146 PHARMACEUTICAL BOTANY 



forms internal phloem (or intraxylary phloem), thus giving rise to 

 bicollateral bundles. 



In Strychnos Nux Vomica internal phloem exactly as in Gelse- 

 mium, etc., appears but in addition interxylary phloem is developed. 

 In the wood region of this plant axis the cambium starts at a certain 

 age to lay down patches of phloem which become wedged in between 

 xylem tissue as interxylary phloem. 



Lenticels and Their Formation. The epidermis in a great ma- 

 jority of cases produces stomata, apertures, surrounded by a pair of 

 guard cells, which function as passages for gases and watery vapor 

 from and to the active cells of the cortex beneath. 



FIG. 73. Cross-section through a lenticel of Sambucus nigra. E, Epidermis; 

 PH, phellogen; L, loosely disposed cells of the lenticel; PL, cambium of the 

 lenticel; PS, phelloderm; C, cortical parenchyma containing chlorophyll. (From 

 Sayre after Strasburger.) 



There very early originate in the region beneath the stomata 

 loosely arranged cells from cork cambium which swell up during 

 rain and rupture, forming convex fissures in the cork layer, called 

 lenticels. 



The function of lenticels is similar to that of stomata, namely, 

 to permit of aeration of delicate cells of the cortex beneath. 



Annual Thickening. In all woody exogenous stems such as 

 trees and shrubs the persistent cambium gives rise to secondary 

 xylem thickening every spring, summer and autumn. Soon a great 

 cylinder of xylem arises which constitutes the wood of the trunk and 

 branches. In the spring, growth is more active, and large ducts 

 with little woody fiber are produced while in summer and autumn 



