No. 467.) STUDY OF THE SALICACEE. 821 
geological, anatomical— is all in one direction, and that is to 
show that the genus Populus is essentially the more primitive 
member of the family, and that it is the genus through which 
we must probably seek connection with ancestral forms. 
It only remains to point out that with respect to a delimita- 
tion of species and varieties, the rule adopted for the gymno- 
sperms (Penhallow, :04c) applies with equal force here, to the 
effect that varieties have no proper status on anatomical grounds, 
and what are designated as such on the basis of external mor- 
. phology must be regarded as species from the standpoint of the 
present studies. 
SALICACEZE 

Transverse.— Growth rings usually devoid of recognizable differentiation 
of spring and summer wood, the outer limits being defined by a usually 
resinous wood parenchyma of radially narrower cells, forming a zone 
upwards of three elements thick. Wood cells variable, hexagonal, and usu- 
ally not in obvious radial rows. Vessels numerous throughout, often pre- 
dominant, and more or less radially compounded. 
Radial.— Medullary rays composed of two kinds of cells, 7. e., those with- 
out (1) and those with (2) pits on the lateral walls opposite vessels. Vessels 
commonly with thyloses, their radial walls with numerous, multiseriate, 
usually localized, oval, round, or hexagonal bordered pits. 
Tangential.— Medullary rays chiefly narrow ; 1- or more rarely 2-seriate 
in part. Vessels with numerous, multiseriate, hexagonal, bordered pits 
throughout. 
1. Populus. 
Radial— Ray cells of two kinds but presenting no essential distinction 
as to length, height, and thickness of the walls ; the pits on the lateral walls 
of cells (2) more definitely rounded, oval or sparingly angled, when they 
become quadrangular and lie in radial series, never merging into scalari- 
form structure. o 
Tangential The two kinds of ray cells not clearly distinguishable. 
Synopsis OF SPECIES. 
Ray cells (2) (tangential) more or less distinguishable 
by differences in 
height, breadth, and less resinous contents. 
