THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 577 



tude of a species, the phytosociologists have proposed the notion of 

 "fidehty" (Braun-Blanquet, 1932, 1951). A perfectly faithful species is 

 virtually not found outside the stands of a certain association. Else- 

 where (1952a, 1957a) I have shown some examples of this situation in 

 various regions.* Thus in the St. Lawrence Valley Symplocarpus foeti- 

 diis and Acer saccharinum are strictly confined to the elm-silver maple 

 flood plain community. 



c. Frequency and constancy. Tests by quadratting or line-transects, 

 or point-quadrats will produce a statistical count of the number of 

 times a certain species occurs within a stand (frequency) or among 

 all the known stands of the association ( constancy ) . 



Although abundance favors high coverage and the latter favors fre- 

 quency and frequency favors constancy, it is well established that these 

 notions can (indeed must!) be applied separately and cannot be de- 

 duced from one another. ( Many examples in Dansereau, 1952a, 1957a. ) 



Structure. The arrangement in space of the different parts of a 

 stand of vegetation can be outlined without reference to its composi- 

 tion: the amount, texture, ramification of hard and soft parts, of light 

 and heavy pieces, of flat and cylindrical organs, of green and other- 

 colored appendices, can all be plotted as part of a mechanical device. 

 The differences between two stands of vegetation can then be evalu- 

 ated in purely structural terms. 



Whereas no biologist would think of limiting his description of 

 vegetation by the exclusive application of this method, it is unfortunate 

 that so very few phytosociologists have not complemented their floris- 

 tic inventories with some kind of structure design. This can be done by 

 considering a minimum of three sets of criteria relating to life form, 

 layering, and spacing. I have written a good deal on this subject (see 

 especially 1951, 1958b, 1960), and I will not attempt here a full dis- 

 cussion of the issues and categories involved but will dwell more in 

 detail on some applications to material already analyzed from the floris- 

 tic point of view. 



1. Epharmonic types. In his monograph (1931) Du Rietz quite 

 thoroughly reviewed the question of life forms and growth forms, and 

 he proposed an elaborate new system which was based on a number of 

 factors. It is rather odd that this remarkable work did not immediately 

 stimulate further endeavors along these lines. It may have come at a 

 time when epharmonic responses did not seem a proper object of study, 

 for at that time natural selection itself was little spoken of. In recent 

 years, however, a renewed interest has been shown (Cain, 1950; Cain 



' Brito da Cunha and Dobzhansky (1954) and Brito da Cunha, Dobzhansky, 

 Pavlovsky, and Spassky (1959) have applied this ecological frame to Dwsophila 

 populations in Brazil. 



