82 PLANT SOCIOLOGY 



The last two decades have brought a great increase in the number of 

 synecological studies, especially of those concerned with agriculture 

 and forestry. Ecologic stations have been established, and it looks as 

 if plant physiologists wished to emerge from the confinement of their 

 autecological studies and enter upon the study of the household eco- 

 nomics of communities. Such a deepening of interest in synecology is 

 undoubtedly one of the most urgent demands of modern plant sociology. 



In the first chapter it was pointed out that for an adequate under- 

 standing of plant communities three causal complexes must be 

 considered. 



1. Historical factors, summarized under paleosyngenetics, paleosynecology, 

 and paleosynchorology. These historical factors have been responsible for the 

 plant populations of the earth's surface and thus have made possible existing 

 combinations of species, genera, and families. 



2. Competition, as outlined in Chap. I. 



3. Operative measurable factors of the habitat. 



These are the forces that really determine communities, govern 

 competition, and from the infinite number of possible combinations 

 of species bring about the selection of a relatively limited number. 



The operative factors of the habitat which are essential for the 

 ecologic description and characterization of associations are best 

 considered under four heads: 



1. Climatic or atmospheric factors. 



2. Edaphic or soil factors. 



3. Orographic or topographic factors. 



4. Biotic factors or the effects of the living environment. 



Each group of factors may be considered separately, with its 

 effects upon the plant covering of the earth. We must not, however, 

 overlook the fact that in nature we always meet not a single factor but a 

 whole complex of them. The single factors have manifold interrela- 

 tions, and the influence of each factor depends upon the whole congeries 

 of factors. These interrelations of factors have received little atten- 

 tion, little synecological consideration, although they offer a rich field 

 for investigation. The establishment, however, of any orderliness 

 based on definitely known conditions, the elaboration of any positive 

 substitution of one factor for another represents an advance for science 

 and leads to a better understanding of the prodigious complexity of the 

 social life of plants. 



When a plant community, an association, or a subdivision of it is 

 recognized and floristically circumscribed, the investigation 6f the 

 habitat and habitat factors must be undertaken. Obviously such 

 investigations can have a general application only when carried out in 



