808 THE AMERICAN NATURALIST. (Vor. XXXIX. 
eralized type, and they possess no value for differential pur- 
poses. The wood parenchyma is always characterized by the 
presence of resinous matter which is generally deposited in 
large quantity, and it thereby gives a distinguishing feature , 
to the elements which admits of their very ready recognition. 
This is especially true of them in the region of the medul- 
lary sheath, but it is also equally true in some other cases. 
The distribution of the resinous wood parenchyma as dis- 
Li : 
- 
* 
NIIT 
LEI PR 

Fic. 7.— Populus —« Transverse section showing the characteristic structural 
u of the genu C $2. 
played in transverse section, presents features of special value 
for diagnostic purposes. In all of the Salicacez it forms a 
thin, limiting zone on the outer face of the growth ring. This 
zone varies somewhat in thickness and continuity, but as a 
rule it is composed of a continuous series of cells forming à 
layer from one to three elements thick (Figs. 6, 7). But in the 
