THE STEM. 



140. Lenticels. In steins 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 , 



v IG. 130. A bit of a transverse section of the 



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



in the formation or a lenticel. I he cortical 

 cells under a stoma have divided tangen- 

 tially and are forming a loose tissue which 

 has already torn apart the guard cells. 

 (See fig.[i3i.) Magnified izodiam. After 

 Stahl. 



this poxvdery mass of cells 

 through a usually biconvex 

 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, s, the cork cambium. 

 Compare fig. 130. Magnified So cliani. 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 



