302 PLANT SOCIOLOGY 



form, larger and smaller physiognomic units of vegetation (formations, 

 synusiae) which are entirely independent of floristic composition. 



Synusia. — By "synusia" is meant a natural community of species 

 belonging to the same life-form groups and with uniform ecological 

 requirements. A cover of crustose lichens, a pure carpet of moss or of 

 dwarf shrubs, the tree layer of a fir stand are ecological synusiae. 



In the simplest case the association may coincide with a synusia 

 (fruticose lichen carpet, algal cover) (Fig. 152). 



As a rule, however, an association includes several, and in the case 

 of a forest association many, ecological synusiae. With advances in 

 the organization of the community the number of snyusiae increases. 

 On one and the same locality several synusiae may follow one another 

 in the course of a season. These synusiae then correspond closely to 

 the seasonal aspects of the association. The Juncus capitatus-Isoetes 

 duriaei association of southern France includes a winter synusia of 

 liverworts and a geophyte-therophyte synusia in the vernal aspect. 



Formation. — The combination of two or more synusiae with uniform 

 physiognomy and more or less uniform habitat requirements constitutes 

 a "formation." The formation is entirely independent of floristic 

 composition and may, therefore, occur anywhere under similar external 

 conditions. It depends upon geographic factors only as far as they 

 directly affect the climate. Externally closely corresponding forma- 

 tion in widely separated parts of the earth, with similar physiognomy 

 and in similar climates, are called homologous. To illustrate, the 

 Californian chaparral and the Mediterranean sclerophyll scrub are 

 homologous formations; so also are the north African Artemisia herba- 

 alba, the Aralo-Caspian A. maritima and A. pauciflora, and the A. 

 iridentata steppe of the United States. Formations with similar 

 physiognomy compose "groups of formations." These are arranged 

 in classes, and the latter finally in "types of vegetation. " The vegeta- 

 tional types of the earth have been used ever since Grisebach's time in 

 the rough characterization of the various parts of the world. They 

 cannot, however, be used as permanent bases in the nomenclature and 

 taxonomy of plant communities, and it will be wise to use the terms 

 "formation," "groups of formations," "vegetational types," etc., with 

 caution in the field of plant sociology. 



