39° BOTANICAL GAZETTE [November 



up of Andropogon furcatus and Sorghastrum nutans, in fine-grained 

 moist soil of stream bottoms in the floodplain, should also be noted. 5 

 It probably formerly occupied considerable areas. 



THE PRAIRIE -GRASS ASSOCIATION 



Prairie-grass formation. — Pound and Clements 13, p. 348, in part. 

 Prairie-grass formation. — Shantz 18, p. 23, in part. 

 Prairie-grass formation. — Harvey 8, in part. 

 Prairie-grass formation. — Harshberger 7, pp. 522, 527, in part. 

 Prairie-grass associations. — Vestal 19, pp. 354, 355. 



The prairie-grass growth within the region is mesophytic, 

 meadow-like, in spring, becoming quite dry in late summer, most 

 of the plants being then dead. It is typically developed in rich 



moisture 



summer 



Such soil is regularly found in alluvial lower slopes 



gner mesas 



The Poa-Koeleria consocies is dominated by Poa Buckleyana 

 Nash and by Koeleria cristata (L.) Pers. Stipa viridula Trin., Poa 

 pratensis L. ? Agropyron violaceum (Hornem.) Vasey, and Agropyron 

 Smithii Rydb. are occasionally present. The growth is tall and close. 

 In alluvial fans at the base of certain slopes, facing to the north 

 usually, the Arnica society of the prairie-grass growth is developed 



(fig. 6). 



'phala 



fulg 



in part). The Delph 

 Delphinium Nelsonii 



ostium campestre Greene ar 



other well defined societies. 



The Stipa viridula con: 



There are several 



dominated by Slip 



Trin., with occasional other grasses. It is usually on the outer 



5 The occurrence of bunch-grass in such different habitats as its mesa and flood- 

 plain stations bears out the view that the plant growth itself, rather than the habitat, 

 should be used in delimitation of associations within even such a small area as the one 

 studied. The resultant conditions essential to the plant growth, that is, continually 

 available moisture, are fulfilled in both situations. Shantz has shown (18, p. 55) 

 that the Andropogon sco pari us consocies is found in gravelly soil near the mountains, 

 in sand and loose loam in eastern Colorado, in Fort Pierre clays in South Dakota, and 

 in loam soil in the eastern part of the prairie region. Probably the vegetation itself 

 is a very good expression of the resultant of climatic, local, and historic factors; the 

 complete environment. 



