PLANT MIGRATIONS ACROSS THE NORTH ATLANTIC OCEAN 



177 



died out in areas other than those which they now occupy. The long-distance 

 dispersal hypothesis will be considered first. 



Plants can be spread over long distances in different ways. Anemochorous 

 species are carried by wind and have very light and small seeds or spores 

 (less than 0.2 mm) or have wings or hairs attached to seeds or fruits which 



Fig. 2. Number of taxa from the eastern Amphi-Atlantic element in the floras of 



diff'erent parts of America. Further explanation in the text. (Base map Denoyer- 



Geppert, by permission). 



reduce the settling velocity in air. Zoochorous species aretransported by animals 

 and have fruits or seeds with either edible, fleshy parts or hooks which attach 

 them to animals passing by. Or, they may be spread by sea currents; it is then 

 important that the seeds or fruits float well in sea water. Unfortunately, not 

 enough data are available for classifying all the plants concerned according to 

 buoyancy of seeds and fruits. But it seems evident that halophilous plants 

 growing on the sea shores in haline environment would have a particular 

 advantage of dispersal in this way since the fruits or seeds would tend to 

 land in an environment favorable for further growth and reproduction. 



