
386 WESTERN YELLOW PINE IN ARIZONA AND NEW MEXICO. 
pine wood from that of such closely related species as shortleaf, long- 
leaf, and other pitch pines. 
In a smooth end view of the wood the pith rays are visible to the 
unaided eye only as faint, narrow, radially disposed lines, but they 
are plainly visible under a hand magnifier. (Fig. 7 a, p. 7.) The 
microscopic characters of the pith rays furnish the most reliable 
means of distinguishing the wood of western yellow pine from that 
of closely related species. 
MICROSCOPIC CHARACTERS. 
In a transverse section + of the wood (fig. 7, a), the inner and outer 
boundaries of each annualring are distinctly outlined (fig.7 a, a. 7.), 
while within the ring is seen 
a very gradual transition 
(, e. w. and t. w.), the latter 
from early to late wood (fig. 
usually occupying from one- 
fourth to one-third of the 
width of the annual ring. 
This gradual transition is in 
striking contrast to the ab- 
rupt changes from early to 
late wood seen in shortleatf 
and longleaf pine. Trache- 
ids (fig. 7 a, t.) in early wood 
uniform in size, and arranged 
in regular radial rows _ be- 
tween the pith rays, while in 
late wood they are compressed 
radially so that the cell cavi- 
ties appear elongated (fig. 
7 a, l. w.), or are sometimes 
completely obscured (fig. 7 a, 
Fic. 8.—Transverse section of a resin duct with | Foes : 
the surrounding tissue magnified 250 diame- ii .). The very prominent 
ters; r. d., resin duct; ep. c., epithelial cells; resin ducts oceur chiefly in 
Ww. p. C., wood parenchyma cells; ¢., tracheids ; 2 ee 
b. p., bordered pits; p. r., pith rays; c. w., the early wood (figs. ( and 8, 
G208S (ELS r. d.). The main passage in 
these, a continuous channel, is lined moderately thin-walled much- 
flattened resin-secreting (epithelial) cells (fig. 8, ep. ¢.). The pith 
rays (figs. 7 and 8, p. 7.) are of two kinds, one with resin ducts and 
one without ducts. Those without resin ducts are more abundant and 
always one cell wide, and are separated from one another by from 1 
to 20 rows of tracheids. 



1 The section must be cut precisely at right angles to the vertical axis of the tree. 
are round or polygonal, quite 




