76 BETULACEAE. 



Populus trichocarpa T. & G. Cottonwood. Large tree, 10-30 m. tall, 

 with rough fissured bark; leaves broadly ovate, rounded at the base, acuminate, 

 5-12 cm. long, puberulent when young, whitish beneath; petioles slender, 

 terete, about equalling the blades; staminate aments dense, the bracts long- 

 hairy; stamens 50-60; pistillate aments becoming loose; capsules nearly sessile, 

 globose, puberulent or glabrate. Banks of streams. 



Populus angustifolia James. Medium sized tree, 10-20 m. high with thick 

 rough fissured bark; leaves oblong-lanceolate to lanceolate, acute, crenulate, 

 5-12 cm. long; petiole not flattened, less than one third as long as the blade; 

 staminate aments oblong-cylindric, the bracts fringed; stamens 12-20; 

 capsules ovoid, glabrous. A rare tree in Washington known as yet only from 

 Spokane, Professor Bonser. 



Family 25. BETULACEAE. Birch Family. 



Trees or shrubs; leaves alternate, petioled, simple, with de- 

 ciduous stipules; flowers small, monoecious, in linear-cylindric, 

 oblong or subglobose aments; staminate aments pendulous; 

 pistillate aments erect, spreading or drooping, spike-like or 

 capitate; staminate flowers 1-3 in the axil of each bract, with a 

 membranous 2-4-parted calyx, or calyx none, and 2-10 hypogy- 

 nous stamens ; pistillate flowers with or without a calyx adnate to 

 the solitary 1-2-celled ovary; ovules 1-2 in each cavity; fruit a 

 small compressed or ovoid-globose, mostly 1-celled and 1-seeded 

 nut or samara; endosperm none. 



Stamens 2; filaments 2-cleft, each fork bearing an anther 



sac; fruiting bracts 3-Iobed or entire. 105. Betula, 76. 



Stamens 4; anther sacs adnate; fruiting bracts woody, 



erose or 5-toothed, persistent. 106. Alnus, 77. 



IDS. BETULA. Birch. 



Trees and shrubs; leaves dentate or serrate; buds scaly; 

 flowers ail in aments, expanding before and with the leaves; the 

 pistillate aments erect or spreading; staminate flowers about 3 

 in the axil of each bract, with a membranous usually 4-toothed 

 perianth and 2 stamens, and subtended by 2 bractlets; pistillate 

 flowers 2 or 3 (rarely 1) in the axil of each bract; perianth none; 

 ovary sessile, 2-celled; nut small, a samara, shorter than the 

 bracts. 



Branchlets pubescent, not glandular warty. B. papyrifera. 

 Branchlets warty with resiniferous glands. 



Pistillate aments stout, 2-3 cm. long. B. microphylla. 



Pistillate aments slender, 5 cm. long. B. piperi. 



Betula papyrifera Marsh. Tree with a single trunk, 8-20 m. high, erect; 

 bark grayish-bronze, the erect or spreading branchlets dull gray; leaves 

 broadly ovate, attenuate-acute at base, somewhat lobed at the ends of the 



