30 BOTANY 



those of the castor oil plant, that the root does not 

 remain single but very speedily begins to give off 

 branches. By this process of branching a very large 

 root system is made possible. The main root of Di- 

 cotyledons usually persists and remains longer and 

 stronger than its branches. Such a main root is called 

 a tap root. The branches in turn branch and we get 

 roots of the second, third, and higher orders. If we 

 trace the formation of these roots, as we can do by 

 cultivating a seedling in water, or a dilute solution of 

 the necessary mineral compounds, we find that they 

 arise in constant succession as the main root grows, 

 the youngest thus being always nearest the growing 

 point of the main root. Each branch root has the same 

 appearance as the one from which it springs, and 

 similarly bears near its apex a band or zone of root 

 hairs. The branches orginate in the interior of the old 

 root and bore their way outwards. They arise in 

 definite positions, in relations to certain internal 

 structures which will be discussed a little later. 



The branches are sensitive to the same stimuli as 

 the main root, but they respond rather differently to 

 the action of gravity. Instead of growing vertically 

 downwards, the first branches stand out nearly at right 

 angles to the main root and persist in growing in this 

 direction. The branches which in their turn they bear 

 do not grow in such definite positions, but extend 

 symmetrically round the one from which they spring. 

 If by accident the main root is killed, its place is taken 

 by one of its strongest branches, which alters its response 

 to gravity and grows vertically downwards. 



By this course of development the root system of a 

 plant comes to occupy considerable space in the earth 

 and to fill the interstices of the latter very completely. 

 Two advantages are thus secured: a very firm grip of 

 the soil is secured by the attachment of the root hairs 



