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GEORGE E. NICHOLS Vol. IV, No. 2 



Where to draw the line between the association, considered in the abstract, 

 and the association-type is a question which can no more be answered dog- 

 matically than the question of where to draw the corresponding line between 

 genus and species. Thus, in the case of the genus Pyrus, Gray's " Manual " 

 places under one genus the species which Britton's ''Flora" separates into 

 four. In so far as an association-type is limited to a geographically continu- 

 ous region, the various concrete associations by which it is represented are 

 commonly coordinated floristically, in greater or less degree, by the general 

 occurrence of certain "binding species." 32 Thus the mesophytic deciduous 

 broad-leaf forests of the eastern United States are characterized throughout 

 their range by the presence of the beech and the sugar maple. It is quite 

 possible in such a case, by interpreting the floristic criteria very broadly and 

 laying stress only on these binding species, to refer all the individual associ- 

 ations of such a type to a single abstract association ; in other words, to apply 

 the term association in the abstract to exactly the same unit, for this region, 

 as the term association-type. 33 



But, on the other hand, leaving out of consideration the binding species, 

 these same individual associations exhibit very considerable floristic diversity 

 in different localities and the general tendency, among those who have given 

 most study to the floristic phase of plant sociology, 34 seems to be to delimit 

 associations in the abstract rather narrowly on a purely floristic basis. As 

 concepts, the relation between the association abstract and the association- 

 type seems pretty clearly defined, and when it comes to the application of 

 these concepts the ecologist has to use individual judgment and common sense, 

 taking into consideration both the facts and the point of view from which he 

 is working, just as the taxonomist has to do in applying taxonomic concepts. 

 Strictly speaking, it may be stated that an association-type commonly embraces 

 several to many floristically defined abstract associations; but in plant soci- 

 ology, as in taxonomy, when it comes to the treatment of particular cases, it 



[= association-type] — termine biologico — corrisponde essatamente Vassociasione — termine 

 floristico." In a recent letter (4 Jan., 1922) Negri expresses the view that " The plant 

 association can be denned only by its floristic composition ; to physiognomy and ecological 

 structure corresponds the concept of plant formation. The two terms express two differ- 

 ent appreciations of the same plant population of a definite piece of ground subject to a 

 definite local climate. 



33 This term is taken from an unpublished manuscript by Prof. W. S. Cooper. 



33 In my own work I am inclined to thus emphasize the association-type at the expense 

 of the association abstract. I tend to refer the individual concrete associations to par- 

 ticular types, distinguishing the abstract associations only where these are clearly and 

 unmistakably differentiated. 



34 Recent work on the part of European plant sociologists has contributed much to- 

 ward the detailed understanding of the association, more especially with reference to its 

 delimitation from the floristic point of view. See in this connection recent papers by 

 Pavillard ('19), Du Reitz et al. ('20), Du Rietz ('21), and Braun-Blanquet ('21). 



