and Old World Birches. 169 



vegetation and are rapidly spread by means of their thin- 

 winged or hairy fruit. In fact, this relation of the Alaskan 

 willows to those of the Old World is brought out by Mr. F. V. 

 Coville in his scholarly monograph on the Willows of Alaska,* 

 in which he recognizes 23 species, 2 of them circumpolar in dis- 

 tribution, and 9 others occurring in Siberia. That is, of the 

 23 willows recognized by Mr. Coville in Alaska, only 12, or 

 scarcely more than one-half, are endemic North American 

 species. A similar detailed study of northeastern Scdices will, 

 it seems to the writer, likewise show a closer relationship be- 

 tween certain other American and Old World forms than has 

 been generally recognized. 



The examination of the birches by the writer has led to the 

 uniting of some well-known American and Old World trees 

 and shrubs. The conclusions reached in his studies, and cer- 

 tain details of the studies themselves, may best be presented 

 by discussing separately the species which have been specially 

 examined. 



§ Allbse. Trees or shrubs: the wing two or three times as 

 wide as the achene ; rarely only a little broader. 



Betula alba. 



The original Betula alba of Linnseusf seems to have em- 

 braced both the common white birches of northern and central 

 Europe. In 1788, however. Roth distinguished the two trees : 

 " Betula alba. B. foliis ovato-acuminatis inciso-serratis scabris, 

 ramis erectis strictis " ; and '^ Betnls, pendulo . B. foliis ovato- 

 acuminatis inciso-serratis glabris, ramis flaccidis pendulis.":}; 

 This separation of the two trees by Both has been very gen- 

 erally ignored by recent European authors, nevertheless, and 

 the species with pubescent (" scabrous ") leaves and upright 

 branches, the true Betula alba as interpreted by Both, is uni- 

 versally known abroad as B. j}icbesGe?is, Ehrh.,§ while the 

 smooth-leaved species with pendulous branches, the Betida 

 jpendnla., Both., is ordinariW known by its later name, B. ver- 

 riccosa^ Ehrh. 



By some authors, as Brantl|| and Guerke,!" these two white 

 birches are treated as distinct species, while by others the 

 entire group is regarded as a polymorphous species with many 

 subspecies and varieties. Behder, in Bailey's Cyclopedia of 

 American Horticulture, disposes of the forms in this way, 



* Proc. Wash. Acad. Sci., iii, 297-362 (1901). f Sp.-, ii, 982 (1753). 



X Roth, Fl. Germ., i, 404, 405 (1788). § Beitr., v, 160, vi, 98 (1790-91). 



I Prantl in Engler & Prantl, Nat. Pflanzenf., iii, pt. 1, 45. 



IT PI. Eur., ii, 47, 48. 



