THE STEM. 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 _, . , . , 4 * t * 



r Fig. 130. — A bit of a transverse section of the 



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



7 L in the formation of a lenticel. 1 he cortical 



this powdery maSS Of Cells <*ft un <J er a stoma have divided tangen- 



r J tially and are forming a loose tissue which 



through a usually biconvex h c as alread V°™ a P. a , rt , the j> uard fg*- 



o ' (See ng.^i.) Magnified 120 diam.— After 



rift, whose shape suggested Stahl - 



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, s, the cork cambium. 

 Compare fig. 130. Magnified 80 diam. — 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 



