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THE AMERICAN NATURALIST [Vol. XLYIII 



mon habitat, are similar in ecological constitution (eco- 

 logically equivalent) in so far as their presence is deter- 

 mined by the same environmental conditions. It should 

 be pointed out that there are local environmental differ- 

 ences within the area of the association which allow the 

 presence of differently constituted organisms, and that 

 the entire range of environmental conditions within the 

 habitat is usually much wider than that of the environ- 

 mental complex selected by a particular organism. The 

 environmental complex of the organism is not the same 

 as the sum-total of environmental conditions within the 

 association. Each organism differs in greater or less 

 degree from others in ecological constitution, and thus 

 selects a different environmental complex. 



The physical factors of the environment are of great- 

 est importance in the life of the plant. Plants influence 

 one another directly to only a slight extent. There is 

 usually very li'ttle of the social relationship among eco- 

 logically similar plants which will compare with such 

 relationships as seen in animals. Competition among 

 plants is mainly a struggle to determine which plants 

 are to be most favored by physical conditions, and it is 

 probably most severe for the physical factor present in 

 minimal quantity. In desert associations plant competi- 

 tion is almost exclusively for water, and extensive root 

 systems are developed. In grassland it is very largely 

 for above-ground space; in forests it is principally for 

 light. The influence of the animal-environment is prob- 

 ably of greater importance than has commonly been 

 realized by plant ecologists; the study of economic ento- 

 mology and of the effects of grazing upon grasslands is 

 helping to bring about a realization of the importance of 

 animal influence upon plant life. 



The structures of plants show frequent and great 

 modification in response to the physical conditions of the 

 environment. These modifications are most frequent and 

 important with respect to the factor present in minimal 

 quantity. Characters which may be associated with 



