176 EILIF DAHL 



taxa belong here and Fig. 1 gives the number of taxa in this element in the 

 different parts of Europe. The highest number is found in north and central 

 Scandinavia, a fair number also in Scotland, whereas the number decreases 

 eastwards and southwards, in the south they are mainly confined to the 

 mountains. 



Within the western Amphi-Atlantic element sub-elements with more res- 

 tricted distribution can be discerned. Four taxa grow only in Spitsbergen east 

 of the Atlantic Ocean. Another sub-element grows in Scandinavia, a few 

 species of it also in Novaya Zemlya and Scotland, but it does not reach Ural 

 or the Alps. This element has been called "West Arctic" by Blytt (1876) and 

 34 taxa belong to it. Finally there are five species restricted to the British 

 Isles east of the Atlantic Ocean. 



Corresponding to the western Amphi-Atlantic element in Europe, there is 

 an eastern Amphi-Atlantic element in America. Its species are native in 

 Europe and also in eastern America, but not farther west than the Great 

 Plains and a fine east of the Rocky Mountains from the Great Plains to the 

 mouth of the Mackenzie River. Figure 2 gives the number of members 

 belonging to this element in America. It will be seen that the highest number 

 is found in south Greenland with a fair number also in Newfoundland, and 

 that the number drops westwards and southwards. The number is also rela- 

 tively low farther north, but this is not pronounced if the element is taken as a 

 percentage of the native flora. 



Within the eastern Amphi-Atlantic element in America sub-elements with 

 more restricted distribution can be recognized. Eight taxa are confined to 

 northeast Greenland on the west side of the Atlantic Ocean and an additional 

 30 taxa do not reach farther west than Greenland. Twelve taxa have a very 

 restricted distribution along the Atlantic seaboard. The three sub-elements 

 mentioned form an American counterpart of the West Arctic element in 

 Europe (see Lindroth, 1957, p. 237). Around the Gulf of St. Lawrence and in 

 Newfoundland a European element of five taxa form a counterpart to the 

 American element in the British Isles. 



From the data given it will be evident that the Amphi-Atlantic element 

 essentially is Arctic-Sub-Arctic in character and very few plants from the 

 temperate regions exhibit an Amphi-Atlantic distribution pattern. 



F:5 THE LONG-DISTANCE DISPERSAL HYPOTHESIS 



Different hypotheses may be invoked to explain the present-day distribu- 

 tion patterns of the Amphi-Atlantic plants. One main hypothesis suggests that 

 the Amphi-Atlantic species once migrated across the North Atlantic Ocean, 

 either by long-distance dispersal or across a former land connection between 

 America and Europe. The other main hypothesis suggests that they once 

 migrated across the Bering Strait between Asia and America and subsequently 



