186 SALICACEAE 



Ovary and fruit glabrous, but slightly warty, ovoid. 



9. P. hastata. 

 Fruit pedicelled, 2-carpellary, glabrous; capsule ovoid. 



Twigs, petioles, and veins glabrous. 10. P. balsamifera. 



Twigs, petioles, and veins of the lower surface of the leaves more or less 



pubescent. 11. P. candicans. 



Petioles about one-third as long as the blade or less. 



Leaves cordate at the base. 12. P. Tweedyi. 



Leaf-blades lanceolate, usually acute at the base. 13. P. angusti folia. 



Leaves white-tomentose beneath, often lobed. 14. P. alba. 



1. P. cercidiphylla Britton. A small tree, with light grayish bark; petioles 

 1.5-2 cm. long; leaf-blades orbicular or somewhat reniform, slightly cordate at 

 the base, dull green, somewhat paler beneath, entire or slightly undulate, about 

 3.5 dm. wide. Hoback Basin, Wyo. Submont. 



2. P. tremuloides Michx. A slender tree, with light green or whitish bark, 

 up to 30 m. high; petioles 4-6 cm. long; leaf -blades rounded or subcordate at 

 base, 2-6 cm. broad, green and glabrous, ciliate on the margin; bracts silky, 

 deeply 3-5-cleft into linear lobes; stamens 6-12; stigma-lobes linear; capsule 

 conic, warty. The western tree may be distinct, and if so should be known as 

 P. aurea Tidestrom. QUAKING ASPEN. Cold places, sub-alpine forests: Newf. 

 N.J. Tenn. N.M. Nev. Alaska. Submont. Subalp. Mr-My. 



3. P. Wislizeni (S. Wats.) Sarg. A tree about 15 m. high, with gray trunk 

 and yellow branches; petioles glabrous, 4-7 cm. long; leaf -blades truncate or 

 broadly reniform, or sometimes broadly cuneate at the base; aments 5-10 cm. 

 long; bracts light red, fringed, with linear lobes; capsule ovoid, papillose. COT- 

 TONWOOD. Valleys and river banks: Tex. Colo. Ariz. n Mex. Son. 

 Mont. Mr-My. " 



4. P. Fremontii S. Wats. A tree occasionally 35 m. high, with dark brown 

 trunk and gray branches; petioles 5-7 cm. long; blades broadly cordate-deltoid, 

 with an open rounded sinus at the base, 4-7 cm. long, 5-10 cm. broad; aments 

 5-10 cm. long; capsule 8-12 mm. long. COTTONWOOD. Along rivers and in 

 canons: Calif. s Utah 'Ariz. L. Calif. Son. F-Mr. 



5. P. Sargentii Dode. A tree sometimes 30 m. high, with gray trunk and 

 straw-colored or light yellow branches; petioles about as long as the blades; leaf- 

 blades broadly deltoid or cordate, abruptly acuminate at the apex, glabrous and 

 shining, 5-10 cm. long; aments 5-8 cm. long; capsules ovoid, muricate, on pedirels 

 4-6 mm. long. P. occidentalis Rydb. WESTERN or RIVER COTTONWOOD. River 

 bottoms: Sask. Kans. Ariz. Alta. Plain -Submont. Mr-Ap. 



6. P. Besseyana Dode. A tree about 10 m. high, with gray trunk and 

 yellowish terete branches; petioles about as long as the leaf -blades; blades abruptly 

 acuminate, 5-10 cm. long, glabrous and shining, slightly paler beneath; aments 

 7-8 cm. long; pedicels shorter than the fruit; capsule ovoid, about 5 mm. long, 

 muricate, 3-valved; stigma dilated. Valleys: Mont. Ida. Submont. Ap-Je. 



7. P. acuminata Rydb. A tree up to about 20 m. high, with brownish or 

 gray trunk and straw-colored branches; petioles 3-7 cm. long; leaf-blades 

 rhombic-lanceolate to ovate, usually cuneate at the base, finely crenate, green 

 and glabrous on both sides; aments 3-5 cm. long, or the pistillate in fruit 10-15 

 cm.; capsule ovoid, blunt, 6-8 mm. long, papillose; pedicels in fruit 3-5 mm. 

 long. RYDBERG'S or SMOOTH-BARKED COTTONWOOD. River banks: Sask. 

 Tex. 'Ariz. Mont. Plain Submont. Ap-My. 



P. acuminata X Sargentii Sarg. Intermediate between the parents, with the fine 

 serrations, cuneate base, and texture of the leaves of P. acuminata, but with the broader 

 leaves and darker branchlets of P. Sargentii. P. Andrewsii Sarg. Colo. 



8. P. trichocarpa T. & G. A tree sometimes 60-70 m. high, with gray 

 trunk and orange or gray, pubescent branches; petioles 2-6 cm. long; leaf -blades 

 broadly ovate to ovate-lanceolate, acute or short-acuminate at the apex, cordate, 

 truncate, or rounded at the base, 8-12 cm. long, finely crenate, dark green above, 

 pale beneath, usually pubescent when young; aments 4-7 cm. long. WESTERN 

 BALSAM POPLAR. River banks: Alaska Mont. 'Calif. Plain Submont. Ap- 

 My. 



