Transition Prairie-Forest District. 519 
Physalis heterophylla Nees. Sisyrinchium angustifolium Mill. 
» virginiana Mill. Solidago nemoralis Ait. 
Polygala verticillata L. » rigida L. 
Psoralea argophylla Pursh. » rigidiuscula T. & G. 
» esculenta Pursh. » rupestris Raf. 
> tenuiflora Pursh. Sporobolus asper Michx. 
Rosa arkansana Porter. » heterolepis A. Gray. 
Ruellia ciliosa Pursh. Stipa spartea Trin. 
Salix humilis Marsh. Verbena stricta Vent. 
Senecio plattensis Nutt. Vernonia gigantea Walt. 
»  integerrimus Nutt. » fasciculata Michx. 
Silphium laciniatum L. 
The vegetation center of the prairies is found in Nebraska, Iowa, 
Kansas and the Dakotas'). From this center, the typic plains flora slowly 
shades out toward both south and north. The prairie region naturally sepa- 
rates itself into several districts; Transition Prairie Forest District; Prairie 
District; Sand Hill District; Foothill District. The prairies and sandhills being 
in close conjunction and only edaphic modifications are to en. united in the 
same paragraph (B.). 
A. Transition Prairie-Forest District. 
The passage from the heavily wooded region of the north and east into 
the treeless plains of the west is a gradual one and the disappearance of the 
underwood and the predominance of oak openings, or groves of oak and other 
forest trces, not crowded together, but scattered over the surface at a con- 
siderable distance apart without any low shrubs or underbrush is the charac- 
teristic appearance of the prairies in Illinois, eastern Iowa, parts of Wis- 
consin and Minnesota and in fact along the eastern border of the prairies 
where they abut upon the eastern deciduous forcsts’”). 
ı. The forests of the Transition District. 
Throughout the central part of Illinois, the prairie represents the most 
primitive plant formation which has been invaded by a forest along the drain- 
age lines where the succession of plant associations is hastened by the change 
in soil and topography due to stream action. As there are no water courses 
ı) POUND, ROSCOE and CLEMENTS, FREDERIC E.: The Vegetation Regions of the prairie Province. 
Botanical Gazette XXV: 384. June 1898; and the important papers by Hırcacock, A. S., Eco- 
logical plant Geography of Kansas; Transactions of the Academy of Science of St. Louis (1898) 
VII: 55—69 and GreAson, H. A., Some unsolved Problems of the Kan, Bulletin Torrey 
Botanical Club XXXVI: 265— 
2) PAmMEL, L. H.: A a rare Study of the Vegetation of swamp, clay and sandstone 
Areas in western Wisconsin, southeastern Minnesota, northeastern, central and southeastern Iow wa. 
Proceedings Davenport Academy of Science X: 32—126. 1905. 
