232 MINNESOTA BOTANICAL STUDIES 



plants so well described by Pound and Clements under the "Arte- 

 misia filifolia Formation." 



Still another common prairie shrub, Amorpha canescens, is 

 noted at many stations in the sandhills. The dominion of this 

 species is also local and so partakes of the general effects produced 

 by the same plant in other portions of the prairie-grass formation. 

 The prairie shoestring, as it is commonly called, is characteristic of 

 firm soils. With Kuhnistera villosa this species forms extensive 

 societies of low bush in the harder soils north of the Niobrara River. 

 Within the heart of the sandhills it occasionally assumes a dominant 

 role over small local areas. The low, finely branched plants with a 

 dense cover of silvery leaflets always stand out as very prominent 

 objects of the prairie flora. This species as it occurs in the sand- 

 hills seldom becomes so socially exclusive as do Rosa and Ceanothus. 



The open nature of the bunch-grass association is a condition 

 which makes possible the presence of a great many secondary 

 species. The most common of these widely distributed interstitials 

 have been enumerated in the list of secondary species of the associa- 

 tion. Besides the above woody species that are of common occur- 

 rence in more or less density as characteristic societies, the following 

 species have a wide distribution and frequently occur as closely 

 aggregated patches or societies: Lygodesmia jtincea, Lathyrus 

 ornatus incanus, Psoralea lanceolata, Anemone caroliniana, Helian- 

 thus petiolaris, H. snhrhomhoideus, Lesquerella argentea, Froelichia 

 Horidana, and Eriogonum annuiim. Certain of these species are 

 notably seasonal in their appearance and for short periods they are 

 conspicuous, locally dominant species of the association. Thus 

 Latliynis ornatus is a distinctive species of very sandy hilltops and 

 ridges where in late spring or early summer it fairly covers the 

 ground with a prostrate tangle of slender vinelike stems and ten- 

 drils. This plant is a profuse bloomer and when at the height of its 

 flowering period is one of the most delicate and beautiful species of 

 the whole state. Froelichia floridana and Eriogonum annuum 

 usually occur singly or in loose patches scattered among the bunch- 

 grasses, but occasionally dense communities of these are seen. 

 The latter is especially common on valley floors and bottoms of 

 pockets where the soil is quite firm. 



THE MUHLENBERGIA ASSOCI.VTION 



This association occupies upland habitats similar in topography 



