584 PLANT GROWTH AND PLANT COMMUNITIES 



3. Spacing and coverage. Stratification is not all, of course, and is 

 scarcely separable from coverage. The components of coverage are the 

 size and spread of individual components (whether considered sub 

 aspectu speciei or sub aspectu formae biologicae), their spacing, and 

 the availability of light, moisture, etc. However, none of these consid- 

 erations is necessary to a plotting of the space occupied, and they are 

 undesirable if the correlation with site or site factors is indeed q.e.d. 



The mobile scale I have used ( 1951, 1958b ) would seem to allow 

 an indefinite number of stratification-coverage combinations. I have 

 argued that this is not the case and that most formation types fall into 

 one of the following ten ( which I have defined and illustrated ) : for- 

 est, woodland, savanna, scrub, prairie, meadow, steppe, desert, tundra, 

 and crust. Some of these terms (used in order to avoid the coining of 

 new words ) are employed in a narrower or broader sense than is gener- 

 ally accepted. At all events, it is worth pointing out that they have no 

 obligate geographical connotation! 



The 14 stands of vegetation have been labeled in this way in Table 

 II. It may or may not appear that identical formation types have differ- 

 ent inner dynamics, and this, precisely, is the subject of physiological 

 rather than anatomical investigation. 



It might remain, in terms of structure, to define further some bio- 

 topic peculiarities, such as various niches within the general matrix. 

 The system already used provides for epiphytes and crusts to be shown. 

 To this could possibly be added microrelief features of the kind illus- 

 trated in Jovet (1949) and Dansereau (1957a, Figures A-4 and A-6). 



Physiology of the plant community 



The plant community is a part of the ecosystem: its living vege- 

 table matrix rests upon and penetrates the inorganic and the dead 

 organic material. Roots, stems, leaves, and fructifying parts carry on a 

 latent or variously active exchange with the gases, liquids, and solids 

 that surround them. The functions of the unit as a whole are governed 

 by the aptitudes of the component species, many of which are bound 

 to have nearly identical requirements and/or responses at the same 

 time. Such functions must therefore be analyzed, much in the same 

 way as the forms mentioned above, in order to establish their relative 

 importance in the community and to draw comparisons with other 

 communities. Here again, each one must be assessed independently 

 of all others. 



Periodicity and dispersal. Most plant communities are adjusted to 

 seasonal variation. This has been detected even in wet tropical areas 

 (Veloso, 1945). In the so-called temperate zone, periodicity is over- 

 whelmingly in evidence. Temperate forests and tropical savannas, in 



