128 TREES OF NORTH AMERICA 



dark red-brown filiform lobes; disk of the staminate flower cup-shaped, slightly oblique, 

 short-stalked; stamens 12-20, with short filaments and large light red anthers; disk of the 

 pistillate flower shallow, cup-shaped, slightly and irregularly lobed, short-stalked; ovary 

 ovoid, more or less 2-lobed, with a short or elongated style and 2 oblique dilated irregu- 

 larly lobed stigmas. Fruit broadly ovoid, often rather abruptly contracted above the 

 middle, short-pointed, thin-walled, 2-valved; pedicels often \' long; seeds ovoid or obovoid, 

 rather obtuse, light brown, nearly $' long. 



Fig. 121 



A tree, 50-60 high, with a trunk rarely more than 18' in diameter, slender erect branches 

 forming a narrow and usually pyramidal head, and slender glabrous or rarely puberulous 

 branchlets marked by pale lenticels, at first light yellow-green, becoming bright or dark 

 orange color in their first season, pale yellow in their second winter, and ultimately ashy 

 gray. Winter-buds very resinous, ovoid, long-pointed, covered by usually 5 thin concave 

 chestnut-brown scales; terminal \'-\' long and nearly twice as large as the axillary buds. 

 Bark f '-!' thick, light yellow-green, divided near the base of old trees by shallow fissures 

 into broad flat ridges, smooth and much thinner above. Wood light brown, with thin 

 nearly white sap wood of 10-30 layers of annual growth. 



Distribution. Banks of streams usually at altitudes of 5000-10,000 above the sea; 

 southern Alberta to the Black Hills of South Dakota and northwestern Nebraska (basin 

 of Hat Creek) westward through Wyoming, Montana and Idaho to Yakima County, 

 Washington, and southward to central Nevada, southwestern New Mexico (Silver City, 

 Grant County) and northern Arizona; the common Cottonwood of northern Colorado, 

 Utah, Wyoming, southern Montana, and eastern Idaho; on the mountains of Chihuahua. 



7. Populus acuminata Rydb. Cottonwood. 



Leaves rhombic-lanceolate to ovate, abruptly acuminate, gradually or abruptly nar- 

 rowed and cuneate or concave-cuneate, or rarely broad and rounded at the mostly entire 

 base, coarsely crenately serrate except near the apex, dark green and lustrous above, dull 

 green below, 2'-4' long, f '-2' wide, with a slender yellow midrib, thin remote primary veins 

 and obscure reticulate veinlets; petioles slender, nearly terete, l'-3' long. Flowers: 

 aments slender, short-stalked, 2'-3' long, the pistillate becoming 4' or 5' long before the 

 fruit ripens; scales scarious, light brown, glabrous, dilated and irregularly divided into 

 filiform lobes; disk of the staminate flower wide, oblique, and membranaceous; stamens 

 numerous, with short filaments and dark red anthers; disk of the pistillate flower deep 

 cup-shaped; ovary broad-ovoid, gradually narrowed above, with large laciniately lobed 

 nearly sessile stigmas. Fruit pedicellate, oblong-ovoid, acute, thin-walled, slightly pitted, 



