of the Distribution of Arctic Plants. 339 



coverable between the isothermal lines (whether annual or monthly) 

 and the amount of vegetation, beyond the general fact that the 

 scantiness of the Siberian flora is associated with a great southern 

 bend in Asia, and its richness in Lapland, with an equally great 

 northern bend there, of the annual isotherm of 32^. Yet" the 

 same isotherm bends northwards in passing from Eastern Ame- 

 rica to Greenland, the vegetation of which is the scantier of the 

 two ; and it passes to the northward of Iceland, which is much 

 poorer in species than those parts of Lapland to the southward of 

 which it passes." A glance at the supposed former state of things 

 would suggest the explanation of all that is anomalous here.' 



" The June isothermals, as indicating the most eflfective tempe- 

 ratures in the arctic regions (when all vegetation is torpid for nine 

 months, and excessively stimulated during the three others) might 

 have been expected to indicate better the positions of the most 

 luxuriant vegetation. But neither is this the case ; for the June 

 isothermal of 41°, which lies within the arctic zone in Asia, where 

 the vegetation is scanty in the extreme, descends to lat. 54° in the 

 meridian of Behring's Straits, where the flora is comparatively 

 luxuriant." The aridity of the former, and the humidity of the 

 latter district here ofi"ers an obvious explanation ; also the great 

 severity of the winter in the former, and its mildness in the latter. 

 And Great Britain, in which a far greater diversity of species are 

 capable of surviving without protection than in the Eastern United 

 States under the same annual isotherms, indicates the advantage 

 of a anean over an extreme climate in this respect, if only there 

 be a certain amount of summer heat. For lack of that, doubtless, 

 very many of the introduced denizens of Britain would soon dis- 

 appear, if deprived of human care.' 



" The northern limit to which vegetation extends varies in every 

 longitude ; the extreme is still unknown ; it may, indeed, reach 

 to the pole itself. Phrenogamic plants, however, are probably 

 nowhere found far north of lat. 81^. Seventy flowering plants 

 are found in Spitzbergen ; and Sabine and Ross collected 9 on 

 Walden Island, towards its northern extreme, but none on Ross's 

 Islet, 15 miles further to the north." 



" Saxifraga oppositi folia is probably the most ubiquitous, and 

 may be considered the commonest and most arctic flowering plant." 

 There are only eight or nine phienogamous species peculiar to the 

 arctic zone, and only one peculiar genus, viz. : the grass, Pleura- 



