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GEORGE E. NICHOLS Vol. IV, No. I 



be distinguished, but the one generally recognized as constituting the funda- 

 mental unit of vegetation is the plant association. The subject-matter of this 

 paper falls more or less naturally into two parts, the first (sections II, III) 

 dealing with the plant association as a vegetation-unit and in its relation to 

 environment, the second (sections IV- VII I) with the ecological classification 

 of plant associations. 



Use of Terms. As a preliminary step to the classification of plant com- 

 munities it is necessary that the various concepts upon which the classification 

 is to be based should be definitely formulated; and for convenience in ex- 

 pressing concepts of any description it is imperative that a rather definite 

 terminology should be adopted. In this connection, however, it should be 

 emphasized that a distinction must always be drawn between a concept and 

 the term by which a concept is designated. In itself a concept may be sound, 

 while the choice of terms may be unfortunate. The two must be judged 

 independently. Given a sound concept, its terminology can be determined 

 only by general usage or definite agreement. It is believed that the concepts 

 advanced in the present paper are logical ; time alone will decide whether the 

 choice of terms has been a happy one. 3 



Acknowledgments. Among those to whom I feel indebted for stimulating 

 and suggestive advice in connection with the preparation of this paper are 

 Prof. W. S. Cooper, Prof. A. W. Evans, Dr. H. A. Gleason, Dr. A. F. Hill, 

 Dr. Ellsworth Huntington, Mr. Barrington Moore, Dr. A. G. Tansley, and 

 Mr. Norman Taylor. 



II. The Plant Association as the Fundamental Unit of Vegetation 



A. The Essential Characteristics of the Association 

 The Association as a Concrete Piece of Vegetation. Expressed most 

 simply, a plant association may be described as a group or community of 

 plants which occupy a common habitat. Technically speaking, however, not 

 all plant communities of this description are recognized as associations ; it is 

 only those in which the vegetation, taken in its entirety, exhibits certain more 

 or less definite characteristics. To be regarded as an association, a piece of 

 vegetation must be essentially similar throughout its extent in physiognomy, 

 in ecological structure, and in floristic composition. In other words, essential 

 homogeneity in these three particulars is the fundamental criterion of the 

 association. Thus conceived, a plant association may well be epitomized as a 

 "qualitatively and quantitatively homogeneous plant community" (Raunkiaer, 

 '12; Nordhagen, '19). 



Criteria of the Association. By the physiognomy of a plant community 



3 For the most part, the terms here used are already more or less familiar. Some 



of these have had the misfortune to be variously employed by different writers (the 



term formation, for example), but it has seemed better, on the whole, to retain these 



rather than to introduce new terms. 



