Butler : The western Aivierican birches 435 



14. Betula papyrifera Marsh. Arbust. Am. 19. 1785 



B. papyracea Ait. Hort. Kew. 3: 337. 1789. 



B, alba var. or subsp. of Regel and others. 



A forest tree 15 to 25 m. high with slender, graceful branches 

 and smooth chalky white bark with large conspicuous brown or 

 yellow lenticels, easily separated into thin paper-like layers, peel- 

 ing naturally upon the tree into narrow horizontal plates, the ex- 

 posed red or orange-brown inner bark making a striking contrast 

 to the white, the bark near the base of the tree thicker and rough, 

 nearly black and irregularly furrowed; branchlets more or less 

 pubescent, densely so on vigorous young shoots in the western 

 forms, but not usually resinous-dotted, greenish or light brown at 

 first with paler lenticels, becoming dark shining brown, sparingly 

 hairy or glabrous ; leaf-blades 4-8 cm. long, 2-5 cm. wide (or more 

 oi\ vigorous yowng shoots), narrowly ovate to oval, with acute or 

 acuminate apex, and rounded or cuneate, sometimes truncate, 

 often oblique base, finely or coarsely serrate, sometimes slightly 

 lobed, hairy when young, becoming glabrous, dull bronze-green, 

 mostly free from resin-dots ; fruiting aments 2—5 cm. long on 

 slender resinous stalks ; bractlets about 5 mm. long, middle lobe 

 narrow, elongated, rounded, lateral lobes more or less ciliated, 

 obliquely rhombic, acute or obtuse, claw short, cuneate j wings of 

 samara broader than nutlet. [Figure 14.] 



The leaves of the eastern forms of this species are usually 

 densely resin-dotted, the bractlets are mostly glabrous and the 

 bark peels very readily into thin papery layers. The western form 

 usually differs in these respects, the bark often remaining close and 

 firm, peeling only with difficulty and then into small scale-like plates. 

 Nelson describes one of the western forms from Colorado (Bot. 

 Gaz. 43: 279. 1907) and proposes the name Betula Andrewsi 

 should it prove distinct from those of the East I do not think 

 there is sufficient ground for separating the two. I would not in- 

 clude, however, all the western forms heretofore known as B. 

 papyrifera^ at least three distinct species having been included 

 under this name, B, papyrifera proper, B, occidentalism and B, stib- 

 cordata. The last two are more common in the Rocky Mountain 

 region, B. papyrifera being rare and, so far as known, not found 

 west of the Bitter Root Mountains. It ranges from western Mon- 

 tana and Alberta to Colorado and eastward across the continent. 



