234 MINNESOTA BOTANICAL STUDIES 



vegetation from that of the bunch-grass association and sufficiently 

 different from the blow-out association to be worthy of separate 

 classification and ti:eatment. Successionally the Muhlenbergia asso- 

 ciation is very evidently intermediate between these two upland as- 

 sociations but with a closer relationship to the blow-out association. 

 There is much evidence in addition to that cited above that this as- 

 sociation preceded the bunch-grasses during the initial invasions 

 that marked the early population of the sandhill region by plants. 

 Furthermore it appears that when overgrazing or other causes have 

 reduced the bunch-grass association to a very low degree the two 

 types of vegetation which then make their appearance are the blow- 

 out association and the Muhlenbergia association. I have found 

 wherever this association is most highly developed that overgrazing 

 or repeated firing or both have been the forces that have probably 

 made possible its origin. In such places, then, we naturally find 

 the maximum expression of the association where it occupies the 

 less severe or slightly more stable uplands surrounding points of 

 greatest exposure whereupon typical blow-outs are developed with 

 the physical and vegetative features to be noted elsewhere in this 

 paper. 



Some idea of the peculiar typical appearance of this association 

 may be obtained from a brief description of the dominant species. 

 Muhlenbergia pungens is a low grass characterized by tufted stems 

 with very glaucous and extremely rigid, narrow, involute-setaceous 

 leaves from three to five inches in length. These leaves are arranged 

 in two ranks upon the short erect culms, which arise from creeping 

 rootstocks, and which are inclined to be grouped into small tufts 

 or cushions that always lie close to the sand. The decumbent bases 

 of the culms bring the plants down to a height of less than four 

 inches. These low irregular clusters a few inches in diameter 

 closely crowded together present a remarkable spiny, carpet-like 

 cover wherever the species is controlling. This growth relation is 

 frequently so emphatic as to exclude nearly all other species from 

 areas often many square yards in extent. From a distance such 

 areas might be mistaken for short-grass land covered with a carpet 

 of Bouteloua if the very sandy substratum was not evident. Muh- 

 lenbergia pungens is an excellent sand-collector. The rhizome type 

 of propagation and dissemination is so well developed that the plants 

 recover very rapidly from sand burial and are thereby enabled to 

 maintain a foothold against active sand movement. We have here 



