BIRCH FAMILY " 203 



Trees, often very large, never shrub-like; bark c'kshy pray or brown, rough- 

 ened; leaves ovate, sometimes lobed, 4-7 cm. long, flnely serrate, dull 

 bronze-green, acute at the apex, truncate at the base; twigs but slightly 

 glandular-resiniferous. 8. B. montanensis. 



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

 ferous; slirubs or slirub-like trees. 

 Leaves oval or rhombic-ovate. 



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

 or dentate; apex acute, base cuneate; twigs finely pubescent, not glandu- 

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

 obliquely rhombic; fruiting aments 2-4 cm. long. 9. D. 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, cmieate at the base; twigs puberulent 

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

 Samara- Ming 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 flrst 



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



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 cnn. 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 w^oods: Alta. — Mont. — Ida. — B.C. Mo^d. — 

 Submont, Ap. 



2. B. papyrifera March. 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 brorni, 

 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, m 

 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 cyhndric, 

 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 m age, thm, 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. 



