THE Sl^EM. 



113 



140. Lenticels. — In stems in which the generating layer 

 of the periderm is formed from the epidermis or the cortex 

 adjacent to it, the cork cells 

 produced show certain modi- 

 fications at points correspond- 

 ing to the stomata of the 

 epidermis. Here the cork 

 cells become rounded and 

 loosened from one another 

 (figs. 130, 131). The epider- 

 mis under the strain ruptures „ . , . , 



^ 1^ ir,, 130. — A bit 01 a transverse section of the 



first at the stoma, and exposes cortex of elder, showing a very young stage 



^ m the formation of a lenticel. I'he cortical 



this powdery mass of cells «'',s under a stoma hai-e divided tangen- 



^ tially and are forming a loose tissue which 



through a usually biconvex '^^ already torn apart the guard cells, 



^ ^ (See ng._i3r.) INlagnihed 120 diam, — After 



rift, whose shape suggested ^'*'''- 



for the structure the name lenticel. Lenticels are formed 



either beneath single stomata, or, when the stomata are not 



Fig, 131, — Transverse section through a mature lenticel of elder, .f, tlie cork cambium. 

 Compare fig. 130. Magnified .So diani. — After Stahl, 



uniformly distributed, beneath the clusters of stomata. When 

 the generating layer of cork is deep-seated the lenticels pro- 

 duced are without relation to the position of the stomata. 

 141. 2. The formation of secondary wood and bast. — 



The position of the internal generating layer (the stelar cam- 

 bium) is not subject to the same variations as the external 



