262 MINNESOTA BOTANICAL STUDIES 



because of the low, silvery, silken plants that often become densely 

 aggregated with a height about equal to that of the short-grasses. 

 The pepper-grass and the plantain are two species that often invade 

 the dense patches of Bulbilis. All of these variations are evanes- 

 cent and the association is soon again characterized by that stability 

 and monotone that is so notable for short-grass land. 



The early summer, however, brings on societies of deeper- 

 rooted species that persist for a longer period. One of the com- 

 monest of these is the Psoralea argophylla society which often 

 covers acres of ground with bushy silvery plants about one foot in 

 height. As indicated above the Aristidas also aid in the local oblit- 

 eration of the short-grass appearance. Frequently these species 

 develop a growth-form that reminds one of the bunch-grasses of 

 the sandy hilltops. The bunches here are, however, always lower 

 and silvery or, as in Stipa, very open and spreading. Psoralea 

 argophylla is about the last species of the season partially to mask 

 the short-grasses over any considerable areas. This species is also 

 frequent in the bunch-grass association but in such habitats it never 

 becomes social. 



Other plants that are commonly seen in greater or less degree 

 of aggregation are Dyssodia papposa, Opitntia polycantha, and 

 Monarda citriodora. The prostrate, radially spreading vines of 

 Cucurbita foetidissima are frequently seen by the traveler on the 

 plains as an odd additional species of this association. The large, 

 thick, dark-green, triangular leaves following out two to a dozen 

 separate, sprawling branches radially disposed, produce a variation 

 in the tone of the association that always compels notice. Again 

 the tall bushy plants of Psoralea tetniiflora- occurring as scattered 

 individuals are noticeable because of their overshadowing tendency. 

 This species often breaks off near the ground level and becomes a 

 "tumble weed." Shantz notes that where this species is distributed 

 uniformly it is indicative of "either a soil which is more easily 

 penetrated by water than normal short-grass land, or else a place 

 that receives the run-off from an extensive area which consequently 

 is penetrated by more water than that which falls upon it as rain." 

 Extensive stands of this sort are not found within the association 

 as it occurs in this state, but lesser communities are common which 

 according to the above investigator indicate "pockets of looser soil 

 or depressions into which water drains from surrounding areas." 



