THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 599 



demonstration would tend to show that Quercus rubra is a physiologi- 

 cal relict, inasmuch as its theoretical optimum simply does not occur in 

 North America at the present time. I had already ( 1953 ) argued some- 

 thing of the sort to explain the observed ecological strategy of die three 

 northern pines in Eastern North America (Finns sfrobus, P. resinosa, 

 P. banksiana) in the face of palynological evidence of their regional 

 dominance in the early postglacial period. 



A preliminary inventory of Laurentian plant associations (Dan- 

 sereau, 1959) shows tliat only a small number are dominants (in the 

 sense used above). Most species, on the other hand, are not only not 

 dominants or constants but are more often companions, and as likely 

 as not these are cornered species, which is most manifest in their posi- 

 tion on the margins between two communities. Such are Hystrix patiih 

 on the edge of deciduous forest and field, Cornus stolonifera on the 

 edge of alder thickets and grassy marshes, etc. 



Conclusions 



The present essay poses many questions and answers very few. In 

 fact, the convergence of criteria that is called for here is relatively new, 

 and the complete data that would allow its application are not avail- 

 able from many parts of the world. The very idea of a functional 

 analysis of vegetation has not been the object of much investigation so 

 far. 



I think I have been bold enough in outlining some generalities, 

 which I shall attempt to restate tentatively. 



1. A multidimensional description and grading of the anatomy of 

 plant communities can be attempted if measures are made which 

 respond to the following criteria: the richness and indicator value of 

 the floristic composition; the distribution of structural elements, show- 

 ing life form, layering, and spacing. 



2. An understanding of the physiology of the plant community 

 will result from recording of periodicity, ecosystematic control, means 

 of exploitation, and degree of stability. 



3. An ordination of plant communities becomes possible through 

 the application of the above. It will turn out that the ecosystematic fit- 

 ness of the species populations assembled will vary so much as to allow 

 very different degrees of cohesion. 



4. The origin and senescence of plant communities is a function of 

 a shifting climatic-edaphic compensation system, which eliminates 

 some elements (species) altogether and allows others to alter their 

 ecological strategy very significantly. By this process, species that over- 

 lap only slightly, either as floristic elements or as members of a valence 



