THE STEM 205 
are cut off by the cork cambium in certain places instead of cork. 
These swell up during rain and pressing outward, form fissures 
in the epidermis, called lenticels. 
The function of lenticels is similar to that of stomata, namely, 
to permit of aération of delicate cells of the cortex beneath. 
Fic. 135.—Representative: portion of a transverse section of a Linden stem, 3 
years old.—1x, 2x, 3x, Successive annual rings of wood. ph, Phloem, with bast 
fibers showing as lighter parts and sieve-tubes as darker parts. c, Cortex containing 
chloroplasts and rosette crystals of CaO. m.r., spreading end of primary medullary 
ray. pe, periderm. (From Small, after Kny.) 
While common to many plants, they are very conspicuously 
prominent in the birches and cherries. 
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 growth is lessened and small ducts and much 
mechanical woody fiber are formed. Thus the open, loosely 
