822 THE AMERICAN NATURALIST. | (Vor. XXXIX. 
Rays (tangential) 2-seriate in part. 
Ray cells (1) somewhat variable, oblong, narrow, or broader and oval; 
unequal. 
Vessels (transverse) broad, oval, 1- to 3-, more rarely 5-compounded. 
I. P. fremontt. 
Rays (tangential) strictly 1-seriate. 
Ray cells (1) (tangential) uniform, narrow, oblong, equal, but occa- 
sional rays with broader, oval, and somewhat variable, unequal 
cells. 
Vessels (transverse) broad, oval, or transversely oval, I- to 2- or 
rarely 4-compounded. ; 3. Fam 
Vessels (transverse) broadly oval, 1- to 4- or rarely 7-compounded. 
P. tremuloides. 
Ray cells (1) (tangential) uniform, narrowly oblong, and equal, but in 
different rays sometimes broadly oblong. 
Vessels (transverse) very numerous, broad, 1- to 3- or rarely 4-com- 
pounded. . : I : ; 5. P. pyramidalis. 
Vessels (transverse) numerous, much smaller toward the outer face 
of the growth ring, 1- to 4- or rarely 5-compounded. 
. P. grandidentata. 
Ray cells (2) (tangential) not distinguishable from (1). 
Rays (tangential) all strictly 1-seriate. 
Rays (tangential) with diminishing terminal width. 
Vessels (transverse) at first predominant, gradually diminishing in 
size and number, 1- to 3-, rarely 4- to 5-compounded. 
P. wislizeni. 
Rays (tangential) of uniform width throughout. 
essels (transverse) numerous, oval, r- to 4- or rarely 5-com- 
pounded. . : : : : i P. balsamifera. 
Vessels (transverse) at first predominant, gradually diminishing in 
Size and number; 1- to 4^ rarely 6-compounded. | 
. P. angustifolia. 
Vessels (transverse) numerous throughout, naar pe i the 
Outer portions of the growth ring; 1- to 4-compounded. 
10. P. trichocarpa. 
ring sometimes 3-compounded. PUS 
Rays (tangential) 2-seriate in ae * . 11. P. heterophy 
m m uniform in width throughout 
region of the Ait broadly oval, abruptly fewer in the 
er wood, chiefly single, or less frequently 
2- to 3-compounded. : : j P moaier. 
