BETULACEAE. 77 



prominent nerves, serrate, acuminate, very sparsely hairy on both sides, 

 scarcely glandular, 4—7 cm. long, on slender petioles 2-3 cm. long; pistillate 

 aments cylindrical, 3 cm. long, 1 cm. thick, usually drooping on slender 

 peduncles; bracts pubescent, the central lobes longest. Blue Mountains 

 and throughout northern Idaho, but not abundant. 



Betula microphylla Bunge {B. fontinalis Sargent). Small tree, S-8 m. 

 high, the several trunks rarely erect, the bark dark bronze; branchlets drooping, 

 grayish, warty; leaves broadly ovate, obtuse or acute, coarsely serrate-dentate, 

 glandular and sparsely hairy on each surface, 2-4 cm. long, on slender petioles 

 half their length; staminate aments about 2 cm. long; pistillate aments solitary, 

 cylindric, 2-3 cm. long, drooping with the branchlets, sessile or on short pe- 

 duncles; fruiting bracts pubescent and ciliolate, the central lobe largest and 

 narrowest; wings as broad as the obovate nutlet. Common along streams. 



Betula piperi Britt. Graceful tree, 8-15 m. tall, with drooping branches; 

 bark dark bronze, even the branchlets scarcely gray, these latter very glan- 

 dular; leaves ovate, obtuse or acute at the base, coarsely doubly serrate, 

 acuminate, shining green above, glandular on both surfaces, 2-4 cm. long, on 

 petioles half as long; pistillate aments cylindrical, 5 cm. long, 5 mm. thick, 

 often flexuous, mostly in twos, dark green; staminate aments narrow, flexuous, 

 4-8 cm. long. Springy hillsides near Almota. 



106. ALNUS. Alder. 



Trees or shrubs; leaves dentate or serrulate; buds few-scaled; 

 both kinds of flowers in aments, expanding before, with or after 

 the leaves; the staminate pendulous; the pistillate erect, clus- 

 tered; staminate flowers 3, sometimes 6, in the axil of each bract, 

 consisting of a mostly 4-parted perianth and 4 stamens, and sub- 

 tended by 1 or 2 bractlets; filaments short, simple; anther-sacs 

 adnate; pistillate flowers 2 or 3 in the axil of each bract, without 

 a perianth, but subtended by 2-4 minute bractlets; ovary sessile, 

 2-celled; styles 2; bracts woody, persistent, 5-toothed or erose; 

 nut small, compressed, wingless or winged. 



Leaves simply denticulate, not at all lobed. A. rhombifolia. 



Leaves doubly dentate or with short lobes at the ends of the 

 main veins. 



Peduncles slender, exceeding the cones; leaves shining. A. sinuata. 



Peduncles stout, shorter than the cones; leaves dull. A. tenuifolia. 



Alnus rhombifolia Nutt. Tree 10-15 m. tall, with white bark which on 

 older trees becomes broken into rectangular blocks; leaves ovate, elliptic or 

 obovate, mostly cuneate at the base and obtuse at the apex, glandular-den- 

 ticulate, finely puberulent on each surface, somewhat glandular beneath, 5-8 

 cm. long; petioles 1 cm. or less long; fruiting aments oblong-ovate, 1.5-3 cm. 

 long, on short stout peduncles; nutlets broadly obovate, the margins thickened. 

 Along streams in warm valleys. 



Alnus sinuata (Regel) Rydb. Small tree or shrub, 3-S m. tall, erect or 

 ascending; bark rather dark, except in old stems; twigs chestnut brown; buds 

 smooth, gummy; leaves ovate, acuminate, obtuse or cuneate at the base, 

 bright green, doubly dentate, glabrous above, nearly so beneath, thin, very 

 gummy when young, 5-8 cm. long; petioles slender, 1-2 cm. long; fruiting 

 aments 1-1.5 cm. long, on slender peduncles as long or longer. Along streams 

 in the mountains. 



