370 PLANT SOCIOLOGY 



3. Ecologic differentiation. The more varied the union in respect to layering, 

 life forms and adaptations to social life the more complex and highly developed is 

 the social organism. Communities made up of simple and ecologically similar life 

 forms stand low in the scale of organization (therophytes). 



4. The stabiHty and duration of the communities. Simple pioneer and transi- 

 tion stages are of shorter duration and lower stability than the climatic climax. 



5. Sociologic independence. Dependent communities, such as the algal 

 communities of the Sphagnetum and epiphytic communities, are usually short- 

 lived and simple in structure. 



The higher units (circles of vegetation, classes, orders, alliances) 

 offer no insuperable difficulties to classification according to sociologi- 

 cal progression. The sequence of the circles of vegetation is deter- 

 mined by the level of organization of the dominant climax communities. 

 First come the extraregional circles of air, water, and perhaps of the 

 soil. Then follow the circles of polar regions, deserts, steppes, grass- 

 lands, and scrub; and finally those of scrub and of forest, each accord- 

 ing to its own stage of sociological development. 



In describing the plant communities of a circle of vegetation, 

 or of any delimited area, the arrangement of the units down to alliances 

 should be according to sociological progression. But there is little 

 use in extending this arrangement to the associations and their 

 subdivisions. The steps are here too close together, and the gradations 

 too minute. Floristic relations suffice to determine the sequence of 

 the lower units. 



4. Classification of Plant Communities According to Sociological 



Progression 



A. Extraregional one-layered communities of ecologically simple, similar, 

 lowly, and unadapted organisms, with extremely loose union. 



I. Floating communities of unstable composition. 



1. Composition of communities subject to rapid, often sudden, change 

 (Aeroplankton). 



2. Composition more stable, often subject to cyclic rhythms (Hydro- 

 plankton); 



II. Communities with more or less stable constituents. 



1. Populations of few species, on the surface of ice or snow, characterized 

 by great mobility of the constituents (Cryoplankton). 



2. Bacteria, algae, and fungi of soil, often with active interdependence 

 (Phy toedaphon) . 



B. Regionally delimited distinctly layered communities, mostly of rather high 

 organisms with stronger mutual dependence. 



I. One-layered communities without root competition. 

 1. Free swimming (Pleuston). 



a. Oceanic. Sargassum-like communities. 



b. Limnetic. Lemnoid communities of Lemna, Azolla, Salmnia, ete. 



