BIRCH FAMILY 203 



Trees, often very large, never shrub-like; bark ashy gray or brown, rough- 

 ened: leaves "ovate, sometimes lobed, 4-7 cm. long, finely serrate, dull 

 bronzt'-fircen. acute at the apex, truncate at the base: twigs but slightly 

 glandular-i-fsiniferous. 8. B. montancnsis. 



Twigs and bninclilcts more or less pubescent, not or sparingly glandular-resini- 

 ferous: shrubs or slu"ub-like trees. 

 Leaves oval or rhomliic-ovate. 



Leaves narrowly oval or rhombic-ovate, 6 cm. long or more, sharply serrate 

 or dentate: ape.x acute, base cuneate: twigs finely pubescent, not glandu- 

 lar: middle lobe of the bractlets triangular, obtuse; lateral lobes rounded, 

 obliquely rhombic: fruiting aments 2-4 cm. long. 9. B. Sandbcrgii. 

 Leaves broadly oval, less than 4 cm. long, acute at both ends, serrate or 

 serrate-crenate ; twigs pubescent and sparingly glandular; middle lobe 

 of the bractlets rounded, as broad as or broader than the ascending 

 lateral ones; fruiting aments less than 2 cm. long. 10. B. Elrodiana. 

 Leaves obovate, rounded at the apex, cuneate at the base; twigs puberulent 

 and with scattered coarse hairs, sparingly glandular. 11. B. obocata. 

 Samara-wing narrower than the body; shrubs mostly low. 

 Twigs and branchlets pubescent, slightly glandular. 



Leaves finely crenate or crenate-serrate, oval, acute at both ends; twigs at first 



puberulent, at length glabrate. 12. B. crenata. 



Leaves coarsely serrate, obovate, acute or obtuse at the apex, cuneate at the 

 base: twigs with long scattered coarse hairs. 13. B. glandulifera. 



Twigs and branchlets glabrous, densely glandular-resiniferous ; leaves obovate or 

 orbicular, crenate-serrate. 14. B. glandulosa. 



1. B. subcordata Rydb. A small tree; twigs reddish brown; leaf-blades 

 3-10 cm. long, pubescent when young, in age glabrate and dull bronze-green 

 above, paler and pubescent beneath, irregularly toothed, with broad triangular 

 teeth; pistillate aments short-peduncled on short lateral branches, 2-4 cm. long 

 and 1 cm. thick in fruit; body of the fruit elliptic, the wing of about the same 

 width. White Birch. Mountain woods: Alta. — Mont. — Ida. — B.C. Mo"<.— 

 Submont, Ap. 



2. B. papyrifera Mar(jh. A tree up to 25 m. high; leaf -blades 3-11 cm. 

 long, irregularly or doubly* serrate, glabrous and dark green above, pubescent 

 beneath; staminate aments 5-10 cm. long, 2 or 3 at the ends of the branches; 

 pistillate aments peduncled, solitary on small lateral leafy branches, 2-4 cm. 

 long. B. Andrewsii A. Nels. Paper Birch. Cold woods: Lab. — N.J. — Colo. 

 — Alaska. Submont. — Subalp. Ap-My. 



3. B. occidentalis Hook. A tree sometimes 30-40 m. high; twigs brown, 

 at first pubescent or puberulent; leaf -blades acute at the apex, rounded or some- 

 times subcordate or acute at the base, more or less glandular-resiniferous, 

 pubescent along the veins and in their axils; staminate aments 7-10 cm. long, 

 usually several together; pistillate aments 1-4, on lateral leafy branches, in 

 fruit 3-4 cm. long; terminal lobe of the bractlets lanceolate, longer than the sub- 

 rhomboid, ascending lateral ones. Banks of streams and lakes: B.C. — w Mont. 

 — Wash. Submont. My. 



4. B. alaskana Sarg. A tree 10-12 m., rarely 25 m. high; twigs red-brown, 

 verrucose with conspicuous resinous glands; leaf -blades from cuneate to cordate 

 at the base, acuminate at the apex, entire at the base, glabrous and glandular- 

 resiniferous; staminate aments clustered, 2-4 cm. long; pistillate aments in fruit 

 2-3 cm. long, 8-10 mm. thick; terminal lobe of the bractlets lanceolate, shghtly 

 longer than the rounded-obovate ascending lateral ones. River banks and 

 woods: Alaska — Sask. — Mack. Boreal. My-Je. 



5. B. Piperi Britton. A tree up to 15 m. high; leaf-blades broadly ovate, 

 thin, sharply and irregularly serrate, acute or short-acuminate, rounded or acute 

 at the entire base; staminate aments 6-8 cm. long; pistillate aments cylindric, 

 in fruit 3-5 cm. long; terminal lobe of the bractlets triangular-lanceolate. River 

 banks: Wash. — Mont. — Ida. — Ore. Submont. My-Je. 



6. B, utahensis Britton. A tree 4-6 m. high; leaf-blades ovate to nearly 

 orbicular, sharply toothed, with broad teeth, glabrate in age, thin, acute, 3-5 

 cm. long, acute or truncate at the base, dull green; staminate aments about 5 

 cm. long; pistillate ones in fruit 3-4 cm. long; terminal lobe of the bractlets 

 triangular-lanceolate. Canons and woods: Utah — Wyo. — Mont. Submont. — 

 Mont. Ap-Je. 



