60 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



Noteworthy among them are Taxodium distichum* Rhamnus caro- 

 liniana, Gleditsia aquatica, Nyssa, aquatica, Ilex decidua, Catalpa 

 speciosa, Fraxinus profunda, Celtis mississippiensis, Quercus Phellos, 

 Leitneria floridana, and a host of other shrubs and herbaceous species. 

 Soil conditions can not be cited as the cause of this range, because the 

 soils in which these plants grow is all alluvial, washed down by the 

 streams from the glaciated region, and parallels them without any 

 essential difference for miles within the glacial boundary. Neither 

 can climate be cited, since two of them, Taxodium distichum and 

 Catalpa speciosa, are commonly, and others occasionally, cultivated 

 200 to 300 miles north of their natural limits. 



Northern Limits of Southern Plants. — The rocky hills of the Ozark 

 uplift in southern Illinois also carry a number of southern plants 

 which reach here their northern limits, such as Pinus echinata^, Bato- 

 dendron arboreum, Ulmus alata, Azalea nudifiora, Bumeli-a hjcioides, 

 B. lanuginosa, and many others. These also have failed to utilize the 

 extensive strip of similar habitats along the rocky bluffs of the Missis- 

 sippi river for further migrations into the glaciated region. Still other 

 species have interesting ranges correlated in some way with the glacial 

 boundary. Thus Heuchera parviflora occupies isolated areas along the 

 boundary only, with the exception of its stations in the lower Allegheny 

 mountains. Trichomanes Boschianum preserves an outpost in southern 

 Illinois, where it has recently been discovered by Cowles. Sidlivantia 

 Sullivantii occurs only along the glacial boundary from Ohio to Illinois 

 and reappears in the driftless area 300 miles to the north. Saxifraga 

 Forbesii is strictly limited to two stations along the glacial boundary 

 in southern Illinois and southeastern Missouri. Phlox Stellaria is 

 chiefly confined to certain limestone cliffs along the glacial boundary 

 in Illinois and not far beyond it in Kentucky. 



Undoubtedly there are species which reach their northern limit 

 near the glacial boundary because of climatic conditions. This might 

 be true of Phoradendron flavescens, whose host trees are abundant north 

 of the boundary, but which has not crossed itself. But with numerous 

 species with coincident range margins at this line, and several species 

 with isolated stations near it, an easier explanation is that they re- 

 mained here during the glacial period and have not migrated north- 

 ward since then, indicating that the northern boundary of the arcto- 

 tertiary forests lay parallel with and close to the ice margin. 



There is at present no evidence whether this forest flora at that 

 time extended westward toward the Ozarks, leaving northern Missouri 



* A single colony aceurs in Knox County, Indiana, within the glaciated area. 



