7 
370 I-tinois NATURAL History SurvEY BULLETIN 
this study; for their help I am truly grate- 
ful. 
Also, I wish to acknowledge here the 
assistance given by Mr. W. H. Phegley 
and Mr. J. J. Steibel of Prairie du 
Rocher, both of whom are now deceased. 
Some of the vegetation studies for this in- 
vestigation were accomplished through 
their kind co-operation. I wish to thank 
Dr. G. W. White, Department of Geol- 
ogy, University of Illinois, and Dr. H. 
B. Willman and Dr. G. E. Ekblaw, IIli- 
nois State Geological Survey, for infor- 
mation concerning the geology of hill 
prairies; Mr. B. M. Woods, University 
Library Map Room, for his efforts to lo- 
cate suitable aerial photographs; Mr. 
Julian Neill, East St. Louis, for assist- 
ance in the study of hill prairies in that 
region; Dr. J. L. Forsberg, Dr. P. F. 
Hoftman, Mr. J. W. Curfman, and Mr. 
R. E. Teegardin, while members of the 
Natural History Survey staff, for assist- 
ance with the statistics, the hydrogen-ion 
determinations, and the maps, diagrams, 
and drawings; and Mr. J. S. Ayars, also 
a Survey staff member, for his assistance 
with editorial problems. 
Others I wish to thank for their con- 
tributions to this investigation are Dr. M. 
W. Sanderson, Dr. H. H. Ross, Dr. L. 
J. Stannard, Illinois Natural History 
Survey; Mr. and Mrs. F. W. Evers, 
Quincy; Mr. Raymond Hatcher, Mur- 
physboro; Dr. J. W. Hall, University of 
Minnesota; Mr. and Mrs. B. C. Trees, 
East St. Louis; Frederick Evers, Clara 
V. Evers, and Marilyn Briggs Ellerman, 
Champaign; and Miss Virginia Frank, 
Chicago. 
Mr. Dewey Clark, Quincy, took the 
pictures for the frontispiece and fig. 16. 
Mr. W. E. Clark, Natural History Sur- 
vey staff, took the photographs used as 
figs. 3, 17, 18, 22, and 24. All other 
photographs were taken by the author. 
The plant nomenclature used in this 
study is for the most part that of Hitch- 
cock (1950) for grasses and Fernald 
(1950) for other plants. Where the no- 
menclature in this report does not con- 
form to that in the manual of Hitchcock 
or that of Fernald, the manual name is 
included in brackets in the section on the 
flora of hill prairies. In instances in which 
Vol. 26, Art. 5— 
a name of long standing has been changed © 
in recent manuals, the former name, in- 
brackets, also appears. Common names not 
from the above manuals are from Deam 
(1940) or Jones (1950). 
q 
| 
ENVIRONMENT | 
The occurrence and distribution of 
plants, and consequently plant communi-— 
ties, is determined largely by climate and 
other environmental conditions, including 
the soil or substratum in which they grow. — 
Some of the environmental conditions fa- 
voring the existence of hill prairies in Ili- | 
nois are discussed in the following sec~ 
tions. 
The climate in practically any part off 
Illinois permits growth of either prairie 
or forest. The circumstances (aside from 
those of accident and of history) which 
tip the balance and thus determine the 
details of local distribution of prairie and 
forest are chiefly the controls exerted by 
topography. 
Climate 
The following information on climatic 
conditions applies to the Mississippi River 
valley along the western border of Illi- 
nois, the region of the majority of hill 
prairies observed for this report. With 
few exceptions, the climatic data, taken 
from Page (1949), are from weather sta- 
tions located along the Mississippi River. 
Exceptions are the data from the Mount 
Carroll, Carbondale, Anna, and Green- 
ville weather stations; these stations are, 
respectively, about 9, 11, 12, and 33 miles 
from the nearest hill prairies. 
Temperature.—The average January 
temperature along the western border of 
Illinois varies from 19.6 degrees Fahren- 
heit in northwestern Illinois (Dubuque, 
Iowa, weather station) to 36.2 in south- 
ern Illinois at Cairo. The average July 
temperature is 74.6 degrees in northwest- 
ern, 80.2 in west-central (Quincy weather 
station), and 79.8 in southwestern and 
southern Illinois (St. Louis, Missouri, 
and Carbondale, Illinois, weather sta- 
tions). In Cairo, at the southernmost 
weather station in Illinois, the average 
July temperature is 79.5 degrees. The re- 
