132 STRUCTURE AND UNITS OF VEGETATION. 



opportunity to migrate over the whole region; Psoralea tenuiflora is found, in 

 fact, from Illinois and Minnesota to Texas, Sonora, Arizona, and Montana. 

 Hence the presence of the society over large stretches and its absence in other 

 places must be a matter of habitat control. In this, naturally, competition 

 must often play a dominant part, and there can be little question that exact 

 analysis will some day enable us to distinguish some societies upon this basis 

 (Woodhead, 1906 : 396; Sherff, 1912 : 415). At present, such a distinction is 

 impossible or at least without real meaning. Hence, while societies are readily 

 seen to range from complete dominance, often greater than that of the con- 

 sociation, to mere characteristic, it is highly probable that these merely repre- 

 sent different degrees of habitat response. This is often not obvious, for the 

 decisive effect of the factors which control a society may be felt only at the 

 time of germination for example, and might easily escape one who failed to 

 use the exact methods of quadrat study throughout the entire growing-season. 

 Perhaps no better evidence of the relation of societies to habitat can be fur- 

 nished at present than their striking variation in abundance from one area to 

 another, when such areas show no visible habitat differences. As a conse- 

 quence, while it is possible to regard some societies as dominant, and others as 

 only characteristic, it is felt that such a distinction is merely one of degree. 

 It is necessarily superficial in the present state of our knowledge, and has the 

 further disadvantage of being too easily subjective. An experimental study 

 of dominance might well furnish a real basis for distinctions here, but further 

 analysis must await such study. 



Kinds of societies. — ^There may well be differences of opinion as to the desira- 

 bility or necessity of distinguishing various types of societies. Those who are 

 more interested in other phases of vegetation than in its development and 

 structure will naturally not need to use finer distinctions. On the contrary, 

 those who wish to trace in detail the response of the community to its habitat 

 will find it helpful to recognize several kinds of societies. Even here, however, 

 it is undesirable to outrun our present needs and to base distinctions upon 

 differences which are subordinate or local. Thus, while it is convenient and 

 natural to recognize layer societies, it would result in a surplus of concepts 

 and terms to distinguish societies upon the basis of the six or eight layers 

 present in well-lighted forests. Accordingly it seems desirable to regard all 

 societies as due to habitat control, more or less modified by competition, and 

 to establish subdivisions only upon the following bases: (1) aspects, (2) layers, 

 (3) cryptogams. In addition, there are the relict and nascent societies of 

 various serai stages, which will be considered under developmental societies. 

 Finally, there are the related questions of changes of rank or dominance, which 

 are dealt with below. 



Aspect societies. — Since most societies are composed of subdominant herbs, 

 i. e., dominant within a dominance, their chief value usually appears as they 

 approach maturity, and especially when they are in flower. Astragalus crassi- 

 carpus, for example, is present in the prairie from early spring to frost. But 

 it dominates hillsides only in the spring, before the taller herbs have grown, 

 and this dominance is a conspicuous feature only when the plants are in bloom. 

 There is, then, a seasonal change of dominance which marks the aspects of 

 the vegetation. In open woods a similar change of dominance results from 

 the successive appearance of the layers, the earlier lower layers being masked 



