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 and are gradually sloughed off as old bark, 



exposing the inner younger cork 

 growth beneath. This sloughing or 

 peeling off 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 oaks, 

 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 suberization 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 



!'!,, 2S -Trunk of a birch tree show- Cutin (P^ 6 ? 8 ) and is Water-proof, 

 ing the large lenticels. /-^ / , i i_ j 



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 lenticels 



