70 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



occurs not far from the Mississippi and in one county in southeastern 

 Iowa. The occurrence of boreal relics in the driftless area of north- 

 eastern Iowa has already been noted. 



Ponds were abundant on the Wisconsin drift in Illinois half a 

 century ago, and were notable for the complete lack of such hardy 

 boreal plants as Comarum palustre, Dulichium arundinaceum, and 

 Menyanthes trifoliata. They were mostly shallow and easily drained 

 and have long since disappeared. They were also characterized by 

 the absence of deep peat deposits, indicating that they had never been 

 occupied by boreal vegetation and were of comparatively recent origin. 

 The carefully prepared reports of the Illinois soil survey present 

 interesting figures concerning them. Ten counties of central Illinois 

 contain only 1,986 acres of deep peat, or 0.04% of their area, and 

 of this 1,779 acres lie in the floodplain of the Illinois River. Three 

 counties of north-central and northern Illinois contain 0.15% of deep 

 peat, one county in the Kankakee valley 0.83%, while three counties 

 of northeastern Illinois contain 4.67%. Even in southern Michigan, 

 far within the boundary of the Wisconsin deposits, only the deeper 

 lakes are bordered by tamarack bogs. This lack of hydrophytic boreal 

 relics toward the southwestern angle of the Wisconsin glaciation is 

 best explained by the assumption of the xerothermic period, as already 

 noted, during which hydrophytic habitats were obliterated toward the 

 west, except in the deepest depressions or in local areas affected by 

 subsurface water, 39 thereby restricting the relic colonies chiefly to the 

 xerophytic types. 



Early Migration of the Prairie Flora. — An advance of the 

 prairie vegetation toward the east and northeast followed immediately 

 behind the coniferous forests, displacing the rearguard of the forest 

 by successional processes. There is now no place in the Middle West 

 where grassland is succeeding forest, and it becomes difficult to picture 

 the detailed steps by which such succession proceeded. In bogs, as 

 the climate grew warmer, and the ingress of water was reduced with 

 increasing distance from melting ice, the gradual drying may have 

 inhibited the growth of seedlings and permitted the entrance of prairie 

 species, This process is now seen in modified form in southern Michi- 

 gan, where partial drainage of tamarack bogs leads to the displace- 

 ment of the usual bog shrubs by Dasiphora fruticosa. On uplands, 

 the exposure of the marginal trees to warm and dry winds during the 

 summer, with consequent injury through excessive transpiration, may 



39 Gates, F. C. A bog in central Illinois. Torreya 11: 205-211. 1911. 



