196 A TEXTBOOK OF GENERAL BOTANY 
continuous band in the outer part of the cortex. As previously 
explained, this distribution of collenchyma is connected with 
the resistance which stems have to offer to bending stresses. 
As roots are not subject to the same type of stresses, there is 
not the same necessity for the development of collenchyma. 

Fr1G.198. Cross section of central portion of root of mungo bean (Phaseolus ' 
radiatus) after secondary xylem has been formed from the cambium 
e, endodermis; p, pericycle; ph, phloém; ca, cambium; pz, primary xylem ; 
sx, secondary xylem. (x 115) 
Roots that are in the ground cannot carry on photosynthesis, 
and in such cases chlorophyll is not developed in the cortex. 
In roots which normally are exposed to the light, however, 
chlorophyll is frequently developed in the parenchyma cells of 
the cortex, and in this case the cells are chlorenchyma cells. 
Branch roots. ‘he branches of stems originate as superficial 
outgrowths in which both the epidermis and the underlying 
tissues take part. The cortex and epidermis of the branch and 
those of the main stem are therefore continuous. In roots, on 
