19 1 4] VESTAL— PRAIRIE VEGETATION 395 



The Aristida longiseta consocies is dominated by the wire-grass 

 {Aristida longiseta). It is important (in sandy loam and broken 

 soils) in parts of eastern Colorado and west-central Kansas and 

 Nebraska (Shantz 18, p. 48). In the mesa area Aristida does not 

 occur except in association with other grasses, chiefly Stipa comata. 

 For the present the two growths have been considered together, on 

 account of similar habitat preference, similarity of secondary 

 species, and regularity of occurrence in association, at least in the 

 region studied. More extended study may show the two growths 

 to be representatives of different associations, which intergrade 

 within the region. 



, THE SAND-HILLS MIXED ASSOCIATION 



Sand-hill formations. — Pound and Clements 13, p. 352, in part. 

 Sand-hills mixed association. — Shantz i8, p. 58. 



Within the region studied no truly sandy areas of any extent 

 have been found. Local areas of sandy soil occur with sandstone 

 outcrops and in certain dry stream beds of the plains. The sand- 

 hills mixed association is perhaps not typically developed within 

 the area, though well shown on sandy bluffs of the South Platte at 

 Globeville, on the outskirts of Denver; the usual growth may be 

 referred either to a variation of the short-grass mixed association, 



or to a primitive open growth made up chiefly of plant species 

 abundant in sand-hills. Sandy soil may be recognized by the 

 presence, often in abundance, of Mentzelia nuda (Pursh) T. and G., 

 Abronia fragrans Nutt., Cleome serriilata Pursh, and Artemisia 

 filifolia Torr. Yucca glauca Nutt. is often extremely abundant in 

 sandy soil. 



THE HORDEUM JUBATUM ASSOCIATION 



A pure growth of the squirrel-tail grass (Hordeum jubatum L.) 

 typically occupies moist soil of depressions, or a zone of moist soil, 

 of variable width, surrounding swampy areas or standing water, or 

 bordering gently sloping ditch banks. The growth appears to be 

 native in such situations, and geographically widespread. 



THE CHRYSOTHAMNUS-SARCOBATUS ASSOCIATION 



Chrysothamnus growth of the sage-brush formation.— Pound and Clements 



*3, p. 372. 



