LENTICELS AND THEIR FORMATION 



27 



Lenticels and Their Formation. — The epidermis in a great majority 

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



,- 000 

 ^QOqO 



Fig. 20. — Cross-section through a lenticel of Sambucus nigra. E, Epidermis; 

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

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

 Sayre after Strasburger.) 



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

 to the active cells of the cortex beneath. 



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 con- 

 stitutes the wood of the trunk and 



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

 little woody fibre are produced while in summer or autumn growth is 



Fig. 21. — Part of a transverse sec- 

 tion of a twig of the lime, four years old. 

 m, Pith; ms, medullary sheath; x, secon- 

 dary wood; Ph\ phloem. 2, 3, 4, annual 

 rings; c, cambium; pa, dilated outer ends 

 of medullary rays; b, blast; pr, primary 

 cortex ; k, cork. {From Sayre after Vines.) 



