272 THE NORTH AMERICAN GRASSLAND 



in the western half. With respect to life form, the bunch grasses make 

 a large majority, Agropyrum and Muhlenbergia alone forming a defi- 

 nite sod. 



In view of its extent, it is significant that most of the dominants 

 occur throughout the true prairie. All of them range through its en- 

 tire width and the major number of them over the whole area from 

 north to south. Stipa spartea and Muhlenbergia cuspidata find their 

 southern limits along the boundary between Kansas and Oklahoma; 

 Sporobolus asper extends northward only into Central Minnesota and 

 North Dakota, and Carex pennsijlvanica but little farther. In conse- 

 quence, there may be distinguished three faciations, a central major 

 one containing all the dominants, and a northern and a southern one, 

 each lacking a eudominant, respectively Sporobolus asper and Stipa 

 spartea. Such a subdivision is reflected in the distribution of the large 

 number of subdominants, as indicated below. 



Subdominants. The general unity of the true prairie is reflected as 

 strongly, and even more visibly, by the perennial forbs as by the 

 grasses. The rainfall is regularly sufficient to reduce the competition 

 of the dominants to a point where a wealth of subdominants find op- 

 portunity for development. More than a hundred species are con- 

 cerned in characterizing the various aspects, and the abundance of 

 individuals is often such as to obscure the grasses more or less com- 

 pletely. The prevernal aspect is marked by a few rosette forms such 

 as Anemone patens, A. caroliniana, and Antennaria campestris; the 

 vernal stage comprises a much larger number of species, mostly of 

 modest stature, that are disposed to form pure societies over consid- 

 erable areas. Chief among these are Astragalus crassicarpus, Viola 

 pedata, V. pedatifida, Tradescantia virginiana, Phlox pilosa, Senecio 

 aureus, and Baptisia leucophaea. 



The estival aspect is the most varied in composition, with about a 

 third of its components drawn from the pea family, nearly as many 

 from composites, and the remainder widely scattered in relationship. 

 Erect forms of moderate height are the rule, in response to the com- 

 petition of the grasses. Probably the most characteristic species is 

 the shrubby Amorpha canescens, with Psoralea tenuijlora, P. argo- 

 phylla, Petalostemon candidus, P. purpureus, Erigeron ramosus, Echi- 

 nacea angustifolia and Glycyrhiza lepidota also of the first rank. The 

 serotinal aspect is distinguished by an all but complete ascendency of 

 composites, represented by Aster, Solidago, and Helianthus with six 

 species each, and Artemisia, Liatris, Silphium, and Vernonia by three 

 each. The characteristic species widely present in great abundance 

 are Aster multiflorus, A. ericoides, levis and novae-angliae, Solidago 



