THE STEM 3 61 
on the outside cork, whilst on the inside it adds to the 
cortical tissue. As cork does not allow of the passage of 
water, it follows that the tissues outside the cork are cut off 
from the supply of water, and wither; the term “bark” 
is applied to all the dried-up tissues outside it. These 
may include different tissue-systems and cells of different 
forms. : 

Fic. 30.—CLosep LENntTICcEL, TRANSVERSE SECTION. (High power.) 
¢.c, cork cambium ; ck, cork ; e, ruptured epidermis ; cort, cortex. 
If the cork formed a thick ring all round the wood of the 
_ tree it would be impossible for the oxygen of the air to reach 
the cells inside. Special structures, answering to stomata, are 
formed in the cortex; these are called lenticels. 
Lenticels which occur in branches of one yeat’s growth look 
like brownish or whitish specks. They often originate just 
beneath the stomata and in cases where the cork is thick, 
_ they may form almost deep canals. 
Ul 
In the spring these lenticels are filled with cork cells, which 
are very loosely packed, so that gaseous interchange between 
the plant and external air is possible. In the winter these 
lenticels are closed ; the cork cells which have no intercellular 
Spaces are tightly packed, so as to exclude air (Fig. 30). 
