532 
Jamesui, Lesquerella, argentea, and Opuntia fragilis on the sand dunes 
of Illinois many miles from their nearest reported station in the west. 
This brief summary of the prairie habitats of the state emphasizes 
the fact that the chief prairie region reached its greatest development on 
postglacial pre-erosion topography. The associations of prairie plants 
found in these habitats may be placed in two distinct groups: the hydrarch 
successions found on flood-plains, in morainal depressions, and on the 
old lake bed of Lake Chicago ; and the xerarch successions found on sand 
and xerophytic upland glacial soils. 
SUCCESSION OF PRAIRIE PLANT ASSOCIATIONS 
1. THE HYDRARCH SUCCESSIONS 
The Succession (a) on the River Flood-plains 
The largest and least disturbed example of this type of prairie stud- 
ied is located on the flood-plain of the Mississippi River south of Sa- 
vanna. The exact area of this prairie was not determined, but it is 
known to include several thousand acres. The associations of prairie 
plants found on this flood-plain are usually distinct and readily differen- 
tiated. The list of associations found and their successional relations 
are represented diagramatically in Figure 1. See also Plates XLVIJI— 
LIII for illustrations of the individual associations, and pages 558—568 
for a list of representative species. 
The associations in the diagram are given in order from swamp to 
prairie. On these Savanna prairies the plants representing the various 
associations usually occupy from 85 to 95 per cent. of the area of the 
association which they represent and are in every case the dominant 
plants of their respective associations. The areas of the individual 
associations vary from a few square rods to tens of acres. Where the 
slope gradient is steep they occur as narrow strips of only a few feet 
in width and some of them may be eliminated entirely. One Panicum 
virgatum association was found occupying nearly a hundred acres 
(Plate LII), and similar areas of Spartina Michauxiana are not infre- 
quent. Andropogon furcatus is the dominant grass of the drier portions 
of the flood-plain, but owing to the fact that habitats which have 
reached this stage of development are suitable for agriculture very few 
such areas remain undisturbed. ‘ 
Some of this prairie is now being grazed and is losing its natural 
aspect. Hundreds of acres, however, are disturbed only by late autumn 
mowing and occasional fires, while occasional local areas may be found 
that are seldom disturbed by man. It is in these least disturbed tracts 
where the dead grass remains from year to year that the dominant 
plants maintain their purest stand. The secondary species become insig- 
nificant, being represented only as scattered individuals here and there 
throughout the association. It is in such situations also that the transi- 
tion zones between the various associations are most clearly defined and 
readily recognized. 
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