72 H. A. GLEASON VEGETATIONAL HISTOKY OF MIDDLE WEST 



preserved its hydrophytic environment since an early period, being 

 watered by springs, and consequently contains a considerable number 

 of boreal species, notably Betula pumila. Opuntia humifusa, occurs 

 near the same place. ,Species of distinctly western affinities do not 

 occur on Presque Isle, near Erie, Pennsylvania, 46 and the abundant 

 development of boreal species and the typically Alleghenian aspect 

 of the flora in northeastern Ohio make it reasonably certain that 

 extensive prairies were not developed east of Cleveland. 



The unglaciated areas of southern Ohio, Indiana, and Illinois, and 

 the Ozark region of southern Missouri are now populated with a forest 

 flora that presents indications of great antiquity, and in their greater 

 topographical relief offer better opportunities for forest species to 

 endure unfavorable atmospheric conditions. It seems reasonably cer- 

 tain that the prairies did not encroach on this area at any time. Prob- 

 ably the southern boundary of the glaciated area marked the division 

 between forest and prairie, and probably forest belts followed even 

 then the courses of the rivers northward well into the prairie region. 

 The forest belts of the Mississippi may have been derived from the 

 southern species of the Mississippi embayment; at the present time 

 several of them follow the course of this river northward, such as 

 Gleditsia aquatica and Hicoria Pecan. No evidence is now at hand 

 concerning the possible extent of the prairie into the Ozark region 

 of southwestern Missouri and northwestern Arkansas, What rela- 

 tion the xerothermic period may have had to the barrens of Tennessee, 

 the prairie belt of central Alabama, the prairies of eastern Arkansas, 

 and the coastal prairies of Louisiana must be left to the deductions 

 of observers who have had personal experience with the vegetation of 

 these areas. 



In further support of the idea of a xerothermic period, it need only 

 be recalled that the discontinuous distribution of such plants as Opuntia, 

 fragilis and 0. humifusaj Sporobolus heterolepis, Cristatella Jamesii, 

 and Callirrhoe triangulata can be adequately explained only by assum- 

 ing a former period of climate sufficiently drier than the present to 

 permit their continuous migration. This, with the existence of relic 

 prairie colonies at the east and the prevailingly xerophytic nature of 

 the relic boreal colonies at the west, seems to be sufficient evidence 

 of the actuality of a post-Wisconsin xerothermic period. 



46 Jennings, Otto E. A botanical survey of Presque Isle, Erie County, Pennsyl- 

 vania. Annals Carnegie Mus. 5: 289-421. 1909. 



