THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 583 



Such categories are certainly useful, inasmuch as they take notice 

 of certain outstanding qualities of the parts ( species populations ) that 

 make up a stand of vegetation. The biological spectra constructed from 

 these data reveal how many or what percentage of the taxonomic units 

 contribute to a given form. However, in order to project the spatial im- 

 portance of each category (now divorced from its taxonomic appurte- 

 nance), a quantitative coffiecient is required. On the scale of plant com- 

 munities, a very striking difiFerence can be shown between the per- 

 centage of life forms specieswise and masswise, as Table VI and VII 

 have indicated. Another instance can be quoted: in three aquatic com- 

 munities ( Dansereau, 1945 ) the rush-like life form was represented by 

 four or five species, which ran to percentages of total flora of 18.5, 

 17.9, and 20.0. The use of a quantitative coefficient, however, raised 

 the value by ten times for one of the three communities. A similar dis- 

 crepancy was noted in two field communities (Dansereau and Gille, 

 1949), where percentagewise the therophytes rated 17.54 and 9.53 per 

 cent, whereas masswise they were respectively reduced to 8 per cent 

 and 3 per cent! 



2. Layering. Once the elementary pieces have been detected, spe- 

 cies by species, and lumped according fo the category they belong to, it 

 remains to assemble them vertically (layering) and horizontally (cover- 

 age ) . Although it may seem artificial to separate stratification from cov- 

 erage ( see Cain and Castro, 1959, pp. 223-233 ) it has seemed useful to 

 me to employ independent scales so that any number of actual com- 

 binations can be represented. 



I have reviewed elsewhere ( 1960 ) the different approaches to the 

 methodology of measuring layering. I shall be concerned here only 

 with the system I have myself applied in various publications and in 

 the tables of the present contribution. 



Layering is an adjustment not only to the qualities of the site but 

 also to the interactions among the species and even the life forms. The 

 ceiling imposed upon an individual is a function of its morphogenesis, 

 but the time and rate at which this maximum vertical extension can be 

 attained are controlled by complex immediate conditions, involving 

 root competition as well as light, crowding, nutrients, etc. The more 

 the reason to measure layering independently. 



The scale already used (Dansereau, 1958b) provides for seven 

 layers. This rigid frame, although it does not reflect the complex reality 

 of some stands, is very useful for comparison purposes. If a necessary 

 distinction is made between layer and synusia (see Dansereau and 

 Arros, 1961), the dynamics of layering immediately become apparent. 

 However, dynamics are not primarily involved: it is the spatial distri- 

 bution at a certain time and place that is of major interest and that this 

 graphic method purports to illustrate. 



