1908] HARVEY— PRAIRIE-GRASS FORMATION 295 



Among the more important of the secondary species may be men- 

 tioned Viola pedatifida and Oxalls 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 undulatus, and Polygala alba, which occur on middle and 

 lower slopes; Solidago rigidiuscida and S. nemoralis upon lower and 

 middle slopes, and S. missouriensis upon upper slopes mainly; and 

 finally Aster multijlorus 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- 

 tensis estabHshes 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 u j 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 

 pioneer 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 90 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 

 MeHlotus, Hordeum juhatum on lower slopes and moist soil; Panicum 

 capillare, Verbena bracleosa, and Amaranthus graecizans of the more 

 open 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 



