No. 467.] STUDY OF THE SALICACEE. 807 
radius of the vessel; when forming isolated tracts, they tend to 
assume the form of the associated wood cells ; but when occur- 
ring in the limiting zone of a growth ring where they are chiefly 
to be met with, they are invariably compressed in a radial direc- 
tion, thereby assuming a form which serves to distinguish them 
from the associated mechanical elements. 
In their longitudinal aspects there is little that is not common 
to such elements wherever they may be found. They are cylin- 


i l i } i i t f . X . 
Fic. 6.— Salix alba. T t g the genus 52 
drical elements with somewhat thick and pitted walls, and abrupt 
or transverse terminations. In the medullary sheath they are 
generally only a few times longer than broad, but within the 
region of the secondary xylem, and especially in the limiting 
zone of the growth ring, they become much longer and propor- 
tionately narrower. Such features, however, whether exhibited 
in transverse or longitudinal section, are of an altogether gen- 
