
THE ROOT 197 
the other hand, a branch originates in the pericycle and forces 
its way through the cortex and epidermis, breaking these tis- 
sues as it grows (Fig. 191). Owing to their method of origin, 












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C058 onan oe Os 
) HOI ERR T/ 
PEORSS Ae seeeer i: 20. 
e D>. EP SERS Py, Lose > 
SR ERS EE UTE LR nT 
SR SIREN TEE CTY A 
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Fig. 194. Cross section of root of mungo bean (Phaseolus radiatus) after 
all of the cortex except the endodermis has disappeared 
e, endodermis; p, pericycle; ph, phloém; px, primary xylem; sz, secondary 
xylem ; 7, pith ray. (x 80) 
therefore, the epidermis and the cortex of a branch root are not 
continuous with the epidermis and the cortex of the main root. 
Secondary thickening. Dicotyledonous roots, like dicotyle- 
donous stems, increase in thickness owing to the activity of the 
cambium. In very young roots there is no cambium (Fig. 189). 
The beginning of the process of secondary thickening is initiated 
by the formation of a cambium in the stele on the inner side 
of each group of phloém (Fig. 192). Subsequently this cambium 

