1912] GLEASON—PRAIRIE GROVE 45 
Every plant listed as living in the sloughs is by preference a 
prairie species. Throughout the series not one typical plant of 
the forest has been seen. Such common and characteristic plants 
of floodplain swamps and oxbows as Hibiscus militaris, Cephalan- 
thus occidentalis, and Ambrosia trifida are entirely absent. The 
oldest settlers say that there never were either white or yellow 
water lilies. On the contrary, they state that the margins of the 
sloughs were occupied by “‘slough grass” (Spartina Michauxiana) 
tall enough to hide a man on horseback. So it is obvious that these 
were prairie sloughs rather than forest swamps, and that the vege- 
tation must have been entirely distinct from and independent of the 
forest vegetation of the ridges. A reconstruction of the whole 
grove would present a series of prairie sloughs, with grassy vegeta- 
tion, alternating with the series of forested ridges. An interpreta- 
tion of the phytogeographical significance of this condition will 
now follow. 
The forest evidently indicates three stages in a successional series, 
beginning with the oaks and hickories at the south, passing through 
the bur oak stage at the center, and ending with the red oak stage 
near the north end. This succession is the usual one for central 
Illinois, and is caused, at least in part, by the gradual accumulation 
of humus and decrease in light. There are many other places in 
the state where the same series may be observed under different 
ecological circumstances. It is especially typical of the succession 
of forests on uplands along a stream, and is met with in traversing 
such a forest at right angles to the course of the stream. The 
presence of a few trees of basswood at the extreme north end may 
be construed to indicate the approaching development of the 
hard maple-basswood type of forest, the highest type found in 
central Illinois. Along stream courses this normally follows the 
red oak stage, and is located accordingly nearer the stream. The 
chief difference between the forests of a river system and Bur Oak 
Grove lies not in their structure, but in the fact that the former are 
connected with a general forest system extending down the river 
to an indefinite distance, while in the latter the grove is entirely 
isolated from other bodies of forest. The origin of the forests of a 
river system can be explained by the gradual and continuous 
