226 MINNESOTA BOTANICAL STUDIES 



scoparius. They are stray pioneer groups of vegetation more espe- 

 cially characteristic of other associations. Later in the season when 

 the onward march of xerophilous conditions has enveloped the hills, 

 Koeleria and Stipa wane and Andropogon pushes rapidly to the 

 maximum and most traces of the former species have completely 

 disappeared. During the late summer Calamovilfa longifolia 

 becomes quite dominant in situations where blow sand is still abun- 

 dant. Eragrostis trichodes and Eriocoma cuspidata occur at such 

 times near sand-draws or blow-outs wdiere the sand still has a 

 tendency to slide. It is thus seen that, although these species are 

 frequently noted within this association, they are largely "left-overs" 

 from blow-out conditions. They are rather efficient sand binders 

 and hence they serve as convenient intermediary species between 

 RedHeldia of the blow-out association and Andropogon. 



Andropogon hallii is a tall, coarse grass that forms loose, open 

 bunches of a few large stems that are often conspicuous objects in 

 this association. This species is more frequent and abundant 

 toward the tops of hills and ridges especially over the eastern 

 stretches of the sandhills in Holt, Garfield, Wheeler, and Antelope 

 counties. Usually coming to full development later than A. 

 scoparius and Stipa comata, the tufts of few tall, glaucous stems and 

 broad, blue leaves scattered thinly over slopes and hill tops, are note- 

 worthy features of many hills as the vegetative season approaches 

 its close during the last week of July or the first week of August. 

 The spiny blow-out grass, Muhlenbergia pungens, persists for a 

 number of years as small, low, cushion-like mats after the bunch- 

 grass association has become controlling. The hairy grama, Bou- 

 teloua hirsuta, is also a frequent member of the bunch-grass asso- 

 ciation in many parts of the hills. The small, low, bunchlike mats 

 four inches or more in diameter occur at widely scattered intervals 

 throughout the region. The sand-enduring, wind-tolerant nature 

 of this species is remarkable. 



Two sedges, Cyperus schweinitzii and Carex stenophylla, are 

 common bunch-grass associates. The former species is charac- 

 terized by low tufts composed of a few spreading or grasslike 

 divergent stems, thus producing a very open type of the bunch- 

 grass form. These are widely scattered and are interesting because 

 of their extreme xerophilous nature. Carex stenophylla on the 

 other hand becomes controlling in certain spots. If one views the 

 hills during the early days of May one is impressed by the appear- 



