422 Butler : The western American birches 



The following key recognizes seventeen species. About these 

 types as characterized maybe grouped all the western forms known 

 to me at the present time. 



Key to tlie "western American l>irclies 



Bark separable into layers or sheets, the very thin outer layer peeling into small shreds; 



trees. 

 Bark chalky white or silvery gray. 



Leaves broadly ovate, apex acute, base mostly subcordate, 



teeth spreading; twigs glabrous or slightly puberulent ; 



bark silvery gray, often dark ; middle lobe of bractlet 



acute, slightly longer than the rounded lateral ones, 15- -^- suhcordata. 



Leaves narrowly ovate,^apex long-acute or acuminate, base 



rounded or cuneate, teeth directed toward apex ; twigs • 



pubescent; bark chalky white; middle lobe of bractlet 

 narrow, rounded, about twice as long as the acute or obtuse 



lateral ones. 

 Bark yellowish or reddish-brown, often very dtirk. 



14. B, papyrifera. 



Twigs very glandular; middle lobe of bractlet triangular, 



acute, much longer than the obliquely ovate or rhom- 

 bic, spreading, lateral ones. 

 Leaves ovate, very thin, dark green, apex acute, finely 

 and Irregularly doubly serrate with slender long- 

 pointed teeth. 16. B. occidenialis, 

 Leaves deltoid or ovate, thick, bronze-green, apex long- 



acuminate, coarsely serrate. 13. B. alaskana. 



Twigs not or but slightly glandular, middle lobe of bractlet 



rounded, equaling or but slightly longer than the 

 rounded ascending lateral ones. 

 Leaves broadly ovate, apex acute, base cuneate. lo. B, kenaica. 



Bark not separable into layers, outer bark not peeling into shreds; 



trees and shrubs. 



Twigs and branchlets glandubr-resiniferous, not hairy. 

 Samara wings broader than the nutlet. 



Lateral lobes of bractlets spreading, obliquely ovate, 



auricled at the base ; trees. 

 Leaves very thin ; fruiting ament narrowly cylin- 

 drical, elongated, less than I cm. thick ; lateral 

 lobes of bractlet obtuse or rounded, claw much 

 shorter than middle lobe. 12. B. Piperi, 



Leaves thick ; fruiting ament broadly cylindrical, 

 stout, I cm. or more thick ; lateral lobes of bract- 

 let mostly acute, claw as long as middle lobe. 



II. B, utahensis 

 Lateral lobes of bractlets ascending, obliquely rhombic, ' 



not auricled at the base ; shrubs or trees. 

 Shrubs or small shrub-like trees ; bark shining, 

 dark red-brown, smooth ; leaves broadly ovate, 



IVY 



