442 
determine the occurrence of prairie on the 
upper slope of a bluff. The west- to 
southwest-facing position of the slope, 
which exposes the slope directly to the hot 
rays of the afternoon sun and to prevail- 
ing southwest summer winds, and the alti- 
tude of the bluff and width of the adja- 
cent bottomlands help to provide the ex- 
tremely xeric conditions under which this 
type of prairie thrives. Rapid and exces- 
sive drainage, due both to slope and to 
permeable loess substratum, is a reinforc- 
ing condition. 
5. The vegetation of Illinois hill prai- 
ries is the bunch-grass type, with 4 ndro- 
pogon scoparius the dominant species in 
most stands. Bouteloua curtipendula is 
usually present, in a few prairies dom- 
inant. In a few hill prairies, Sorghastrum 
nutans and a few other bunch grasses oc- 
cur frequently and may dominate small 
areas within the prairie, or, rarely, the 
entire stand. 
6. For detailed studies of the vegeta- 
tion, plots were staked in unpastured and 
in pastured variants of the same prairie 
slope. Data were obtained from plot sizes 
ranging from 1/256 milacre to 9 milacres. 
The smallest size found to be effective 
was 1/64 milacre. The largest area staked 
in each prairie was 75 milacres. One mil- 
acre in each prairie—pastured and unpas- 
tured—was mapped. All plant individ- 
uals were charted and counted in each of 
these milacres. Species lists were compiled 
for plots of the several sizes. 
7. In the mapped milacre of the un- 
pastured prairie, the estimated number of 
plants was 1,949; 1,404 of these were 4n- 
dropogon scoparius. In the mapped mil- 
acre of the pastured prairie, the estimated 
number of plants was 1,341; 849 of these 
were 4. scoparius. 
8. The ground space occupied by 
plants in the mapped milacre of the un- 
pastured prairie was 1,884.1 square 
inches, of which 1,781.0 square inches 
were occupied by Andropogon scoparius. 
In the mapped milacre of the pastured 
prairie the ground space occupied by 
plants was found to be 1,403.3 square 
inches, of which 1,107.0 square inches 
were occupied by 4. scoparius. In the un- 
pastured prairie, at ground surface, 69.96 
per cent of the mapped milacre was bare 
ItLrnois NaTuRAL History SurveEY BULLETIN 
Vol. 26, Art. 5 
loess; in the pastured prairie, 77.63 per 
cent. 
9. In the mapped milacre of the un- 
pastured prairie, foliage covered 4,988 
square inches, approximately 80 per cent 
of the milacre. In the mapped milacre of 
the pastured prairie, foliage covered 2,525 
square inches, about 40 per cent of the 
milacre. In both pastured and unpastured 
prairie, Andropogon scoparius was the 
species with the largest foliage area. In 
the unpastured milacre, approximately 20 
per cent of the ground was not covered by 
foliage; in the pastured milacre, about 60 
per cent. 
10. The available space per plant in 
the unpastured milacre was found to be 
3.22 square inches; in the pastured mil- 
acre it was 4.68 square inches. 
11. For the study of frequency of oc- 
currence of species in plots within hill 
prairie, a good distribution of species was 
obtained with quadrat sizes of 1/64 to 
1/4 milacre in unpastured prairie; 1/16 
to 1/4 milacre in pastured prairie. 
12. From _ species-area curves, the 
smallest representative area—the smallest 
one-piece area having some claim to be 
representative—was determined for the 
unpastured prairie as 0.76 milacre, and, 
for the pastured, 1.26 milacres. The min- 
imum area for assignment to type—an 
area that is large enough to include all the 
important and a moderate number of mi- 
nor species—was determined as 3.80 mil- 
acres for the unpastured and 6.30 mil- 
acres for the pastured prairie. The fair- 
sized stand—an area that is 50 times as 
large as the smallest representative area 
and contains twice as many species—was 
38.0 milacres for the unpastured and 63.0 
milacres for the pastured hill prairie. 
13. The flora of 36 hill prairies was 
used as a basis for presence studies. The 
“constants” (species in 29, 80 per cent, or 
more of the 36 locations) of this type of 
prairie in Illinois were Andropogon sco- 
parius, Erigeron strigosus, Bouteloua cur- 
tipendula, Petalostemum purpureum, Eu- 
phorbia corollata, Penstemon pallidus, A. 
gerardi, Verbena stricta, and Kuhnia eu- 
patorioides. Twenty-five species were 
found in 18 or more of the 36 prairies 
used in this study of presence. Species 
most characteristic of the hill prairie type 
