158 CONTRIBUTIONS FROM THE NATIONAL, HERBARIUM. 



Pedicels much shorter than the capsules. 



Capsules 5 mm. long or shorter 7. P. arizonica. 



Capsules 6 to 10 mm. long. 



Leaf blades usually broadly cuneate or rounded at the base, long- 

 pointed, glabrous or nearly so 8. P. mexicana. 



Leaf blades mostly truncate or subcordate at the base, short-pointed. 



Petioles and leaves glabrous or nearly so 9. P. fremontii. 



Petioles and lower surface of the leaves densely short-pilose or 

 tomentose 10. P. macdougalii. 



1. Populus trichocarpa Torr. & Gray; Hook. Icon. PL 9: pi. 878. 1852. 



San Pedro Martir Mountains of Baja California at an altitude of about 1,350 

 meters. Northward to Alaska ; type from Santa Clara River, California. 



Tree, sometimes 60 meters high, but in Baja California much smaller, with 

 a narrow ci'own ; bark light gray, deeply fissured in age; wood soft, weak, 

 brown, its specific gravity about 0.38. 



In the United States the wood is used for barrel staves, tubs, bowls, etc. 

 Among the Indians it was a favorite tree for making canoes, and the roots 

 were used in basketry. The sterile Mexican specimens seen by the writer have 

 very small leaves. This species is known in the United States as black cotton- 

 wood. 



2. Populus angustifolia James in Long, Exped. 1: 497. 1S23. 



Along streams, mountains of northern Chihuahua. Northward to Canada ; 

 type from the Rocky Mountains. 



In Chihuahua said to be a tree 4.5 to 7.5 meters high, but farther north often 

 much larger, sometimes attaining a height of 20 meters and a trunk diameter 

 of 40 to 50 cm. ; bark rough or fissured ; leaves 5 to 12 cm. long ; catkins 2 to 

 6 cm. long; wood weak, soft, light brown, its specific gravity about 0.39. 



3. Populus monticola T. S. Brandeg. Zoe 1 : 274. 1890. 



Sierra de la Laguna of Baja California at altitudes of 660 to 1,550 meters. 



Tree, 15 to 22 meters high, the trunk 60 to 90 cm. in diameter, the bark 

 often smooth and white; branchlets at first densely tomentose; leaf blades 

 coarsely dentate ; wood light reddish. " Huirigo." 



The wood is used locally for making furniture and other objects. It has 

 been stated by Bailey ' that this is a form of the Old World P. alba which has 

 become naturalized in Baja California, but the writer is convinced from study 

 of specimens that this is not the case. 



4. Populus tremuloides Michx. Fl. Bor. Amer. 2: 243. 1803. 



Mountains of Chihuahua, Sonora, San Luis PotosI, and Durango. Widely 

 distributed in the United States and in Canada (type locality). 



Usually a small slender graceful tree, but sometimes 12 meters high or even 

 up to 18 meters, the trunk sometimes 60 cm. in diameter; bark thin, smooth, 

 pale green or grayish ; wood soft, weak, light brown, its specific gravity about 

 0.40. "Alamillo " (Durango, Pa/oni). 



Large amounts of aspen wood are used in the United States for paper pulp. 

 The tree is one of the first to spring up in lumbered or burned-over regions, and 

 it often covers large areas. The Mexican specimens seen are all sterile. 

 Probably they should be referred to P. aurea Tidestrom,^ but the status of that 

 species is still uncertain. 



' Stand. Cycl. Ilort. 2750. 1016. ' Amer. Mid. Nat. 2 : 35. 1911. 



