Butler: The western American birches 423 



less than 4 cm. long except on young vigorous 

 shoots, coarsely and irregularly serrate, thin, 

 dark green, apex obtuse or acute, base mostly 

 rounded ; twigs densely glandular-resiniferous. 



9. B, fontinalis* 

 Trees, often very large, never slirub-like ; bark 

 dull ashy gray or dull brown, flaked with gray, 



roughened ; leaves ovate, somewhat lobed, 4-7 

 cm. long (or more on vigorous shoots), finely ser- 

 rate, thick, dull bronze-green, apex acute, never 

 obtuse or rounded, base mostly truncate ; twigs 

 but slightly glandular-resiniferous. 17. B^ f?iontanensis. 



Samara wings not broader than the nutlet ; shrubs. 



Leaves crenate-serrate, obovate to orbicular, very 

 resinous, less than 3 cm. long, apex rounded, base 

 mostly cuneate, never cordate. I. B, glandulosa. 



Leaves irregularly serrate, broadly ovate or oval, slightly 

 or not resinous, more than 3 cm. long, apex acute 

 with cordate base, or apex obtuse with cuneate base. 



3. B, IIorneL 

 Twigs and branchlets more or less hairy or pubescent, not or spar- 

 ingly glandular-resiniferous, often glabrous ; shrubs or 



shrub-like trees. 

 Samara wings broader than the nutlet. 

 Leaves oval or rhombic-ovate. 



L 



J 



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

 or less, sharply serrate or dentate, apex acute, 

 base cuneate ; twigs finely pubescent, not gland- 

 ular; middle lobe of bractlet triangular, obtuse, ^ 

 lateral lobes rounded, obliquely rhombic ; fruit- 



r 



ing ament 2-4 cm. long. 8. B, Sandbergi. 



Leaves broadly oval, less than 4 cm. long, acute at 

 both ends, serrate or crenate-serrate ; twigs 

 pubescent and sparingly glandular ; middle lobe 

 of bractlet rounded, as broad as or broader than 

 the ascending rounded lateral ones ; fruiting 

 ament less than 2 cm. long. 4. B, Elrodiana, 



Leaves obovaie. 



Leaves obovate, apex rounded, base cuneate ; twigs 

 puberulent, with long coarse scattered hairs, 

 sparingly glandular; middle lobe of bractlet 

 acute or obtuse, narrower than the spreading, 

 obliquely rhombic, acute lateral ones ; fruiting 

 ament 2-3 cm. long. 5. B, obovata. 



Samara wings not broader than the nutlet. 

 Leaves finely crenate or crenate-serrate. 



Leaves oval, acute at both ends, less than 3 cm. 

 long ; twigs glabrous, puberulent at first, slightly 



glandular ; fruiting ament less than 2 cm. long. 



7. B. crenafa. 



