334 PLANT SOCIOLOGY 



more or less neutrophilous Elynetum into the extremely acidophilous 

 Curvuletum (Fig. 168). The successive increase of the acidophilous 

 Curvuletum species in the Elynetum, as shown by the cuiv^e of the 

 acidophiles, corresponds perfectly with the increasing acidity of the 

 individual stands. The terminal phase of the Elynetum corresponds 

 with the initial phase of the Curvuletum. 



Block Diagrams. — Block diagrams serve to bring out clearly the 

 share of certain dynamically important species or groups of species in 

 different genetically related plant societies. Using this method (Fig. 

 169), Cooper (1922) gives an expressive picture of the changes in 

 abundance of six important shrubs in the secondary succession from 

 pasture land to climax forest in New England. 



Spiraea tomentosa, present in 40 per cent of the pasture plots, is 

 absent from the climax forest, whereas Viburnum alnifolium and Taxus 

 canadensis continually increase throughout the natural course of 

 development, reaching their maximum in the climax forest. 



4. Syngenetic Classification. — The results of more intensive studies 

 of succession have demonstrated the need of arranging the develop- 

 mental series for more ready purview. The units to be classified are 

 the individual seres. On general principles, two arrangements are 

 possible. One possibility is based on the conditions of the habitat, 

 especially the beginning stages. Thus we may distinguish groups 

 of seres on river silt, on limestone talus of alpine levels, on wandering 

 sand dunes, in running water, etc. 



The other possibility rests upon the principle of development itself 

 and especially upon the similarities in development: type and duration 

 of the developmental process, number of stages involved and their 

 parallelism, and the correspondence of climax stages. This type of 

 grouping appears scientifically more satisfying. Thus we would place 

 first the shortest seres with the smallest number of steps, leading to the 

 simplest terminal stages, sociologically and ecologically. The further 

 sequence, according to the sociological progression of the groups of 

 seres, corresponds (1) to increasing differentiation of the terminal 

 stage (climax) through more intense competition and (2) to the larger 

 number of stages and the consequent longer duration of the 

 development. 



Accordingly, four primary groups may be distinguished: 



A. One-layered, primitive terminal communities. 



1. Competition for space only. 



I. Plankton seres, floating societies of air, water, and snow. 



2. Competition for space and nutriment. 



