414 BOTANICAL GAZETTE [DECEMBER 



existing small lake areas of the northern states were covered 

 by the ice during the maximum extension of the Wisconsin ice 

 sheet. As there is no reason to believe that the drift sheets of 

 the preceding epochs, which in many places extend beyond the 

 Wisconsin terminal moraine, contained such small undrained 

 depressions, it follows that the bog societies must have occupied 

 other habitats. 



POSTGLACIAL NORTHWARD MIGRATION. 



With the renewal of a milder climate and the consequent 

 recession of the glaciers, the plant societies would gradually 

 spread in the direction of continuous habitats and generally 

 northward. The bog and tundra types would be the first to 

 push into the barren ground left by the retreating ice. The 

 area over which they spread in early postglacial times must 

 have been very much more extensive even than that now 

 occupied by them. In the smaller glacial depressions where 

 absence of wave action would favor littoral vegetation, the bog 

 plants would become firmly established. On the western and 

 eastern sides of the glaciated area the tundra would be closely 

 followed by the conifer forests. 



In the west the spreading of the conifers to the north was 

 followed by their gradual destruction in the southwest, due to 

 the increase in temperature as compared with the rainfall. It is 

 possible that the rainfall in Nebraska was never any greater 

 than at the present time. But the decrease in transpiration 

 accompanying decrease in temperature might account for the 

 presence there during glacial times of trees which cannot live 

 under present conditions. The bog plants perished with the 

 conifers and their southwestern boundaries today correspond 

 with that of the forest. 



In the east, among the highlands, exceptional circumstances 

 were afforded for the preservation of these northern forms. 

 Many relicts still crowd the higher elevations as far south as 

 western North Carolina. 



But in the northern Ohio valley, with its scant conifer vegeta- 

 tion, the areas which at that time supported the bog societies 



