CHAPTER II 

 THE SMALLER SOCIAL UNITS 



The ultimate "atomistic" particles of any community are the 

 individual plants. The combination of these creates the social organ- 

 ism. The countless individuals may be grouped in two distinct ways: 

 under the concept of the taxonomic species or under the concept of the 

 growth forms or life forms (c/. chapter on Life Forms). The Brussels 

 Congress (1910) rightly decided in favor of the species as the funda- 

 mental unit of the plant community. The concept of "life form" is 

 indefinite, has not been adequately defined, and cannot be considered 

 as a sufficient basis for a science of vegetation. Species, however, are 

 groups of individuals with uniform inheritance and have been for many 

 years the objects of careful investigation. 



In the species are embodied certain definite adjustments to and 

 demands upon the environment. Hence the species have come to be 

 regarded as conspicuous indicators of certain conditions of life. The 

 most exact indicators are often, indeed, not the "good Linnaean 

 species" but rather the elementary species or races, the "ecotypes" of 

 Turesson (1925). These forms require more narrowly circumscribed 

 life conditions and therefore are socially more sharply speciahzed. A 

 knowledge of the species usually gives at once an idea of the life form in 

 which it falls. 



Precise recognition of species is therefore the first and indispensable 

 requirement for the phytosociologist. Studies of the structure of 

 vegetation without accurate knowledge of the species concerned are 

 scientifically worthless. The goal of these studies is to establish 

 precisely the significance of the species in the organization of the com- 

 munity and to discover the order which prevails in the grouping of the 

 species into communities. 



Habitat and Plant Community. — By the term "habitat" we mean, 

 following Yapp (1922), the dwelling place of a species or of a community 

 including all of the operative factors, except competition, that influence 

 the plants themselves. The exact place of occurrence is called the 

 locality or "station." 



A given plant community may occur in many localities. But it 

 exists usually in only one well-defined and ecologically characteristic 



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