PLANT ORGANS AND ORGANISMS 
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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 majority 
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. 46. — 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 
