No. 467.] STUDY OF THE SALICACEZ. 809 
genus Salix, other features of importance for differential pur- 
poses are presented.‘ Thus in S. cordata the cells occur in large 
numbers throughout the woody zone, and are distributed in such 
manner as to form rather conspicuous and often somewhat exten- 
sive tracts disposed without any reference to an orderly arrange- 
ment. In S. sessilifolia and S. longifolia, it not only occurs in 
the limiting zone, but it is also definitely associated with the 

Fıc. 8.— Populus tremuloides. Tangential section. X 52- 
in S. cordata, forming about them a narrow but 
discontinuous layer, or as isolated cells at various parts ve ” 
periphery. All other species of Salix, so far as € Y 
present investigations, have the wood parenchyma p n 
the limiting zone of the growth ring, à rule which also seem 
to apply to Populus without exception. 
The a The chief diagnostic value o 
vessels as 
f the vessels is to 
