292 A TEXT-BOOK OF BOTANY. 



156). They usually arise as the product of a meristem situated 

 beneath the stomata of the epidermis, the stomata being replaced 

 by them when cork is developed. Several types of lenticels are 



FIG. 157. Bark of Rhamnus Purshiana showing large whitish patches of lichens, 

 and numerous lens-shaped lenticels, 



distinguished. They are quite characteristic and prominent in 

 a number of barks, as those of species of Betula, Prunus, Rham- 

 nus (Fig. 157), etc. 



