590 



PLANT GROWTH AND PLANT COMMUNITIES 



writes of oxyphytia, hyperxerophytia, or lithophytia, he refers to the 

 whole metabohsm and to the chain of cychc responses of a certain 

 group of plants which show various degrees of well-being under these 

 more or less stressful conditions. This scheme, which comprises 25 

 alternatives plus a number of combinations thereof (see del Villar, 

 1929; Cuatrecasas, 1934), is non-geographic and therefore lends itself 

 to a completely independent application. 



Del Villar himself plotted it on a small-scale map of Europe ( 1929, 

 p. 208). But it can just as well be used on a larger scale, as Cuatrecasas 

 ( 1934 ) has done in grading the mountain zones of Colombia and as I 

 have done for the St. Lawrence Lowlands ( Dansereau, 1960 ) . Table II 

 shows an application to the 14 stands already graded for other features. 



Again the mere label "tropophytia" or "oxyphytia" is nothing more 

 than a key to the prevalent influence in a stand, an association, or a 

 region. It remains to demonstrate, by evaluating the ecological ampli- 

 tude of all component species, how strongly tropophytic or oxyphytic 

 the area is, and this mostly by comparison with other comparable areas. 



Here a quantitative plotting of index values will provide the kind 

 of evidence needed. For instance, the prevalent reaction to light and 

 moisture in the Laurentian maple forest species (Dansereau, 1943, 

 1946) or in Quebec pasture associations (Dansereau and Gille, 1949) 

 or in Wisconsin forests ( Curtis, 1959 ) allows the elaboration of cumu- 

 lative indices which fairly characterize diflFerent kinds of vegetation 

 and place them in some linear or multidimensional order reflecting 

 their ecological affinities. 



Exploitation and turnover. Much attention has been given in recent 

 years to the yield of communities, and various experiments have been 

 performed to implement Lindeman's (1942) concept of the "trophic- 

 dynamic" nature of the plant community. Biogeochemical studies are 

 increasingly numerous and have cast a great deal of new light on the 

 metabohsm of ecosystems. H. T. Odum (1956, 1957, 1960) has ad- 

 vanced considerably along these lines by either estimating or actually 

 measuring energy flow. I can only agree that what we are seeking is a 

 bioenergetic law, but I am rather uncertain that our understanding of 

 ecosystematic anatomy and physiology is such as to allow us, at this 

 time, to measure the right things. I have argued elsewhere (1958a) 

 that we should have in mind: (1) the total amount of resources of each 

 kind and their microdistribution; (2) the percentage of bound and free 

 resources; (3) the degree of utilization of all resources by the living- 

 members of the ecosystem; (4) the relative capacity of plants (of 

 different physiological kinds) to liberate resources, to transform them, 

 and to feed them back into the ecosystem. 



The major taxonomic groups, of course, provide us with some ob- 

 vious differences in the tapping mechanism of plants: algae, lichens. 



