300 GEORGE E. NICHOLS 



sents the fundamental unit of vegetation. The association 

 may be defined as: any group or community of plants, taken in 

 its entirety, which occupies a common habitat (see Cowles 6, 

 p. 939). In terms of dynamic plant geography it may be fur- 

 ther defined as: any stage in a given successional series. 



Uniformity of habitat, then, affords the criterion of the as- 

 sociation. The word habitat commonly has been applied some- 

 what loosely, but from an ecological standpoint it is desirable 

 that it should be dehmited as precisely as possible. As con- 

 ceived by the writer, the habitat may be defined as: any unit 

 area in which the combined influence of the various external 

 factors which determine the ecological aspect of the vegetation 

 is such as to produce an essentially uniform environmeiit. It 

 is the environment which determines the ecological aspect of 

 a plant community. The nature of the environment, in turn, 

 is determined by a complex of physical and chemical factors 

 which, in a general way, may be classified as (1) climatic, (2) 

 edaphic, and (3) biotic. (1) The cli7natic factors comprise all 

 those atmospheric conditions through whose widespread uni- 

 formity the character of the regional climate is determined. 

 These include: (a) atmospheric humidity, (b) precipitation, 

 (c) temperature, and (d) light. (2) The edaphic factors in- 

 clude all conditions which are attributable, directly or indirectly, 

 to soil or topographic agencies. Their influences may be exerted 

 either through the medium of the ground or through that of 

 the atmosphere. The ground influences are attributable to: 

 (a) soil factors (the physico-chemical nature of the soil, and the 

 ground-water relations); (b) slope factors (the inclination of 

 the ground's surface, i.e., the degree to which it departs from 

 the level) ; and (c) dynamic physiographic agencies, where these 

 are operative (as seen in the phenomena of erosion and deposi- 

 tion). The edaphic atmospheric influences are seen in the local 

 modification of the climate associated with differences in ex- 

 posure, which in turn are attributable to variations in topog- 

 raphy: thus, a north-facing slope commonly possesses a some- 

 what different "local climate" from a south-facing slope; 

 similarly, the "local climate" of a ravine differs from that of 



