THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 569 



ing pattern. At all events, increasingly sharp tools are being used for 

 the measurement of plant communities. The work of Evans (1952), Cain 

 (1943), Goodall (1954), Gounot (1957), and many others promises a 

 more precise means of defining the ways of vegetation and the scope 

 of its units. 



In the present contribution, however, I do not pretend to assess the 

 "objective reality" of the association and even less to present a new 

 synthesis of the problem. I am frankly taking the view that the plant 

 association does indeed exist. I have myself made studies of a certain 

 number of them in the field in many different areas, and I can at least 

 point out the significant elements that are worth considering as valid 

 criteria and mechanisms. In so doing I propose to review the anatomy 

 and physiology of the plant community and to provide a tentative ex- 

 planation of how some associations are defined sharply, others very 

 loosely— and why. 



The space and time allowed hardly permit a redefinition of all the 

 terms employed in this contribution, even less a justification of all the 

 assumptions, although I hope that the latter are mutually consistent. It 

 is also impossible to elaborate upon the various criteria to be used in 

 the analysis of communities, and I shall all too often have to refer to 

 previous papers (especially 1951, 1952a, 1956a, 1956b, 1957a, 1958b). 



The parts that make up a plant community are populations of one 

 or more species. In a given stand, each species has its own (complete 

 or incomplete) cycle, its growth form, its minimum and maximum 

 height and breadth, and is represented by a more or less variable num- 

 ber of individuals. Each specific population is arranged according to a 

 pattern which reflects its relative fitness to exploit the total resources 

 of the environment. The biomass as a whole also shows a greater or 

 lesser stability. 



In order to understand such a dynamic equilibrium, it is necessary 

 to examine the relative ecosystematic fitness of the species populations. 

 This can only be done by taking the pieces apart to study their nature 

 and structure and also to observe the behavior of the parts and of the 

 whole. Thus an analysis of floristic composition and of structure will re- 

 veal the "anatomy" of the plant community, and a study of its periodic 

 fluctuations, methods of tapping resources, and tendencies to floristic 

 and structural change will reveal its "physiology." 



Such an analysis will serve to define, one by one, the units of the 

 plant cover. It has been well established, in a large number of cases, 

 that such units are to be found in essential integrity througliout a fairly 

 large geographical area. If we dare to pose the problem of their origin 

 at all, we must therefore look to past vegetational and climatic history 

 in order to account for the present-and perhaps temporary, although 



