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 Engelm., 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. 1 



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 a 

 rather short, clear, straight, often rapidly tapering trunk. In favor- 

 able situations, and when they have reached their full growth, the 

 frees are from 60 to nearly 100 feet high and from 16 to 20 inches 

 in diameter, or exceptionally from 28 to 36 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 1J 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 reddish-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. 



1 So far as is known Pinus strobiformis has not been introduced into cultivation in 

 Europe, nor has it been cultivated in eastern United States. Judging from the be- 

 havior of Pinus flexilis 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. 



