples. Adventitious 

 roots develop also from the 

 stems of many plants like 

 the poison ivy and trumpet 



creeper and act as holdiastS Fig. g6. stages in the development of a corn 



in supporting these climbers seedling. P is the primary root, S a secondary 



, ,, . , root, and A an adventitious root from the first 



on trees and walls. Adven- node of the stem. 

 titious roots may arise also 



at any point on a primary or secondary root, following in- 

 juries. For example, when we plant pieces of horseradish 

 or dandelion roots, adventitious roots develop. During the 

 dry season in deserts the younger parts of the root systems 



