PLANT MIGRATIONS ACROSS THE NORTH ATLANTIC OCEAN 1 83 



2. There is evidence of over-representation of types adapted to long- 

 distance dispersal within western Low Arctic elements penetrating to 

 Greenland and Iceland. 



From this it can be concluded either that the adaptations recognized as 

 favoring long-distance dispersal are of no biological significance, or that the 

 problem of Amphi-Atlantic plant distribution is not a matter of long- 

 distance dispersal. 



The first conclusion is difficult to accept. As to the anemochorous species 

 the reader is referred to the work by Wilhelm Schmidt and others (cf. Geiger, 

 1961, p. 50 etc.) regarding probable travelling distances of seeds and spores as 

 a function of turbulence and settling velocity in air. These considerations are 

 borne out by finds of numerous far-travelled spores and pollen. Also, the 

 re-colonization of the flora of the island of Krakatoa shows that species 

 apparently adapted to long-distance dispersal had a considerable advantage 

 in colonizing the island after its flora was destroyed by a volcanic eruption in 

 1883 (cf. Dahl, 1959). Thus, it is concluded that, whatever is the explanation of 

 the Amphi-Atlantic distribution pattern, it is not a matter of long-distance 

 dispersal. 



DID THE AMPHI-ATLANTIC PLANTS MIGRATE ACROSS THE 



BERING SEA? 



Only two hypotheses now remain to be considered in order to explain the 

 distribution of the Amphi-Atlantic plants. One is the hypothesis of a former 

 migration across the Bering Sea, and the other concerns migration across a 

 land connection between Europe and America. 



The first hypothesis involves very extensive plant dispersals with the 

 result that populations on both sides of the Atlantic Ocean must have been 

 isolated genetically for a long time. Especially in polymorphic groups of 

 species, and in taxa of lower rank than species, one would expect to find a 

 diff'erentiation of the populations on both sides of the North Atlantic Ocean. 

 On the contrary, if plants had dispersed directly across the North Atlantic 

 Ocean one would expect to find closely related taxa on both sides of the 

 Ocean differing from the populations farther east in Asia, in the Pacific area, 

 and in western America. 



The Eastern Amphi-Atlantic element in North America can be sub-divided 

 into the following groups, mainly based upon information from Hulten (1958): 



1. Amphi-Atlantic taxa of lower rank than species with 

 vicariants in the Pacific area (Alaska and easternmost 



Asia) 13 taxa 



2. Amphi-Atlantic taxa of lower rank than species with 

 vicariant taxa elsewhere in Europe, Asia, or America but 



absent in the Pacific area 8 taxa 



