DISPERSAL AND SURVIVAL OF PLANTS 201 



form of distribution is able to convey seeds from, for example, Greenland to 

 Spitsbergen, and from there on to Franz Joseph's Land or vice versa cannot 

 be definitely established, but the possibility should not be totally excluded. 



It must be remembered, however, that the sea-ice provides a far from smooth 

 surface over which seeds can slide as easily as over lake-ice. Sea-ice is very 

 rugged, ;^full_ of relief, ridges, etc. (Sverdrup, 1957), that will easily trap the 

 seeds. It is, furthermore, a rough surface even on a minute scale and would 

 act as[an abrasive on the material being pushed over it. Thus, probably only 

 seeds with heavy coats will be able to withstand this treatment for a sustained 

 period of time. 



Not being able to estimate the number of plants dispersed by blowing over 

 the polar ice, we can calculate that at most 10 per cent of the species in our 

 area are able to spread over long distances by air. 



Considering the three media for long-distance dispersal of plants and seeds, 

 it is evident that only a fraction of the present day distribution of plants in the 

 North Atlantic area can be explained as a result of Post-glacial wind-, water-, 

 or animal-dispersal over the present distances. Those plants, which cannot have 

 made use of any of the above-mentioned transportation media, or which have 

 not been brought around by man during very recent times, must therefore 

 have come to the Atlantic islands over land-connections, or, over a system of 

 landmasses at considerably shorter distances from each other than at present. 

 This author does not doubt that such conditions did exist in some form or 

 another at some time or another, but probably so early that it has been neces- 

 sary for the main part of the present, native flora to survive all or at least part 

 of the Pleistocene Ice Age in the area. 



That this survival has resulted in the loss of a great deal of a previous 

 flora is beyond question; it is more surprising that so much has managed to 

 survive. 



In our area it can be said that the Ice Age still reigns in certain parts, as in 

 Greenland, Spitsbergen, and Franz Joseph's Land. Some authors doubt that 

 the climate in the Arctic was as severe as at present during the time when there 

 were continental ice sheets in Eurasia and North America, and it has been 

 designated as an area for possible plant refugia during the Ice Age by, for 

 example, Fernald (1925), Hulten (1937), Marie-Victorin (1938), Ewing and 

 Donn (1956), to mention only a few. Voices to the contrary have of course 

 also been heard (Flint, 1947; Savile, 1961). 



When an area is studied the average temperature is often used as an 

 indicator to the rigor of its climate, but the annual average is a poor figure so 

 long as the amplitude of the temperature variations is not given. If the plants 

 have any frost tolerance at all, it does not seem to matter so much how deep 

 the temperature dips. Three species of cacti easily survive winter temperatures 

 down to —45°C in Manitoba, Canada, Summer temperature, duration of 

 vegetative season, and precipitation are factors that largely determine 



