154. A TEXTBOOK OF GENERAL BOTANY 
usually arises in the outermost layer of the cortex or in the 
epidermis (Fig. 149) by a tangential division of the cells, but 
it may have its origin in a deeper layer (Fig. 150). When the 
phellogen is formed 
by tangential divi- 
sion of the epidermis, 
the outer layer of 
cells remains epider- 
mal, while the inner 
becomes the phello- 
gen. When the out- 
ermost layer of the 
cortex by tangential 
division gives rise to 
Fig. 151. Beginning of lenticel formation under the phellogen, it is the 
a stoma of mulberry (Morus alba). (x 230) outer layer that be- 
comes phellogen. As 
the phellogen is formed by the division of cells which had pre- 
viously lost all the characteristics of meristematic cells, it is 
called secondary meristem. After the phellogen layer is formed, 






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Fig. 152. Lenticel of mango (Mangifera indica) 
Note the phellogen layer extending under the lenticel. (x 95) 
it divides tangentially and gives rise to radial rows of cork cells 
toward the exterior (Fig. 149) of the stem, and frequently to 
parenchyma cells toward the interior. The cork cells soon be- 
come suberized and die. Also, the cutting off of the water supply 

