568 PLANT GROWTH AND PLANT COMMUNITIES 



made on one of the following bases: (1) the boundaries of a regional 

 climate; ( 2 ) a near identity of terrain with respect to the exposure and 

 slope, drainage, and soil texture (at least at the surface); or (3) some 

 outstanding feature of the vegetation itself ( floristic only or floristic and 

 structural). It seems reasonably clear that a single association does in 

 fact describe such communities as cattail in southern Michigan (Se- 

 gadas-Vianna, 1951), aspen groves in Idaho (Lynch, 1955), a red fir 

 forest in the Sierra Nevada (Costing and Billings, 1943), a live-oak 

 forest in southern France ( Braun-Blanquet, 1936), a spurge forest in 

 the Congo (Lebrun, 1947), an Ocotea catharinensis rain forest in South- 

 ern Brazil (Veloso and Klein, 1959). The data collected show a great 

 deal of repetition of the same floristic and ecological features— in fact, 

 a combination thereof which is unique. Because of the existence of 

 such assemblages, many investigators of vegetation have been led to 

 define plant associations largely on the basis of floristic coherence 

 (Braun-Blanquet, 1928, 1932, 1951; Guinochet, 1955) and irrespective 

 of the general structure or spatial assemblage within the individual 

 stands. Others have laid a great deal more stress on structure and have 

 been unwilling to give ultimate pre-eminence to the floristic criterion 

 (Dansereau, 1951, 1958b, 1959). 



A number of workers in the field believe that it is indeed possible 

 to define an association regionally by sampling a number of stands. 

 They think that the interaction of floristic history, geological and pedo- 

 logical processes, and the repeated occupancy of contiguous landscapes 

 by wave upon wave of vegetation in the past have left a residue of 

 species very unevenly fitted to fill the available ecological niches, 

 which are themselves, as often as not, sharply discontinuous. In no 

 other way can the phenomenon of dominance have arisen. 



On the other hand, the view stated above is not mathematically 

 proved, and it has seemed more economical to many ecologists to re- 

 main within Gleason's (1926) reserve and to consider each stand as 

 unique and therefore each concrete community as an entity not refer- 

 able, together with other stands, to an association. Others, such as 

 Curtis and his collaborators ( 1959 ) , take in a broader piece of vegeta- 

 tion than the individual stands and ordinate them in a continuum. I am 

 not sure that the measurements of the continuum produced so far 

 provide proof of the nonexistence of the association. The sample units, 

 after all, are not contiguous, and the statistical treatment to which 

 they have been subjected results in just as "abstract" a construction as 

 the association. Moreover, some of the criteria are apphed as a result 

 of general views of the areas surveyed. 



All students of vegetation probably agree that both continuity and 

 discontinuity exist, and that their interplay is responsible for the emerg- 



