1908] HARVEY—PRAIRIE-GRASS FORMATION 295 
Among the more important of the secondary species may be men- 
tioned Viola pedatifida and Oxalis violacea of middle and lower slopes; 
Meriolix serrulata of higher slopes; the Lithospermums of the more 
xerophytic portions of the formation; Plantago Purshii of middle 
slopes; Linum rigidum in the open association; Potentilla Hippiana, 
Carduus wndulatus, and Polygala alba, which occur on middle and 
lower slopes; Solidago rigidiuscula and S. nemoralis upon lower and 
middle slopes, and S. missouriensis upon upper slopes mainly; and 
finally Aster multiflorus and A. oblongifolia. 
Considering the ground association, the open association prevails 
over ridges and crests and extends down somewhat on slopes, passing 
gradually through a transitional condition into the closed association 
which occupies the valleys, depressions, and base of slopes, working 
always up or outward, displacing the open association. Poa pra- 
fensis establishes the most dense association, but the Boutelouas, 
Koeleria, and Festuca exert perhaps a more extensive influence in 
reducing the open association. In this connection it should be noted 
that the Andropogons are par excellence the pioneers, breaking up the 
Xerophytic open association upon the highest and most xerophytic 
crests, preparing the way for the Boutelouas. It may be possible 
that in some cases the Andropogon bunch-grass stage was not the 
Ploneer society, but that on account of more favorable soil moisture 
Conditions, largely a question of drainage, the Boutelouas were the 
Initial sod-formers. However, upon crests and ridges of excessive 
drainage the Andropogons have invariably preceded the Boutelouas 
and Koeleria. Occurring rarely in the open association is an unde- 
termined xerophytic moss, while two species of the Basidiomycetes 
have been noted in the more mesophytic portions of the formation. 
The enumeration of species includes go forms belonging to the 
formation proper and some 18 ruderals which work into‘the forma- 
tion from the contiguous cultivated regions. The most abundant 
and prominent of the latter are Cassia chamaecrista, two species of 
Melilotus, Hordeum jubatum on lower slopes and moist soil; Panicum 
‘apillare, Verbena bracteosa, and Amaranthus graecizans of the more 
°PeN associations; Onagra biennis of general occurrence; Salsola of 
the open association; and finally Lepidium virginicum, which not 
infrequently becomes quite abundant in the open association of 
