52 



COLLEGE BOTANY 



inside called the phelloderm. All these new tissues outside the 

 cork cambium are called the periderm. The walls of the cork 

 cells become suberized and are, therefore, impervious to water. 

 They eventually die an.d are gradually sloughed oft' as old bark, 



exposing the inner younger cork 

 growth beneath. This sloughing or 

 peeling oft' of the bark is character- 

 istic of most forest trees. Many trees 

 are roughly corrugated, the bark peel- 

 ing off in such small fragments as to 

 be scarcely noticeable, as in the o^aks, 

 while in others it peels off in large 

 pieces, as in the case of the shag bark 

 hickory. The bark of the sycamore 

 cracks and comes off in fragments, 

 while the bark of the birch peels off 

 like paper. 



The suherization is brought about 

 by chemical changes in the cellulose 

 and the addition of suberin layers to 

 the suberized walls, or by the addition 

 of suberin layers only. Suberin is 

 waxy in character, very similar to 

 cutin (page 78) and is water-proof. 

 Of course, the suberized or water- 

 proof epidermis or cork over the surface of the stem is a very 

 great protection against the escape of water from the stem 

 when in a dry atmosphere and also against the absorption of 

 water when subjected to an undue amount of moisture. But 

 there is a provision for aeration of the tissues through lenUcels 



Fig. 28. — Trunk of a birch tree show- 

 ing the large lenticels. 



