826 THE AMERICAN NATURALIST. (VoL. XXXIX. 
Tangential. — Rays numerous, narrow, medium to high, sparingly resin- 
ous, I-seriate. Cells (1) uniform, narrowly oblong, and equal, sometimes 
becoming broader and oval in the central tract so as to form tapering 
extremities to the ray. Cells (2) not readily distinguishable but uniform, 
narrow, oblong, and equal. 
8. P. balsamifera Linn. 
Transverse. — Growth rings medium, uniform ; the limiting wood paren- 
chyma conspicuously resinous, of one or sometimes two rows of celis. 
Vessels numerous, oval, becoming gradually smaller toward the outer limits 
of the growth ring ; 1- to 4- or more rarely 5-compounded. Medullary rays 
numerous, prominent, resinous, one cell wide, and distant upwards of 8 
rows of cells. 
Radial. — Rays strongly resinous, the cells straight. Cells (1), the upper 
and lower walls thin and finely pitted; the terminal walls straight or 
curved, obscurely if at all pitted. Cells (2), the upper, lower, and terminal 
walls thin and obscurely if at all pitted ; the pits on the lateral walls round, 
in radial series, 
Tangential, — Rays very numerous, strongly resinous, low to high, 
I-seriate. Cells (1) uniform in the same ray and oblong, but varying 
between different rays from narrowly oblong to broadly oblong, equal. 
Cells (2) not distinguishable except by their less resinous contents. 
9. P. angustifolia James. 
Transverse.— Growth rings broad ; the limiting wood parenchyma con- 
spicuously resinous, usually upwards of 3 rows of cells thick. Vessels 
at first somewhat predominant, gradually diminishing in number and finally 
its of the growth ring where they are very much 
> ! cell wide, distant upwards of 9 rows of cells. 
Cells (1), the upper and lower walls 
d strongly pitted ; the terminal walls rather thick 
and strongly pitted. Cells (2), the upper and lower walls sparingly pitted ; 
in and sparingly if at all pitted ; the pits on the lateral 
radially seriate. 
ary rays numerous and very variable, low to high, 
^ie. Cells (1) uniform in the same ray and chiefly 
equal, but in some rays becoming twice as broad 
not distinguishable. 
walls round or oval and 
Tangential — Medull 
narrow, resinous, I-seri 
very narrowly oblong, 
nd oblong. Cells (2) 
10. P. trichocarpa Torr. & Gr. 
ee Growth rings very broad ; the limiting wood parenchyma 
Sparingly resinous. Vessels numerous throughout, becoming abruptly 
