352 VESTAL: A BLACK-SOIL PRAIRIE STATION 
Dr. H. A. Gleason, who has accompanied him to the area, for the 
use of some of his notes. The accompanying sketch map shows 
very well the local distribution of the plant associations of the 
immediate vicinity. The length of the area mapped is about 600 
feet. Just south of it is the Great Western railroad; to the east 
lies a blue-grass pasture and a picnic ground, with many of the 
original trees, but with the ground cover replaced by blue-grass; 
on the west the clover field continues; on the north the clover field, 
with its line of prairie sloughs, and the forest, continue also. 
Local distribution of the plant associations of the county line 
station would be of little significance of itself, but there is one 
condition which this area has in common with others in northern 
and central Illinois: the forest is to the east of a line of sloughs, 
which may have served to protect it from the inroads of prairie 
fires. In this region forest areas are much more frequent and more 
extensive just east of streams and sloughs than just on the western 
side. Prairie fires, in former times very prevalent, traveled gener- 
ally from west to east, in the direction of the prevailing winds. 
The bearing of these facts on vegetational history in the transition 
area between interior prairie and eastern forest regions has been 
developed by Gleason.* At the county line station, the narrow 
strip of prairie which separates the forest area mapped into a 
woodland on the north and several small groves on the south, 
may have invaded the formerly more extensive forest by the aid 
of prairie fires which were not stopped by the line of sloughs. It 
is seen on the map that the tongue of prairie extending into the 
forest is in line with the conspicuous gap in the series of oine 
sloughs. 
Established black-soil prairie of the eastern part of the prairie 
region, in its original condition, may be thought of as a luxuriant 
grassland with a large number of plant species and with very 
many local appearances, caused by local dominance or abundance 
of one or several species. The surface is usually slightly undulat- 
ing, and in the recently glaciated areas stream development is 
poor, so that depressions have very wet or submerged soil, and 
elevations may at times be very dry. _The local variation in soil 
* Gleason, H. A. An isolated prairie grove and its Sosa ahi signifi 
cance. Bot. Gaz. 53: 38-49. f. 1,2. 1912. 
