224 MINNESOTA BOTANICAL STUDIES 



a tendency which aU the more influences the conspicuous nature of 

 the bunch-grass form. 



Even though there are numerous other bunch- forming plants 

 associated with the dominant species, and in addition to these, a 

 host of interstitials lacking this growth-form, the fact remains that 

 the typical appearance of the bunch-grass association is that of an 

 open association. Seldom in the characteristic development of the 

 association is the light-colored substratum hidden from view. The 

 proportion of the ground covered by the bunch-grasses and their 

 numerous associates varies widely with the stability of the sub- 

 stratum. This very often means that such variation is regulated by 

 the age of the association, i. e., the interval that has elapsed since the 

 bunch-grasses originally invaded blow-out areas. In some of the 

 more protected "pockets" which fire has not entered for a number 

 of years and in dry meadows the sand is sometimes very nearly 

 completely covered by vegetation. But in such cases as this we 

 have other dominant species present and the areas have lost their 

 typical bunch-grass appearance. 



Apparently the ground cover in our typical sandhill uplands 

 rarely attains to the degree of perfection that Gleason (37) notes 

 for the bunch-grass association on the inland sand deposits of Illi- 

 nois. Farther westward, as noted by Shantz (60) in Colorado, and 

 on the firmer soils on the borders of the association as it occurs 

 here, the bunch-grasses alone occasionally form a cover sufficiently 

 dense to hide the soil and to inhibit the invasion of many of the 

 secondary species. This state of affairs naturally approaches the 

 conditions typical for other associations of the Prairie-Grass For- 

 mation. Toward the opposite extreme in the developmental series 

 the bunch-grass association becomes more open and the bunch- 

 grasses less dominant and the association then approaches the blow- 

 out association. 



The bunch-grass of widest distribution and of most frequent 

 controlling influence is Andropogon scoparius. There are, however, 

 other grasses of wide distribution possessed of similar growth- 

 forms. These seldom occur in sufficient abundance to lend a con- 

 spicuous color to the association, much less become dominant. The 

 species that may become locally important in this manner are Andro- 

 pogon liallii, CalamoviJfa longifoUa, Stipa comata, Koeleria cristata, 

 Eragrostis trichodes, and Eriocoma cuspidata. The latter two are 

 more closelv associated with the blow-out association. When these 



