CLIMATIC ZONES 29 



bergen, 4°C. The period available for any plant growth is only 

 about two months. Nevertheless there is an unexpectedly large 

 number of species. In Spitzbergen 122 species of flowering plants 

 are recorded, and in Greenland between 78° and 83° 88 species. 

 At Cape Chelyuskin, the northermost point of continental Asia, 

 23 species of flowering plants have been noted. 



The existing arcto-glacial flora was derived, presumably, from 

 the much richer flora occupying the region to the south preceding 

 the great ice-age. The limit of vegetation at the period of maxi- 

 mum glaciation was probably far south of its present latitude, 

 but as the glaciers retreated toward the pole, the hardier species 

 moved northward to their present habitat. Such specific changes 

 as have taken place, were comparatively of recent date, and the 

 arctic flora as a whole is a recent one. 



The regions included in the arctic zone comprise the northern 

 coasts of Eurasia and America with the adjacent islands, and the 

 mountain masses immediately south. In northwest Europe, due 

 to the invasion of the sea, and its tempering effect, the south- 

 ern limits of the zone are pushed very far north, but in Russia and 

 Siberia, and North America, arctic conditions prevail in much 

 lower latitudes. 



The limits of the strictly arctic zone have been set between the 

 northernmost limit of vegetation— about 83 °N.— regions with the 

 July isotherm only 2°-4°C, and the regions to the south bounded 

 by the 10°C. July isotherm. 



The general type of vegetation throughout the arctic regions 

 is very similar and most of the species extend all round the world. 

 Where peculiar species occur in any region, they are usually closely 

 related to wide-spread ones, and may be assumed to be of relatively 

 recent origin. Not infrequently species which are apparently 

 recent immigrants from the south are met with; i. e., the arctic 

 American Kalmia glauca. 



While the southern boundary of this zone may be said to corre- 

 spond roughly with the arctic circle, it varies much in latitude, due 

 to the differences in topography in various regions. 



Thus in the great continental areas like Siberia and Canada, 

 it lies much further south than in Scandinavia, Iceland, Green- 

 land and western Alaska, where the proximity of the ocean water 

 greatly tempers the climate. 



