230 MINNESOTA BOTANICAL STUDIES 



these circumstances blow-out rather than bunch-grass conditions 

 prevail. The presence of such species as Redfieldia flexuosa, Pso- 

 ralea lanceolata, and Muhlenhcrgia pimgens clearly indicates this 

 relation. 



In addition to the species that have been mentioned above there 

 are a number of xerophilous bushes and dwarf shrubs that assume 

 considerable importance in the upland vegetation of various portions 

 of the sandhills. The individuals of such species are sometimes so 

 abundant as to dominate relatively small local areas of this associa- 

 tion during certain seasons of the year or sometimes this dominion 

 persists throughout the year. The correlation of these groups with 

 certain physical conditions is not always evident. Chance dissemin- 

 ation accounts for much of the distribution phenomena exhibited by 

 these species although that complex relationship known as competi- 

 tion is very evidently active in a number of cases. 



Prunus besseyi is in reality a more abundant member of the 

 bunch-grass association than Yucca glauca, but because of its 

 smaller stature and the fact that it is never a very leafy plant the 

 species rarely plays the commanding role that is often noticed in the 

 case of Yucca. The sand cherry is, however, to be found on almost 

 every hill in nearly all portions of the sandhills. As with the Yucca, 

 so this species prefers the more open condition of the bunch-grass 

 association. Such habitats commonly reveal during the first two 

 weeks of May the numerous short twigs of the low, bushy plants 

 with their dense umbels of white or pinkish flowers. At this time 

 other fresh vegetative features are not abundant so that the sand 

 cherry becomes a most welcome and obvious floral element spread 

 over the awakening hills. With the falling of its showy flower 

 parts the sand cherry becomes of less conspicuous nature since the 

 leaf development is not so luxuriant as to attract the eye from a 

 distance. Later in the summer or early autumn when the growth 

 period has closed for most sandhill species, Prunus besseyi again 

 becomes especially noticeable because of the many large black fruits 

 that stand out in sharp contrast to the light-colored sand below. 

 Unfortunately a species of Exoascus sometimes completely destroys 

 all of the fruits of the sand cherry. The sand cherry never assumes 

 sufficient dominance to rival the bunch-grasses, being as a rule 

 merely an important principal species. 



Another characteristic shrub is Ceanothus ovatus which 

 becomes especially abundant in the central and southern portions of 



