150 



ROOTS 



Another anatomical feature connected with the older portions 

 of roots is the development of branches, which begin to develop 

 some distance back of the hair zone and 

 in the way shown in Figure 133. In Seed 

 Plants the branch roots, which are called 

 secondary, tertiary, and so on according to 

 their distance from the main root, develop 

 from the pericycle and usually in the re- 

 gion closest to the xylem. In forming a 

 new branch, a few cells of the pericycle in 

 the region where the branch is to appear 

 begin to divide parallel to the surface of 

 the root. The new cells at first appear as 

 a slight elevation on the pericycle, but by 

 rapid growth this elevation of cells soon 

 pushes through the cortex and other over- 

 lying tissues, and becomes a branch with 

 vascular cylinder and other tissues con- 

 tinuous with those of the root of which it 

 is a branch. Of course the farther from 

 the root tip, the older and more fully de- 

 veloped are the branches. 



One important feature in connection with 

 the branching habit is that, when the end 

 of a root is cut away, the remaining por- 

 tion is stimulated to develop branches. It 

 is due to the ability of roots to branch, 

 that trees and other plants with their roots 

 heavily pruned in transplanting are usually 

 able to provide a new root system and become established in 

 their new location. 



Fig. 133. — LengtH^ 

 wise section through 

 a root, showing how 

 branches arise. The 

 branches (6) originate 

 in the region of the 

 vascular cyhndcr and 

 push through the cor- 

 tex, finally reaching 

 the exterior. 



Factors Influencing the Direction of Growth in Roots 



Roots and stems respond very differently in respect to gravity. 

 Primary roots grow toward the center of gravity, while most 

 stems grow in the opposite direction. This earth influence is 

 known as geotropis?n. Geo comes from a word meaning earth 

 and tropism means turning. So the word, geotropism, means 

 earth-turning, and refers to the turning of the root and stem in 



