CLASSIFICATION OF PLANT COMMUNITIES 349 



Even here, however, the edaphic Uniiting factors, at least in 

 some cases, may be of great significance, and it need be only 

 suggested that in an accurate analysis these, as well as the ch- 

 matic limiting factors, must be considered. 



SUMMARY 



By way of summary, it may be stated that the fundamental 

 unit of vegetation is the association. The associations of a 

 unit physiographic area, taken collectively, constitute an 

 edaphic formation. The edaphic formations of a unit climatic 

 area, taken collectively, constitute a climatic formation. The 

 climatic formations of the earth, taken collectively, may be 

 said to constitute the terrestrial formation. The association 

 is a unit determined by habitat, the edaphic formation a unit 

 determined by physiography, the climatic formation a unit de- 

 termined by climate, while the terrestrial formation might be 

 said to be a unit determined by the atmosphere. ''The con- 

 ception of a formation as an ecological genus and an association 

 as an ecological species" (Cowles 10, p. 150) may be fui'ther 

 amplified. If the association is regarded as an ecological species 

 and the edaphic formation constitutes an ecological genus, then 

 the climatic formation may be said to represent an ecological 

 family, while the terrestrial formation might be regarded as a 

 unit of a still higher order. 



In the preceding pages the various ecological units of veg- 

 etation have been treated as an ascending series. Starting 

 with the fundamental unit, the association, the units of a higher 

 order have been treated in the order of their increasing com- 

 plexity. Their relative rank and their relation to one another 

 is brought out by the following synopsis. 



The terrestrial formation: = the climatic formation-complex of the earth 

 The climatic formation : = the edaphic formation-complex of a cliniatic unit 



region 

 The edaphic formation: = the association-complex of a physiographic 



unit area 

 The association: = the plant-complex (community) of a unit 



habitat 



