2 American Midland Naturalist Monograph No. 2 



have been adapted (by permission) from a similar key prepared several 

 years ago by Mr. P. C. Standley, and which appeared in volume 21 of the 

 Contributions of the United States National Herbarium. For convenience, 

 the keys to the families have been divided into sections. Trees and shrubs 

 may be usually keyed out largely on vegetative characters — a considerable 

 advantage in identifying those plants when flowering specimens are not avail- 

 able. The keys to species include, in addition to diagnostic characters, a 

 statement of habitat, time of flowering, suitable common name, and relevant 

 synonymy, the intention having been to correlate the valid name of the plant 

 with other names that may be found in the older manuals. The known distribu- 

 tion in Illinois of the indigenous species was first plotted on a series of outline 

 maps, the data in all instances having been compiled from duly visaed speci- 

 mens, but on account of the necessity for extreme brevity it has been 

 possible to include only the briefest summary of the geographical ranges of 

 the species in Illinois. An attempt has been made to indicate frequency of 

 occurrence by use of the terms common, local, infrequent, etc. For rare plants, 

 specimens are often cited by collector and number or date, although exact 

 localities, even when these are known, are for obvious reasons not mentioned. 

 All species of vascular plants known to the writer to grow spontaneously in 

 Illinois have been included. Further study, however, will probably reveal the 

 presence of additional species. With few exceptions no species has been 

 admitted unless authentic specimens from Illinois have been examined. For 

 the sake of uniformity and convenience the sequence of families is chiefly that 

 of Engler & Diels. 



Although it is now almost a century and a half since the first botanical 

 explorers visited Illinois, our knowledge of the botany of this region is far from 

 complete. In a few areas fairly adequate botanical collections have been made, 

 but more than half of the 102 counties of Illinois, according to the records at 

 present, are almost wholly unexplored botanically. The distribution of the 

 "lower plants" of Illinois is comparatively unknown, and we lack even a check- 

 list of the algae, fungi, and bryophytes. Obviously, much work remains to be 

 done on the botany of Illinois. 



Thanks are due Professor A. G. Vestal of the University of Illinois for 

 aid in writing the account of the vegetational divisions; to Florence Freenian 

 Jones, for help in preparing manuscript; to Professor Alfred Rehder, and Mr. 

 E. J. Palmer, of the Arnold Arboretum of Harvard University, for identifica- 

 tion of certain ligneous plants, and to Dr. Leon Croizat for bibliographical 

 aid. The assistance of these persons is gratefully acknowledged. 



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 



