824 THE AMERICAN NATURALIST. [Vor. XXXIX. 
Radial. — Medullary rays very sparingly resinous. Cells (1), the upper 
and lower walls rather thick, finely but strongly pitted ; the terminal walls 
straight, often diagonal, finely pitted ; equal to about 6 wood cells. straight. 
Cells (2), the upper and lower walls somewhat thinner and not obviously 
pitted ; the terminal walls conspicuously though finely pitted; the pits on 
the lateral walls hexagonal, forming a coarse, sieve-plate structure; some- 
what variable in height and sometimes rather short. 
Tangential. — Rays numerous, very sparingly resinous, medium, I.seri- 
ate. Cells (1) chiefly uniform, narrow, oblong, and equal, but occasionally 
rays with broader, oval, and somewhat variable and unequal cells. Cells 
(2, uniform, narrow, somewhat variable, the terminal ones often much 
higher. 
4. P. tremuloides Michx. 
Transverse. — Growth rings broad, uniform; the limiting wood paren- 
chyma sparingly resinous, in 1 to 2 rows. Vessels at first medium, becom- 
ing larger and finally smaller toward the outer limits of the growth ring, 
broadly oval, 1- to 4- or more rarely 7-compounded. Medullary rays om 
numerous, one cell wide, sparingly resinous, distant upwards of 10 rows of 
wood cells. 
Radial. — Ray cells sparingly resinous, straight. Cells (1), the upper 
and lower walls thin, and finely pitted ; the terminal walls thin, straight 
or curved, and rarely pitted. Cells (2), the upper and lower walls thin 
and sparingly pitted; the terminal walls rarely pitted, often curved ; the 
pits on the lateral walls round, chiefly in radial series. 
Tangential.— Rays numerous, resinous, low to high, r-seriate. The cells 
(1), uniform, narrowly oblong and equal, low rays sometimes showing broader 
and oval cells throughout. Cells (2) not readily distinguishable, but in low 
rays sometimes twice as high and less resinous. 
5. P. pyramidalis Ait. 
ring; 1- ` 
5s in A to A more rarely 4-compounded. Medullary rays prominent, nar- 
: ‚one cell wide, numerous, and distant upwards of 7,more rarely 10 
ows of cells ; somewhat resinous. 
Radial. — Medull | 
finely pitted. Cells and strongly pitted throughout ; the terminal walls 
pitted ; the te © S (2), the upper and lower walls thin and sparingly 
ri CO nn Muf pitted ; the pits on the lateral walls round 
or sparingly angled 
E 
