EARLY MIGRATION OF THE PRAIRIE FLORA 71 



have had the same effect, 40 while severe winter-killing among young 

 plants may have resulted from deficient snow cover. 41 ' 42 . Certainly the 

 fewest boreal relic colonies are now found in southern Wisconsin and 

 southeastern Minnesota, 43 where such atmospheric conditions are still 

 more or less in effect.* Geographically the advance of the prairies was 

 favored by the slow withdrawal of the eastern ice northward and the 

 rapid retreat of the western ice northeastward, thereby opening to 

 invasion first Illinois and then Indiana. A slight change in the nature 

 of the ice retreat might have affected the future vegetational develop- 

 ment of the region very greatly. 



The distance to which the prairie vegetation migrated northward is 

 not definitely known. Very likely it reached in central Michigan as 

 far as the jack pine plains, which still contain numerous prairie species, 

 and if this is true must have reached similar or even higher latitudes 

 in Wisconsin. There is no present reason to believe that prairies were 

 developed on the north shore of Lake Erie. Deciduous forests in 

 Minnesota now occupy a narrow strip between the prairie and the 

 coniferous forests, and it is probable that their entrance was about 

 equally at the expense of the two earlier types of vegetation. If this 

 is true, the prairies did not extend much beyond their present range 

 in that state. 



The eastern migration of the prairie proceeded as a wedge-shaped 

 extension between the coniferous vegetation at the north and the 

 deciduous forests at the south and reached limits considerably beyond 

 the eastern margin of modern continuous prairies. Numerous relic 

 colonies formerly occurred, before they were destroyed by agriculture, 

 in eastern Indiana, northwestern Ohio, and southern Michigan 44 (see 

 also Cooper's description of an oak opening in his novel of the same 

 name). Vegetation closely simulating typical prairie exists along the 

 Scioto River near Columbus, Ohio. Bonser 45 has described a marshy 

 tract near Sandusky, Ohio, which still contains some western species, 

 notably Vernonia fasciculata. This so-called prairie has doubtless 



* Contrast maps in references 37 and 42. 



40 Shimek, B. The prairies. Bull. Lab. Xat. Hist. Univ. Iowa 62 : 169-240. 1911. 



4i Gates, F. C. The relation of snow cover to winter killing in Chamaedaphne 

 calyculata. Torreya 12: 257-262. 1912. 



42 Gates, F. C. Winter as a factor in the xerophily of certain evergreen ericads 

 Bot. Gaz. 57: 445-489. 1914. 



43 Livingston, B. E. A study of the relation between summer evaporation inten- 

 sity and the centers of plant distribution in the United States. Plant World 14: 

 205-222. 1911. 



44Gleason, H. A. A prairie near Ann Arbor, Michigan. Ehodora 19: 163-165. 



1917. 



45 Bonser, Thomas A. Ecological study of Big Spring Prairie, Wyandot County, 

 Ohio. Ohio Acad. Stei. Special Paper 7. 1903. 



