CLIMAX UNITS. l^^ 



these two causes of the production of societies within an association, and to 

 restrict the term society to aggregations due to the latter alone. In this 

 way we should obtain a more logically coherent conception. But the more 

 detailed analysis of vegetation has hardly progressed far enough at present 

 to justify a finer classification of plant communities. While a plant formation 

 is always made up of associations, an association is not always or even neces- 

 sarily made up of societies, which are essentially local discontinuous phenom- 

 ena. Finally, plant-societies are minor features of vegetation, and their pres- 

 ence in certain spots is generally determined by some biological peculiarity, 

 not by the habitat as such." 



Moss (1910 : 48) states that "it is becoming usual in this country to speak 

 of the subdivisions of the association as plant societies" (c/. Clements, 

 1905 : 296), and (1913 : 19) that "a plant society is of lower rank than an asso- 

 ciation, and is marked by still less fundamental differences of the habitat." 

 The facies and " nehenhestande" of many authors are societies, as are also 

 many of the patches of Pound and Clements (1898 : 214; 1900 : 313) and 

 Clements (1902: 19). The concept of the society has further been adopted 

 and applied by Shantz (1906:29; 1911:20), Young (1907:329), Jennings 

 (1908:292; 1909:308), Ramaley (1910:223), Adamson (1912:352), and 

 Vestal (1914'': 383). 



Bases. — ^While the concept of society arose from the dominance of principal 

 species, and thus has always had more or less relation to seasonal aspects, there 

 is no necessary connection between the two. In the prairie association the 

 seasonal appearance of societies is a marked phenomenon. In other com- 

 munities the four aspects, prevernal, vernal, aestival, and autumnal, may be 

 reduced to two or even one, and a society may then persist through much or all 

 of the growing season. Even when the aspects are well-marked, a particular 

 society may persist through two or more. As a consequence, the question of 

 time relations is not a necessary part of the concept, though it may prove 

 desirable to distinguish societies with marked seasonal character. 



The real warrant for the recognition of societies lies in the structure, and 

 hence in the development of the formation also. Areas of characteristic 

 dominance occur within the major dominance of consociation and association. 

 Such commimities can not be ignored, for they are just as truly a part of the 

 plexus of habitat and vegetation as the consociation itself. They are an essen- 

 tial result of the interaction of physical and biological processes, and the 

 explanation of their occurrence is necessarily to be sought in the habitat. As 

 Tansley has suggested (1911 : 12), it may prove desirable to distinguish socie- 

 ties controlled by obvious differences of habitat from those in which such con- 

 trol is lacking or obscure. This seems a task for the future, however, since it 

 depends primarily upon the instrumental study of units, of which we have the 

 barest beginning. Moreover, it appears evident that the vast majority of 

 societies, if not all of them, are expressions of basic habitat relations. This 

 must certainly be true of the societies of climax associations and consociations, 

 and it must also be the general rule in the case of the developmental societies 

 of a sere. The only obvious exceptions are furnished by ruderal or subruderal 

 species which invade quickly and remain dominant for only a few years. In 

 the Great Plains the societies of Eriogonum, Psoralea, Helianthus, etc., which 

 occur and recur over thousands of square miles, have had abundant time and 



