527 
Cowles (4, 5) has shown that the plant associations and their suc- 
cessions in a given region are correlated with its physiographic de- 
velopment. In order therefore to determine the original prairie asso- 
ciations the only feasible point of attack lies in a detailed study of the 
relic associations still to be found in the various physiographic habitats 
of the state. The conditions in these relic associations where least dis- 
- turbed by man approximate closely the conditions which existed in the 
much more extensive association of only a little more than half a cen- 
tury ago. The order of succession of the associations is without doubt 
the same today as in the past, and the composition of the undisturbed 
associations fairly representative. Some of the swamp prairies and 
many of the sand prairies of the state have already been studied on this 
basis by Cowles (5), Sherff (23), Gleason (9, 10), Gates (6), and 
Vestal (27, 28). The present investigation was also carried out on 
this basis with special reference to primitive conditions and an attempt 
to include practically all the prairie habitats of the state. The main 
contribution -of this report is the attempt to determine and classify the 
associations of prairie vegetation in Illinois and point out the succes- 
sions between them. The field work was entirely observational, but 
experimental data of other workers have been drawn upon wherever 
they appear to have a direct bearing upon the topic under discussion. 
Quantitative experimental data on the physiological factors underlying 
the problems of the prairie associations and their successions are too 
meager at present for a full discussion of the underlying causes. A more 
definite knowledge of both the ecological anatomy and physiology of 
many of the plants concerned will have to be obtained before some of 
the more fundamental questions can be answered. 
Most of the investigations were made during the summers of 1916 
and 1916 with the cooperation of the Illinois State Laboratory of 
Natural History, through its director, Professor Stephen A. Forbes, 
and under the direction of Dr. H. C. Cowles and Dr. George D. Fuller, 
of the University of Chicago. This report also contains many data ob- 
tained previous to the above dates and from observations made in many 
sections of the state during the summers of 1917 and 1918. I am par- 
ticularly indebted to Professor Forbes for his cooperation in the survey 
and for the aid he has given in the illustration of the report. I wish 
also to express many obligations to Dr. Edgar N. Transeau, of the Ohio 
State University, who first introduced me to the problems of the prairie 
in the summer of 1910, and helped me with valuable data and criticisms 
during the preparation of this report. To Dr. A. E. Waller, of the Ohio 
State University, I am indebted for criticisms and suggestions on the 
final report. 
LocaTIon oF ExIsTING VIRGIN PRAIRIES OF THE STATE 
The location of most of the prairies studied was obtained by means 
of circular letters of inquiry sent to each of the county surveyors of the 
