THE TRUE PRAIRIE. 127 



so far north explains why the societies of desert plains, short-grass plains, and 

 the mixed prairie are so largely southwestern. The more mesophytic eastern 

 prairie affords a readier area for the invasion of the eastern and southeastern 

 species from regions of greater rainfall. The result is that the prairies are 

 largely characterized by eastern elements. This is particularly true of the 

 spring societies and those of the low prairies, as these are especially meso- 

 phytic. The summer and particularly the autumn societies are increasingly 

 xerophytic, and are accordingly largely southwestern and western in origin. 



Mixed societies. — Subdominants either alternate or mix with each other in 

 the grassland fundament. Their large number explains why mixing is the 

 rule. The degree will vary, however, in the different associations in accord- 

 ance with the water relations, as well as the wealth of the flora. Mixed 

 societies are regularly characteristic of the true prairie, less so of the mixed 

 prairie, and still less so of the short-grass plains, where alternation of pure 

 societies seems rather more frequent. Mixed societies may consist of 2 to 3 

 dominants, one of which is controlling, or of several dominants, of which 2 or 

 3 may be equally important. The most characteristic society of the rolling 

 Stipa-Koeleria prairie about Lincoln consists of Psoralea tenuiflora, Amorpha 

 canescens, Petalostemon candidus, P. purpureus, and Brauneria pallida. At 

 Mandan, North Dakota, this is represented by Psoralea argophylla, Brau- 

 neria, and P. candidus, while along the Rawhide Hills, in eastern Wyoming, 

 Psoralea tenuiflora and P. purpureus form the chief society. In the Pine 

 Ridge region of northwestern Nebraska and South Dakota, Psoralea occurs 

 as a pure society, as is frequently the case throughout the Great Plains. 

 While these subdominants may occur in nearly all possible combinations, the 

 example given illustrates the uniform tendency to reduction wherever local 

 factors or climatic conditions decrease the water-content. As a consequence, 

 the pattern woven by the subdominants upon the grass fundament is a com- 

 plex one and it can be traced only by a careful study of the interaction of com- 

 petition and the water-content. When layer societies occur in the grassland, 

 they are the outcome of competition for light primarily. They are naturally 

 restricted to the prairies. 



Aspects. — The most helpful clue to the structure and grouping of societies 

 is furnished by the study of seasonal aspects and of alternation. These are 

 chiefly expressions of the relation to water-content differences as brought about 

 by season and topography. The success of each subdominant depends upon 

 the extent to which it avoids competition with others. This has led to the 

 multiplication of the number of societies up to the limit set by the water rela- 

 tions. As a corollary, the number of societies and the degree of mixture are 

 indicative of the water relations of a particular area or region. Species have 

 necessarily made use of both methods of avoiding competition, namely, 

 alternation in time and in space. Alternation in time results in periods of 

 maximum development termed aspects (Pound and Clements, 1900 : 143, 

 349; Thornber, 1901 : 57; Shantz, 1906 : 26). Aspects are based primarily 

 on the flowering period of the subdominants, upon the assumption that the 

 total requirements of a plant are greatest at the time of flowering and fruiting. 

 Moreover, the subdominants are most characteristic at this time, and the 

 corresponding societies stand out sharply. The number of aspects naturally 

 depends upon the length of the growing season. At Lincoln, the mean dura- 



