220 MINNESOTA BOTANICAL STUDIES 



considered is very similar to the "Bluestem Type" of the "Bunch- 

 Grass Formation" of the Phytogeography of Nebraska. 



The bunch-grass habit is that aspect of prairie vegetation which, 

 although found in greater or less degree in practically all prairie 

 areas of the United States, has succeeded in a remarkable degree 

 against the rigorous conditions imposed by a very sandy soil and a 

 semi-arid climate. 



The frequency and often the great abundance of interstitial 

 species of the bunch-grass association with an unmistakable alliance 

 with the more extensive type of the prairie-grass formation as it 

 occurs farther eastward, constitute additional argument in favor of 

 the conclusion that the bunch-grass condition of the sandhills is 

 merely a variation of the more typical nature of that formation 

 brought about by the ecological conditions briefly indicated upon 

 preceding pages. I therefore prefer to regard this variation as an 

 association of the great prairie-grass formation instead of a sepa- 

 rate plant formation as suggested by Pound and Clements (57). 

 From Shimek (61) it appears that this is also the typical nature of 

 the vegetation of the sandhills of Iowa. The facts of succession, 

 as we shall see, also favor this subordination. 



The most highly developed and typical expression of this asso- 

 ciation is found where there is the greatest uninterrupted stretch of 

 sandy upland. Conditions of this sort radiate from Thomas and 

 Hooker counties as a center. Northward from the above defined 

 bunch-grass center the bunch-grass association is soon interrupted 

 and dissected by the numerous wet valleys and meadows with low- 

 land associations that are so characteristic of the Cherry County 

 portion of the sandhill region. Here the association is very sharply 

 bounded through the medium of soil differentiation and the result- 

 ing divergence in competition phenomena that distinguish uplands 

 from lowlands which here are very pronounced. Within this por- 

 tion of its range the association is presented in the form of tongues 

 often miles in length and hundreds of yards across, or of smaller 

 isolated ranges with short spurs and islands with bold faces alter- 

 nating with narrower lowlands. This characteristic distribution 

 may be quite easily appreciated from a glance at the topographic 

 sketches found elsewhere in this paper. Such dissected nature of 

 the association persists for the most part throughout Cherry County. 

 The tendency appears to be that as one passes outward from the 

 geographic center of this association the lowland area increases and 



