170 Fernald — delation shijys of some American 



recognizing Betula alba, with the two subspecies, B. pendula, 

 Eoth {B. verrucosa, Ehrh.), and B.puljescens, Ehrh., each with 

 nnmerons marked variations. Mr. Eehder, however, kee^^s 

 separate from the Old World B. alia the American B. jmpy- 

 7'ifera £mdB. occidentalis. 



The most conservative treatment of the group was, in some 

 particulars, that of Eegel in DeCandolle's Prodromus, though 

 many of his varieties have proved of little value. There Kegel 

 recognized two species of white birches (his section Alhce), 

 Betula alha, L., and B. microj)hylla, Bunge. B. alha, how- 

 ever, he divided into nine subspecies : 



verrucosa, with numerous varieties, from Europe, Asia 



aud America ; 

 populifolia, a well-known strictly American tree ; 

 raandshurica, a local Asian tree ; 

 latifolia, with varieties, from central and northern Asia to 



Japan and Kamtschatka ; 

 occidentalis, with varieties, from North America ; 

 papyrifera, with varieties, from North America and 



Siberia ; 

 pubescens, with varieties, from Europe, Asia and North 



America ; 

 tortuosa, from noithern Europe and Siberia ; 

 excelsa, a doubtful form referred to by earlier authors. 



Thus it is clear that Eegel, the most devoted student of the 

 birches, found it impossible to distinguish as clear species the 

 diverse trees and shrubs which pass as white or canoe birches. 

 The recently accumulated material shows that Kegel's course 

 was perhaps the most philosophic of any which has been pro- 

 posed. Yet the primary characters of the trees, first clearly 

 distinguished by Roth, are generally so constant that it seems 

 better to the writer to recognize as species, i. e., as centers of 

 variation, the two forms designated by him. These were the 

 true Betula alba, with mostly stiff and ascending branches, 

 the young brauchlets puberulent or pubescent and the ovate 

 often doubly serrate leaves more or less pubescent beneath, at 

 least when young ; and B. pendula, with more fiexuous 

 branches, the branchlets glabi'ous or verrucose with resiniferous 

 atoms, aud the deltoid or rhombic-ovate often simply serrate 

 leaves glabrous, and more or less glutinous at least when young. 

 These two trees in their characteristic forms are now very well 

 understood abroad, though some of their reduced forms present 

 perplexing problems. But since the days of Eegel the Amer- 

 ican trees have been very generally regarded as endemic species, 

 and it is to the discussion of this question that special atten- 

 tion is here directed. 



