length; anthers ellipsoid; aments longer. 11. S. sessilifolia (B, G). Stigmas short, hardly 2 or 3 times longer than broad. Mature leaves covered below with appressed white silky hairs, those of flowering branchlets entire or barely denticulate. 12. S. exigua (B, F, G). Mature leaves glabrous below, those of flowering branchlets more or less distinctly denticulate. 13. S. longifolia (A, F). Scales of the flowers persistent, dark brown or fuscous, at least toward the apex (in _S. Bebbiana_ more or less straw-colored or tawny). Stamens 2. Ovaries glabrous. Leaves more or less denticulate or serrate; styles short. Base of leaf cuneate or rounded. Leaves acute, oblanceolate to narrowly lanceolate; filaments mostly united below. 14. S. lasiolepis (G). Leaves mostly acuminate; filaments free. Branchlets glabrous, lustrous; leaves oblanceolate to narrowly obovate, up to 2′ in length; pedicels ⅛′—⅙′ long; stipules small. 15. S. Mackenzieana (A, G). Branchlets pubescent; leaves narrowly lanceolate to ovate-lanceolate, 4′—6′ long; pedicels 1.5—2.5 mm. long. 16. S. missouriensis (A). Base of leaf mostly more or less cordate; leaves glabrous; filaments free; pedicels long. 17. S. pyrifolia (A). Leaves entire, oval to broad-obovate; branchlets villose-pubescent during their first season. 18. S. amplifolia. Ovaries pubescent (glabrous often in No. 23). Leaves covered with a soft dense felt-like tomentum, oblong-lanceolate to elliptic-lanceolate. 19. S. alaxensis (B). Leaves glabrous or more or less villose-pubescent below. Bracts of the flowers pale or tawny, often reddish at the tip; pedicels up to ⅕′ in length; leaves elliptic-lanceolate to obovate, reticulate beneath in age, pubescent or glabrate. 20. S. Bebbiana. Bracts of the flowers brown or fuscous. Stipules more or less distinctly developed; pedicels several times longer than the short styles. Leaves elliptic-lanceolate to oblong-elliptic; mostly glabrous in age. 21. S. discolor (A, B, F).