No. 467.] 
STUDY OF THE SALICACEE. 
815 
that in position and form, as also usually in size, there are dif- 
ferential features of well defined value. Thus in Salix longi- 
folia (Fig. 14) there is no room for hesitation 
between the round or transversely oval cells, a, 
of the first order constituting the bulk of the 
structure, and the oblong narrow and terminal 

Fic. 14.— Salix lon- 
acter of the cells; 
th. J Liz 

- J 
transversely oval 
form of the cells of 
cells being sharply 
differentiated. X 
350. 
cells, 4, of the second order. 
Or again in the same spe- 
cies (Fig. 15), the same 
difference appears in the 
2-seriate form of the ray. 
In low rays, the tendency 
is for the cells of the first 
order to be replaced by 
those of the second order 
as also shown in Fig. 15 
on the right. From these 
considerations it will be ob- 
vious that the distinctness 
with which these two forms 
of cells appear in the tan- 
gential section is of impor- 
tance as a basis of generic 
differentiation. 
A. second factor of value 
is to be found in the form 
of the ray cell and its varia- 
tions, In the genus Popu- 
lus, the cells of two kinds 
present little or no differ- 
ence in form by which they 
may be distinguished from one another. 
This is especially true of P. balsamifera, 
P. heterophylla, P. monilifera, etc., while 
in P. tremuloides (Fig. 13), P. alba, P. pyramidalis, P. 
&randidentata, etc., 

there is a somewhat obvious difference in 
many cases which, taken in conjunction with other features, 
Serves to bring such species into a separate group. 
hus 
