Jones: Flora of Illinois 3 



versity of Illinois; to Professor R. H. Mohlenbrock of Southern Illinois 

 University for initial assistance in organizing the manuscript, for the 

 key to Cyperus, and for data concerning some plants in southern Illi- 

 nois; to my colleague Professor Dale M. Smith for advice concerning 

 Asplenium and Hclianthus: to Mr. E. H. Daubs on Lemnaceae; and 

 finally to my wife for help in proofreading and preparation of 

 specimens. 



FLORA AND VEGETATION 



Description of the Area 



Illinois is part of the Great Central Plain of North America, and is 

 situated between 37° and 42° N. lat., and 87° and 91° W. long. It is 

 bounded on the north by Wisconsin, on the east by Indiana, on the 

 west by Iowa and Missouri, and on the south by Kentucky. The 

 maximum length is 380 miles, and the width more than 200 miles. 

 Its area is approximately 57,926 square miles. Physiographically, most 

 of this state except the southern portion lies in the Till Plains Section 

 of the Central Lowland Province. Biogeographically, almost all of 

 Illinois lies in the Austral Zone. The great majority of native species 

 of plants are therefore of southern affinities, and the boreal element is 

 extremely small. The Austroriparian Province enters the state only at 

 its extreme southern end. The average elevation above sea level is 

 about 600 feet. The highest point is 1241 feet altitude at Charles 

 Mound in Jo Daviess County along the Wisconsin- Illinois boundary. 

 Although most of the area has a low elevation and comparatively level 

 surface there is a good drainage system with more than 275 streams, 

 which may be grouped in two river systems, one having the Mississippi 

 River, and the other the Wabash and Ohio rivers as its outlet. The 

 soils of Illinois are remarkable for their fertility, and agriculture is one 

 of the important occupations. The better agricultural districts are 

 characterized by a black loam, and the alluvial soil of the river valleys 

 is especially fertile. On many of the river blufTs the soil is loess. Nearly 

 all the rocks of Illinois are sedimentary and belong to the Paleozoic era. 

 Igneous rocks are found only in a few places, and metamorphic rocks 

 are almost unknown. 



During the Glacial period there were four advances of the ice-sheet 

 into Illinois. The ice of the third, or Illinoian, stage co\ered approxi- 

 mately nine-tenths of the state, and extended southward to the Ozark 

 Ridge, the most southerly latitude reached by the North American ice- 

 sheet. Hence, there are only three districts in Illinois that may have 

 remained untouched by the Pleistocene glaciation. These are 1 ) the 

 seven southernmost counties of the state, 2) an area between the Mis- 

 sissippi and Illinois rivers in Calhoun County, and 3) Jo Daviess 

 County and a small portion of Carroll County. The second and third 

 districts are part of a much larger nonglaciated region known as the 

 Driftless Area, which occupies adjacent portions of Wisconsin, Min- 

 nesota, and Iowa. 



The flora and fauna of Illinois are similar to those of adjacent 



