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 lightest 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. Populus 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. deltoidea, 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, with 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 Ipbed: 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-cylindric ; 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 lighter-colored bark than the preceding: leaves 

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

 dentate, long-petioled and somewhat dropping: 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.* WILLOW 



Leaves mostly narrow, short-petioled. Buds covered by 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 by 1 or 2 little 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 . . I. AMYGDALINAE. 



Petioles and leaf-bases glandular II. PENTANDRAE. 



* This treatment of the genus Salix has been contributed by Mr. Carleton R. Ball, of the 

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



