[Vol. 2 



78 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



to Norway, he supposed that the six different flora-elements 

 had immigrated from the south through Sweden to the places 

 in which they are now found, but during the subsequent 

 change of climate had died out in the intermediate regions, in 

 which they do not grow now. 



Since then, Gunnar Andersson (*96, '06) has discussed this 

 question with special reference to Sweden. He builds more 

 particularly upon paleontological studies of the plants pre- 

 served in peat-bogs. He assumes that the climate, after the 

 melting of the ice, continued to grow warmer until — since 

 Corylus Avellana, according to fossil occurrences, had a far 



more northerly distribution area than at the present time- 

 it showed a mean temperature in August that was about 2.5 

 C. higher than at the present time. The temperature has, 

 therefore, fallen to that of the present day. 



Gunnar Andersson designates the various periods after the 

 Glacial Epoch according to the most characteristic plant, and 

 assumes that the immigration has taken place in the follow- 

 ing order: 



(1) The Dryas Flora includes certain arctic species, e.g., 

 Dryas octopetala, Salix herbacea, S. polaris, 8. reticulata, 

 Oxyria digyna, Arctostaphylos alpina, and others, which are 

 supposed to have migrated into Sweden when the melting of 

 the ice had begun, and followed this northward. The most 

 northerly place, however, where these arctic plants are found 

 in Sweden is in West Gothland, in about the latitude of 

 Gothenburg. They have not been found, from this period, 

 farther north. 



2) The Betula odorata Flora is more subalpine. With 

 it came also Salix aurita, S. caprea, and 8. cinerea, etc. 



(3) The Pinus sylvestris Flora immigrated during a some- 

 what warmer period, which continued to grow warmer. In 

 the lower, and thus older, part of the Pine Zone are found 

 Prunus Padus, Rubus idaeus, Rhamnus Frangula, Sorbus 

 Aucuparia, and Viburnum Opulus; in the upper, and therefore 

 more recent, part, which has had a warmer climate, we find 

 Alnus glutinosa, Cornus sanguined, Crataegus monogyna, 



