KEY TO THE SPECIES 15 



* Petiole flattened laterally; leaves broadly ovate or deltoid. 



1. Populus treiuuloides Michx. (Aspen. American Aspen). A small tree, 

 6-20 m. high, usually in dense groves and rarely attaiuiug the maximum size ; 

 bark smooth, green isb.- white ; leaves small, roundish heart-shaped, with a short 

 sharp point and small regular teeth, smooth on both sides, with downy mar- 

 gins ; petioles slender, flattened laterally, causing movement of the leaves in the 

 lightest breeze; lobes of the bracts linear, silky; stamens 6-20; capsule conic. 

 Very common on moist slopes and valleys in the bills and mountains. 



2. Populus deltoid.es Marsh. (Cottonwood. Necklace Poplar). The largest 

 of the cottonwoods, 1 m. or more in diameter and 20-30 m. in height, with thick 

 rough furrowed bark ; branches angular ; leaves large, broadly deltoid, sometimes 

 heart-shaped, abruptly acuminate ; scales lacerate ; capsule ovate. On stream 

 banks. (See Plant Relations^ p. 70, Fig. 61.) 



* * Petioles sub-terete ; leaves lanceolate. 



3. Populus angustifolia James (Narrow-leavbd Cottonwood). Becom- 

 ing a medium-sized tree, with rough furrowed bark, 10-20 m. high, rarely 1 m. in 

 diameter ; leaves mostly narrowly lanceolate, sometimes broader with rounded 

 base, acute or obtusish at apex, finely crenulate the whole length ; petioles not 

 flattened laterally; branches terete ; capsule ovoid. Usually forming groves on 

 bottom lands. 



4. Populus acuminata Rydb. (Rydberg's Cottonwood). A tall tree with 

 smooth light-colored bark, taller and slenderer than the preceding ; branches 

 terete ; leaves bright green, smooth, lanceolate or broader, long acuminate, 

 rounded at base, crenulate, petioles slender; capsule ovoid, obtuse. From the 

 Black Hills south_through the Rocky Mountains. Largely planted as a shade-tree. 



XII. BETULACEiE (Birch Family) 



Trees or shrubs with alternate simple straight-veined leaves, 

 staminate flowers in elongated aments, fertile flowers in long 

 or short aments, 1-5 sepals or none, 2-8 stamens, and a 2-celled 

 ovary becoming a nut (with or without wings). 



1. Betula. Stamens 2 ; fruiting bracts 3-lobed, fruit winged. 



2. Alnus. Stamens 4 ; fruiting bracts woody, fruit wingless. 



3. Corylus. Stamens 8 ; nut large, bony, enclosed by a ieafy involucre. 



1. BETULA (Birch) 



Trees or shrubs ; bark usually separable in sheets, dotted on the branchlets ; 

 staminate aments long, bearing membranous "flowers with 2 stamens ; pistillate 

 aments short, 2-3 flowers in the axil of each bract, the bracts deciduous with the 

 fruit ; nut small, scale-like, broadly winged. (See Plant Relations, p. "1, Fig. 62.) 



♦ A small tree or tree-like shrub. 



1. Betula fontinalis Sargent (Rocky MotHfTAiN Birch). Ranging in size 

 from a tree-like shrub to a tree 12 m. or more high, usually slender and freely 

 branched ; bark smooth, dark ; branches gracefully drooping ; leaves thin, broadly 

 ovate, with small gland-tipped teeth ; smooth above, lightly pubescent beneath ; 

 wings of the nutlet as broad as the body. On the banks of streams. 



