CLASSIFICATION OF PLANT COMMUNITIES 307 



an exposed hillside; etc. While such local atmospheric dis- 

 similarities represent merely modifications of the climate of 

 the region, a distinction may well be drawn between these 

 local climatic factors, which in the main are attributable to 

 edaphic influences, and the regional climatic factors. For the 

 sake of convenience, any area in which the combined influence 

 of the various edaphic factors is essentially uniform throughout 

 may be termed an edaphic unit area. (3) The biotic factors 

 include all conditions which, directly or indkectly, are at- 

 tributable to plant or animal agencies. Their influence is 

 seen (a) in the ameliorating effect of humus on the water rela- 

 tions, etc. of the soil, and (b) in the effect of shade, as it inhibits 

 the development of intolerant plants, modifies the evaporating 

 power of the air, etc. (in this connection, see Cowles, 11). 



In view of the foregoing remarks, the habitat may be further 

 defined as any unit area in which the combined influence of 

 climatic, edaphic, and biotic factors is essentially uniform 

 throughout. Giving the term this precise interpretation, it 

 becomes evident at once that many an area, such as a pond, 

 a ravine, or a salt marsh, which only too frequently is char- 

 acterized as a '^ habitat,." should be regarded rather as a series 

 of habitats. The various factors which, as above indicated, 

 are responsible for the local variations in the nature of the en- 

 vironment and for the consequent delimitation of distinct 

 habitats, in other words, the edaphic and the biotic factors, 

 taken collectively, may be termed the habitat factors, in contrast 

 to the regional climatic factors, the influence of which is wide- 

 spread and essentially uniform throughout the region. 



Although the natm^e of the habitat affords the actual criterion, 

 plant associations, as a rule are most readily distinguished 

 in terms of their vegetation; thus, in the case of a pond: the 

 Nymphaea association, the Scirpus-Typha association, etc. 



Floristic subdivisions of the association. In its ecological 

 aspect an association is essentially homogeneous throughout. 

 Floristically, however, it is commonly subject to more or less 

 variation. Where the variation concerns the dominant species 

 {fades) it is often possible to distinguish different consociations, 



