PINE TREES OF ROCKY MOUNTAIN REGION. 13 
maintain it as a species and to retain for it Engelmann’s original 
name, P. strobiformis. While the cone scales of P. ayacahuite 
Ehrenberg and P. ayacahuite veitchii Shaw, wholly Mexican pines, 
resemble those of P. strobiformis Kngelm., the seed wings of the 
first two pines are well defined and from two to three times as long 
~as the seeds, while the seed wing of P. strobiformis is very rudi- 
mentary, being scarcely one-eighth of an inch long.* 
DISTINGUISHING CHARACTERISTICS. 
The longer-leafed forms of this tree, grown in protected places at 
low elevations, have somewhat the aspect of the eastern white pine 
(Pinus strobus), while the shorter-leafed trees, found in exposed 
higher situations, have a general resemblance to the limber pine 
(Pinus flexilis). Mexican white pine usually has a narrow conical 
crown with drooping branches, especially the lower ones, and 
rather short, clear, straight, often rapidly tapering trunk. In favor- 
able situations, and when they have reached their full growth, the 
trees are from 60 to nearly 100 feet high and from 16 to 20 inches 
in diameter, or exceptionally from 28 to 386 inches in diameter. 
Commonly, however, the height is from 50 to 60 feet and the diameter 
from 14 to 16 inches. The trunk bark, about 14 inches thick, is dull 
red-brown and rather deeply and irregularly furrowed and narrowly 
ridged, the main ridges being more or less connected by smaller ones 
and composed of small, easily detached scales. The young twigs are 
at first covered with poddich: brown hairs, most of which disappear 
during the first winter. 
The pale blue-green leaves (Pl. VIII), borne in clusters of five, 
are slender, rather stiff, and from 3 to about 4 inches long. Leaves 
of each season’s growth remain on the twigs for about four years; 
some of them, however, are shed during the third year. Retention of 
each season’s leaves until practically the fourth year gives the crown 
a dense, well-clothed appearance. The margins of the leaves have 
minute, sharp, widely separated teeth. A cross section of the leaf 
shows two resin ducts (on the back of the leaf near the border). The 
lower or ventral sides of the leaves are marked with from 3 to 4 lines 
of minute pores (stomata), none being found on the back or dorsal 
sides of the leaves. This characteristic, first pointed out by Coulter 
& Rose in 1886, may serve to separate perplexing forms of this pine 
from its close relative Pinus flexilis, the leaves of which have dorsal 
stomata. 
1So far as is known Pinus strobiformis has not been introduced into cultivation in 
Hurope, nor has it been cultivated in eastern United States. Judging from the be- 
havior of Pinus flevilis in England, to which Pinus strobiformis is similar in its 
requirements of soil and climate, the Mexican white pine also is likely to be adapted 
for growth in that country. The poor showing made by Pinus flexilis in eastern United 
States similarly indicates that Pinus strobiformis would not grow well in that region. 
