170 Fernald—felationships of some American 
recognizing Betula alba, with the two subspecies, B. pendula, 
oth (B. verrucosa, Ehrh.), and B. pubescens, Ehrh., each with 
numerous marked variations. r. Rehder, however, keeps 
separate from the Old World B. alba the American B. papy- 
rifera and B. occidentalis. 
The most conservative treatment of the group was, in some 
particulars, that of Regel in DeCandolle’s Prodromus, though 
many of his varieties have proved of little value. There Regel 
recognized two species of white birches (his section Alba), 
Betula alba, L., and B. microphylla, Bunge. B. alba, how- 
ever, he divided into nine subspecies : 
verrucosa, with numerous varieties, from Europe, Asia 
an merica ; 
populifolia, a well-known strictly American tree ; 
mandshurica, a local Asian tree ; : 
latifolia, with varieties, from central and northern Asia to 
apan and Kamtschatka ; 
occidentalis, with varieties, from North America ; 
papyrifera, with varieties, from North America and 
Siberia ; 
1 > 
pubescens, with varieties, from Europe, Asia and North 
America ; 
tortuosa, from northern Europe and Siberia ; 
excelsa, a doubtful form referred to by earlier authors. 
Thus it is clear that Regel, the most devoted student of the 
birehes, tound 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 Regel’s course 
was perhaps the most philosophic of any which has been pre- 
sed. 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 0 
variation, the two forms designated by him. These were the 
true Betula alba, with mostly stiff and ascending branches, 
the young branchlets puberulent or pubescent and the ovate 
often doubly serrate leaves more or less pubescent beneath, at 
least when young; and B. pendula, with more flexuous 
branches, the branchlets glabrous or verrucose with resiniferous 
atoms, and the deltoid or rhombic-oyate often simply serrate 
eaves glabrous, and more or less glutinous at least when young: 
These two trees in their characteristic forms are now very We 
understood abroad, though some of their reduced forms present 
perplexing problems. But since the days of Regel 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. 
