SALICACE^E 



161 



the elevated oval horizontal leaf-scars. Winter-buds acuminate, resinous, about ' 

 long, with 6 or 7 light chestnut-brown lustrous scales. Bark on young stems and 

 large branches smooth, nearly white, becoming on old trunks pale gray-brown, about 

 \' thick, deeply divided into broad flat ridges. 



Distribution. Banks of streams in the arid eastern foothill region of the Rocky 

 Mountains ; Assiniboia to western Nebraska, eastern Wyoming, and southern Colo- 

 rado. 



Sometimes planted as a shade-tree in the streets of cities in the Rocky Mountain 

 region. 



7. Populus trichocarpa, Hook. Black Cottonwood. Balsam Cottonwood. 

 Leaves broadly ovate to oblong-rhombic, gradually narrowed and usually short- 

 pointed or rarely acute at the apex, broad, rounded or slightly cordate or occasion- 

 ally slightly narrowed and wedge-shaped at the base, finely crenately serrate, coated 



at first with rufous or pale pubescence, becoming thick and firm, dark rich green, 

 glabrous or puberulous and lustrous above, pale and rusty or silvery white and con- 

 spicuously reticulate-venulose below, 3' -4' long, l^'-3' broad; their petioles slender, 

 terete, puberulous, l'-2' long. Flowers : aments stalked, the staminate densely flow- 

 ered, l'-2'long, \' thick, with slender glabrous stems, the pistillate loosely flowered, 

 2^'-3' long, with stout hoary-tomentose stems becoming 4'-5' long before the fruit 

 ripens, their scales dilated at the apex, irregularly cut into numerous filiform lobes, 

 glabrous or slightly puberulous on the outer surface; disk of the staminate flower 

 broad, slightly oblique; stamens 40-60, with slender elongated filaments longer than 

 the large light purple anthers; disk of the pistillate flower deep cup-shaped, with 

 irregularly crenate or nearly entire revolute margins; ovary subglobose, coated with 

 thick hoary tomentum, with 3 nearly sessile broadly dilated deeply lobed stigmas. 

 Fruit subglobose, nearly sessile, pubescent or rarely almost glabrous, thick-walled, 

 3-valved; seeds obovate, apiculate at the gradually narrowed apex, light brown, 

 puberulous toward the ends, ^' long. 



A tree, often 200 high, with a trunk 7-8 in diameter, heavy upright branches 

 forming a broad open head, and stout branchlets terete or slightly angled while 

 young, marked by many orange-colored lenticels, coated at first with deciduous 

 rufous or pale pubescence, light or dark orange-colored and lustrous during their 



