194 CLASSIFICATION OF PLANT COMMUNITIES 



instance, we may say that it is a white oak, using the name in the 

 concrete sense, but if we say that the white oak is common in eastern 

 Illinois we are using the same name in an abstract sense. 



A plant association, viewed in the concrete, may be defined as a 

 plant community characterized by its essentially homogeneous 

 physiognomy and ecological structure and by its essentially homo- 

 geneous floristic composition, at least with regard to dominant 

 species. In the abstract the association may be defined as a vege- 

 tation-unit characterized by an essentially constant physiognomy 

 and ecological structure and by an essentially constant floristic 

 composition, at least with regard to dominant species. • 



Physiognomy, as used in the above definitions, refers to the 

 general outward appearance, or external morphology, of the associa- 

 tion. This is determined almost entirely by the nature of the more 

 prominent plants that enter into its structural composition and more 

 particularly by the nature and abundance of the dominant species. 

 Thus, when we refer to an association as deciduous forest, or ever- 

 green forest, or prairie, we are speaking in terms of physiognomy. 



Ecological structure includes all peculiarities of vegetation that 

 are of ecological significance. It is to the plant association what 

 morphological and physiological structure are to a plant. It takes 

 into consideration not only those peculiarities that are visibly 

 expressed in the growth forms of the plants but also those that are 

 not so expressed. For example, the behavior of the various plants 

 that make up the association in relation to various conditions of 

 light, heat, water, soil, and other factors of the environment would 

 be included under ecological structure. Thus, when we speak of 

 plants as being xeric, hydric, or mesic, or when we speak of them as 

 being tolerant or intolerant of shade we are speaking of character- 

 istics that come under the head of ecological structure but which 

 may or may not be expressed in the growth forms of the plants. 

 Furthermore, while physiognomy is concerned primarily with the 

 dominant species only, ecological structure takes into consideration 

 all of the secondary plants as well. 



Floristic composition refers, of course, to the species of plants 

 that make up the association, but it is the relatively common and 

 conspicuous species, that is, the dominant species, that are of most 

 importance. 



123 Consociations.— A consociation is a morphological part of a 

 plant association and is characterized by having a single dominant 

 species (Fig. 84). There are as many consociations in each associa- 



