THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 37 



manner of growth. The degree of gregariousness of most species is 

 greatly influenced by conditions of habitat and by competition (see 

 also Jenny, 1930, p. 152). Even such cushion and tussock plants as 

 Carex elata and Silene acaulis show a difference in the crowding of 

 shoots according to the community in which they occur and their 

 state of development. From the gregariousness of the species con- 

 clusions may often be drawn as to the nearness of approach to optimum 

 conditions. In the typical Molinietum the individual plants of 

 Phragmites are always isolated (Soc. 1), but in the adjacent Scirpeto- 

 Phragmitetum they occur in troops and crowds (Soc. 3 to 5). Stipa 

 cayillata, Iris sihirica, Veratrum album, Viola tricolor, and countless 

 other species which in general are not considered "social" plants may 

 enter into competition in almost any degree of gregariousness, accord- 

 ing to the more or less favorable conditions of the habitat. In general, 

 vegetative reproduction leads to crowding, that is, to increased socia- 

 bility (cf. Kujala, 1925). 



Because it is so easily modified, the gregariousness of many species 

 changes materially during the course of a succession, as on newly 

 formed land or on neglected cut-over woodland. 



The estimate of sociability is easily made, and on large sample 

 areas it enables us to form a picture of the plant mosaic in much sharper 

 outlines than would be possible from mere density and dominance 

 estimates. 



Statements of sociability are especially desirable where species 

 occur in groups or colonies, whether the organisms are rooted, clinging, 

 or free floating. As Alechin (1926) has pointed out, increased socia- 

 bility is of great service to plants in competition with other species. 

 The sociability of pioneer species on talus is almost always great and 

 may be considered a characteristic of such vegetation (Jenny-Lips, 

 1930). 



The specific number of individuals or shoots or, for attached lichens, 

 mosses, and algae the size or diameter of colonies, may be indicated by 

 definite class numbers (cf. Hayren, 1914). 



Statistically speaking, species and individuals may be distributed 

 either with normal dispersion, or they may have hypo- or hyperdisper- 

 sion. In hyperdispersion the individuals are crowded {Oxalis in beech 

 woods) ; hypodispersion occurs when the individuals are more regularly 

 arranged than would be expected to result by chance {Solanum 

 tuberosum in a potato" field, Juniperus in the Liineburg heath, Fig. 16). 

 In nature normal as well as hypo- and hyperdispersion occurs. The 

 more pronounced the hypodispersion in regard lo species and individ- 

 uals the more homogeneous is the vegetation of the area under 



