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 
Fic. 133.—Length- the root tip, the older and more fully de- 
wise section through veloped are the branches. 
a root, showing how One important feature in connection with 
Deen eee we ane branching habit is that, when the end 
branches (b) originate Z ee 
in the region of the Of @ root is cut away, the remaining por- 
vascular cylinder and tion is stimulated to develop branches. It 
push through the cor- jis due to the ability of roots to branch, 
tex, finally reaching that trees and other plants with their roots 
ene heavily pruned in transplanting are usually 
able to provide a new root system and become established in 
their new location. 
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 geotropism. 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 
