128 SALICACEAE (WILLOW FAMILY) 



short sharp point and small regular teeth, smooth on both sides, with downy 

 margins; petioles slender, flattened laterally, causing movement of the leaves 

 in the Ughtest breeze: lobes of the bracts linear, silky: stamens 6-20: capsule 

 conic. Aspen or American Aspen. — ^Very common on moist slopes and val- 

 leys in the hills and mountains. 



2. Popxilus occidentalis (Rydb.) Brit. Man. Ed. 2. 320. 1905. Becoming 

 a large tree, with thick rough furrowed bark; the branchlets more or less 

 angulate: leaves broadly deltoid, with subcordate or truncate base, abruptly 

 long-acuminate, crenately serrate, the teeth mostly obtuse; petioles often as 

 long as the blade: staminate aments 7-15 cm. long; the pistillate as long or 

 longer, in fruit with several to many ovoid capsules. P. angulata. The com- 

 mon Cottonwood; nearly related to the great Cottonwood, P. delUddea, of 

 the valleys of the Mississippi and its tributaries. — Frequent on stream banks 

 along the eastern base of the Rocky Mountains. 



3. Populus Wislizenii (Wats.) Sarg. Man. Trees N. A. 165. 1905. Becom- 

 ing a large tree, wit!} gray and furrowed bark: leaves subreniform or deltoid, 

 the triangular acumination entire, usually puberulent especially upon the 

 margin, rather deeply sinuate-dentate: staminate aments with slender ped- 

 icels and laciniate bracts, 12 cm. long; the pistillate as long: stigmas 3, di- 

 lated and irregularly lobed: capsule ovoid, the short pedicel abruptly dilated 

 into the broad disk-like receptacle. — Stream banks; southern Colorado to 

 Texas and west to California. 



4. Populus balsamifera L. Sp. PI. 1034. 1753. Becoming a tall tree with 

 nearly smooth bark, with round branches and very resinous-viscid balsamic 

 buds: leaves glabrous, dark green above, pale beneath, ovate, acute or acu- 

 minate, rounded or subcordate at base, crenulate: aments and bracts some- 

 what pubescent: stamens 18-30: capsule 2-valved. BALSAM.-^From Wyom- 

 ing far northward and eastward. 



5. Populus angustifolia James, Long's Exp. 1: 497. 1823. Becoming a 

 medium sized tree, 10-20 m. high, rarely 1 m. in diameter; bark thick, rough, 

 and furrowed: leaves mostly narrowly lanceolate or oblong, sometimes ovate- 

 lanceolate, rounded at base, acute at apex, finely crenulate the whole length; 

 petiole semiterete or furrowed: staminate aments oblong-cyhndric; the bracts 

 obovate, laciniate: stamens 12-20, with large reddish anthers: stigma with 

 broad lobes. — ^The prevailing Cottonwood on the banks of mountain streams 

 at middle elevations; known as the Narrow-leaf Cottonwood. 



6. Populus acuminata Rydb. Bull. Torr. Bot. Club 20: 50. 1893. A slender 

 tree with much smoother and hghter-colored bark than the preceding: leaves 

 rhomboid-ovate, long-acuminate, the base rounded or cuneate, crenulate- 

 dentate, long-petioled and somewhat drooping: pistillate aments slender: the 

 capsules ovoid, pediceled. A clean-looking, well-shaped, and rapid-growing 

 tree now largely grown for shade and ornament; known as the Smooth-bark 

 Cottonwood. Probably short-lived. — Indigenous on some of the streams on 

 the eastern slope of the Rocky Mountains. 



2. SALIX L.* Wiuxjw 



Leaves mostly narrow, short-petioled. Buds covered hy a, single scale. 

 Aments (catkins) mostly erect, appearing before (precocious) or with (coeta- 

 neous) the leaves; bracts entire or merely denticulate. Stamens few, accom- 

 panied bj^ 1 or 2 httle glands. Pistillate flowers with a gland at the base of the 

 ovary; stigmas short. 



Key to the Sections 



CAPSULES GLABROUS. 

 Scales pale yellow, deciduous. 



Stamens 3-5, filaments hairy below; leaves lanceolate, finely 

 serrulate; trees. 

 Petioles and leaf-bases not glandular .... 1. AMYGDALDTAE. 



Petioles and leaf-bases glandular II. PENTANDRAE. 



* This treatment of the genus Salix has been contributed by Mr, Carleton R, BalL of tae 

 U. S, Department of Agriculture, Washington, D. C, 



