Stigmas 2, 2-lobed, their lobes filiform; leaf stalks elongated, laterally compressed; buds slightly resinous. Leaves finely serrate; winter-buds glabrous. 1. P. tremuloides (A, B, F, G). Leaves coarsely serrate; winter-buds tomentose or pubescent. 2. P. grandidentata. Stigmas 2—4, 2-lobed and dilated, their lobes variously divided; buds resinous. Leaf-stalks round. Leaves tomentose below early in the season, broadly ovate, acute or rounded at apex. 3. P. heterophylla (A, C). Leaves glabrous or pilose below. Leaves dark green above, pale, rarely pilose below. Ovary and capsule glabrous. 4. P. tacamahacca (A, B, F). Ovary and capsule tomentose or pubescent. 5. P. trichocarpa (B, F). Leaves light green on both surfaces, glabrous. Leaves lanceolate to ovate-lanceolate. 6. P. angustifolia (F). Leaves rhombic-lanceolate to ovate. 7. P. acuminata (F). Leaf-stalks laterally compressed. Leaves without glands at apex of the petiole, coarsely serrate, thick. Pedicels shorter than the fruit. Disk cup-shaped. Branchlets stout; capsule ⅓′—½′ long. 8. P. Fremontii (G, H). Branchlets slender; capsule not more than ¼′ long. 9. P. arizonica (F, H). Disk minute. Branchlets glabrous; leaves broad-ovate to deltoid, long-pointed and acuminate at apex. 10. P. texana (C). Branchlets pubescent; leaves broad-ovate, abruptly short-pointed or acute at apex. 11. P. McDougallii (G, H). Pedicels 2 or 3 times longer than the fruit; leaves broadly deltoid, abruptly short-pointed. 12. P. Wislizenii (E, F). Leaves furnished with glands at apex of the petiole. Branchlets stout; leaves thick. Winter-buds puberulous; leaves coarsely serrate; branchlets light yellow. 13. P. Sargentii (F). Winter-buds glabrous; leaves less coarsely serrate; branchlets gray or reddish brown. 14. P. balsamifera (A, C). Branchlets slender; leaves thin, ovate, cuneate or rounded at base, finely serrate. 15. P. Palmeri (E).