552 
Scoparius association on sand is pastured, a xerophytic pasture results 
in which the prominent species are Bouteloua hirsuta, Leptoloma cog- 
natum, Panicum pseudopubescens, Paspalum, setaceum, Eragrostis 
pectinacea, and Cyperus Schweinitzti. The presence of the blue-grass 
pastures therefore indicates a rather advanced stage in the development 
of the sand prairie, probably either a late stage of the Andropogon 
Scoparius or mixed grass association, or an Andropogon furcatus asso- 
ciation. 
Apparently this last association was rarely developed on the higher 
sand areas. Gleason estimates that on the sand prairies as a whole at 
least two thirds of the surface was originally occupied by a mixed 
bunch-grass association varying all the way from areas dominated by 
the pioneer grasses to those in which the Andropogons were abundant. 
It would be of interest to both plant ecologists and farmers if a tract 
of the present cultivated sand prairies of the Savanna region could be 
set aside and allowed to revert undisturbed to natural conditions. The 
ecologist could determine more definitely the order of succession of the 
natural grasses on sand. At the same time the agriculturist would have 
a means of determining how long it would be necessary to allow his 
sandy farm to remain undisturbed in order to obtain a permanent blue- 
grass pasture to take the place of present poor crops and shifting sand. 
On the areas of pure sand many years would be required, but on most 
of the area where humus has accumulated from the natural: vegetation 
of former years the time required might be commensurate with the re- 
turns. 
In Figure 9, a summary of the succession of the principal associa- 
tions of the natural prairie plants is given. Associations of weeds and 
cultivated grasses now present on the prairie as a result of the influence 
of civilization are omitted in this summary. 
COMPOSITION OF THE ASSOCIATIONS 
In the preceding discussion of the associations emphasis is placed 
on the dominant species. Following is a list of the associations together 
with some of the more frequent secondary species found on less dis- 
turbed areas. This list made from small relic areas is not meant to be 
complete nor to represent absolutely the composition of the more ex- 
tensive associations before they were disturbed by man. Certain limi- 
tations must be recognized. In the first place, the composition of an as- 
sociation is usually most representative near its center of distribution, 
and least representative near its boundaries where there is an over- 
lapping of secondary species of adjacent associations. The centers of 
distribution of some of the associations have been entirely destroyed by 
cultivation, and only the boundaries remain for study. Under present 
conditions it is impossible in every case to list the secondary species with 
the association in which they were most common. In the second place, 
