THE PRAIRIE-PLAINS GRASSLAND FORMATION. 13 



The vegetation of the true prairie is characterized in the first place 

 by an abundance of tall, sod-forming grasses. Chief among these are 

 Andropogon scoparius, Stipa spartea, Agropyrum glaucum, Stipa 

 comata, Andropogon furcatus, and Kceleria cristata. Others, like 

 Andropogon nutans, Elymus canadensis, Panicum virgatum, and Poa 

 pratensis, while of less importance, agree with most of the preceding in 

 having basal leaves 1 to 2 feet high, and shoots normally 2 to 4 feet 

 tall. The group is one of typical tall-grasses. Since the water-content 

 of the soil is abundant for grassland, large numbers of other herbs find 

 congenial growth conditions and form more or less extensive societies. 

 Different groups of these subdominants help characterize the seasonal 

 aspects, many reaching their maximum development at intervals be- 

 tween the flowering of the dominant grasses. Typical of the more 

 important ones are Amorpha canescens, Psoralea tenuiflora (plate 5, 

 b, c), P. argophylla, Erigeronramosus, Astragalus crassicarpus, Aragal- 

 lus lambertii, Petalostemon candidus, P. purpureus, Glycyrrhizalepidota, 

 and species of Aster and Solidago. Briefly, true prairies occur in re- 

 gions of sufficiently favorable soil and air moisture conditions to permit 

 the growth of large numbers of dominant tall-grasses with which are 

 intermixed a great number of other herbs, the whole forming a luxuriant 

 vegetation. For a general ecological discussion of prairie vegetation 

 the reader is referred to Pound and Clements (1900), Thornber (1901), 

 Harvey (1908), Shimek (1911), Weaver and Thiel (1917), Pool, 

 Weaver, and Jean (1918), Weaver (1919), and Clements (1920). 



THE SHORT-GRASS PLAINS. 



The short-grass plains (Bulbilis-Bouteloua association) are character- 

 ized by a few short, sod-forming grasses, especially Bulbilis dactyloides 

 and Bouteloua gracilis (plates 2 and 9, a). Clements states: 



"The short-grass association ranges from southwestern Nebraska and the 

 western half of Kansas through eastern Colorado into northwestern Texas, 

 northern New Mexico and Arizona. It is also developed to some extent in 

 southeastern Utah and southwestern Colorado." (Clements, 1920:140.) 



The short-grasses grow in a region of light rainfall and are well 

 adapted to a brief growing-season, maturing seed in July. They form 

 such a dense sod and render the soil so compact that run-off is high and 

 water-content consequently normally low. This, together with the 

 intense competition of these short-grasses, practically excludes the 

 taller grasses and other herbaceous vegetation. Moreover, most 

 prairie species find the short-grass plains an uncongenial habitat be- 

 cause of the high evaporating power of the air, and especially the 

 scarcity of water during late summer, just at a time, often, when their 

 demands upon the habitat are greatest. This absence of taller vegeta- 

 tion makes the typical short-grass cover very uniform and monotonous. 



