448 



BOTANY. 



of trees and shrubs (Fig. 329). The layers of the bark are 

 rarely well marked, and they generally become soon obliter- 

 ated by irregular corky growths in the substance of the bark 



Fig. 328. Tangential section of the stem of Ailanthus QlfindulosKs, through the 

 secondary xylem : </, g, pitted vessels ; p, p. xylem parenchyma ; st, $t. medullary 

 rays in cross-section ; If, fibrous tissue (wood cells) ; t, tracheldes. Highly magnified. 

 After Sachs. 



itself. They are, moreover, ruptured by the increase in the 

 diameter of the woody cylinder, and soon decay and fall 

 away. It thus happens that while the annual layers of the 

 wood are constantly increasing in number, reaching in ex- 



