28 OUTLINE OF PLANT GEOGRAPHY 



graphical conditions vary much in different regions. There are 

 vast expanses of barren lowlands that have apparently not been 

 subject to glaciation, especially in northern Siberia and parts 

 of northern Alaska. Most of arctic Eurasia and America, how- 

 ever, was covered by the great glaciers of the Ice Age, and in 

 these regions the lowlands are covered with lakes, swamps and 

 tundras — regions of frozen subsoils covered with a thick mat of 

 mosses and lichens among which grasses, and low prostrate 

 shrubs, like cranberries, rhododendrons, dwarf willows and 

 birches may grow. In Greenland, Iceland, Scandinavia and 

 Spitzbergen there are lofty mountains ice-covered to their base, 

 making it impossible for any plants to grow, except on the mo- 

 raines of the glaciers, or slopes which are free of snow for a brief 

 period in summer. On such exposed places, there may be an 

 unexpected luxuriance of vegetation, grasses and a number of 

 beautiful flowers — poppies, saxifrages, buttercups and others 

 which start into growth as soon as the snow melts. 



This northernmost zone of vegetation has a growing period of 

 only two months, or a little more. Growth begins in June, cul- 

 minates in July, and by the end of August freezing weather has 

 again set in. 



Of course only a limited number of species can survive the 

 excessive cold of these regions. Vegetation consists entirely of 

 perennials, either herbaceous species with perennial underground 

 roots or root-stocks which quickly send up their leaves and flowers 

 during the brief period of sunshine and warmth; or else of low 

 prostrate shrubs, sometimes evergreen, like the cranberries or 

 Lapland rhododendrons, sometimes deciduous, as the dwarf wil- 

 lows and birches. 



The degree of cold which some of these can endure is astonish- 

 ing, since in some cases (as in northern Siberia), they are quite 

 unprotected by snow, and must survive temperatures as low as 

 -60°C. (-76°F.). 



The coldest regions in which there has been found a well-marked 

 flora are in northern Siberia and Grinnell Land, northwest of Green- 

 land. 1 The mean annual temperature of these regions is— 16°C. 

 (4°F.). Even in midsummer the temperatures are very low. 

 In Franz Josef Land, the July mean is only 2°C., and in Spitz- 



1 Drudc, Handbuch der Pflanzengeographie, pp. 352, 353. 



