PINE TREES OP ROCKY MOUNTAIN REGION. 17 



cross section of the leaf shows two resin ducts (near the border on 

 the back of leaf). 



The cones ripen during September and drop their seeds 1 by the 

 middle or latter part of October. Only the scales in the middle of 

 the cones bear perfect seeds. Externally, the cones are pale yellowish 

 or reddish brown. The empty cones fall from the trees mostly in 

 late autumn and early winter. Mature cones (PL X, b, c) which 

 are globelike or egg-shaped and borne on very short stems, vary in 

 length from about 1 inch to 2-J inches, and in breadth from about 1 

 inch to nearly 2§ inches. The seeds (PI. X, e) are from one-half 

 to three- fourths of an inch long by about one- fourth to three-eighths 

 of an inch wide. They vary in color from light yellowish brown 

 to a deep chocolate-brown on the under surface (which is always 

 darker) to cinnamon-brown on the upper side. The pale cinnamon 

 brown wings of the seeds are extremely short (rudimentary) and re- 

 main attached to the scales when the seeds fall (PL X, 5, c, /). 

 Seed-leaves are from 9 (occasionally 8) to 15 in number. 



The wood of Pinus cembroides is very narrow-ringed, and mod- 

 erately soft, but firm. It is the heaviest of all the Rocky Mountain 

 pines, a cubic foot of dry wood weighing about 40^ pounds. In this 

 country the wood is used locally only for fuel and for other minor 

 domestic purposes. 



OCCURRENCE AND HABITS. 



Pinus cembroides grows in poor, shallow, rocky, and gravelly soils 

 on hot mountain slopes, precipitous canyon sides, benches, and foot- 

 hills at elevations between 4,800 and 7,500 feet (map No. 5). Within 

 our border it forms scattered growths, interspersed here and there 

 with Quercus reticulata, Q. hypoleuca, and Q. arizonica. In its 

 more extensive range in Mexico it sometimes occurs in practically 

 pure, but rather open stands. 



The light requirements of this species are insufficiently known at 

 present ; but it appears to be intolerant of shade, except during early 

 youth, and even at that time it endures only partial shade. 



Seed production occurs usually every other year, but heavy crops of 

 seed are borne only at intervals of three or four years. Trees begin to 

 bear cones comparatively early, seed production increasing steadily 

 to old age. A large percentage of the seeds are sound, but their 

 vitality is very transient, so that, except under specially favorable 

 conditions for germination, reproduction occurs sparingly. Seed- 

 lings are generally much scattered, and mainly on washed mineral 

 soil. Sparse reproduction where the tree occurs abundantly is prob- 



1 The seeds are extensively gathered for food in Mexico ; but in the United States few 

 are obtained for this purpose because of the limited occurrence of the tree. 



61354°— Bull. 460—17 2 



