22 PLANT SOCIOLOGY 



habitat. From this concept have come the long-continued efforts to 

 dehmit and sharply define the habitat. The hope was to arrive thus at 

 a classification of the plant communities themselves — a hope which has 

 not been realized. 



The more sharply we have sought to dehmit the problem of the 

 habitat the more complicated and involved its definition has become. 

 The effective external factors are so numerous and so variable, the 

 possible combinations so manifold, the overlappings so frequent that 

 a clear and unequivocal delimitation of habitats according to operative 

 external factors appears quite unattainable. It may be added that the 

 relation of habitat to plant community is not a simple and reversible 

 function. In the first place, the flora of a region is historically the 

 result of a long process of natural selection. 



On account of this difficulty it is becoming more and more necessary, 

 in investigating the communities, to go directly to the vegetation itself. 

 We then arrive at the point from which we should logically have started 

 out: the natural grouping of plants. The natural units of vegetation 

 come thus into the foreground of our study, and, temporarily ignoring 

 the habitat, we seek to recognize and define the floristic individuality 

 of the communities. 



From the exact floristic analysis of individual communities of 

 vegetation we proceed to the synthesis of plant communities. This 

 analysis and synthesis should afford the basis for conclusions regarding 

 the combination of species, the numerical relations of the individual 

 species, and the significance of each species in the origin, development, 

 maintenance, and decline of plant communities, especially of those 

 fundamental units of vegetation: the associations. 



Fundamental Unit of Plant Sociology. — "An association is a plant 

 ■ community of definite floristic composition" (Flahault and Schroter, 

 1910). In this statement the Third International Botanical Congress 

 at Brussels recognized as fundamental the floristically uniform char- 

 acter of this unit of vegetation. The definition is at once too narrow 

 and too broad : too broad, because not only the association but all the 

 lower and higher units, variants, facies, societies, alhances, etc., are 

 characterized to a greater or lesser extent by definite floristic composi- 

 tion; too narrow, because, with few exceptions, no two bits of vegeta- 

 tion have precisely identical floristic composition. 



The possible combinations of plant species are indeed endless. To 

 attribute to every actual combination in nature the value of a type 

 would result in a chaotic splitting up of the units of vegetation. On 

 such a basis every quarter of a square meter of a meadow community 

 would form a separate unit. We are obliged, therefore, to institute 



