64 



"The formation is the unit of vegetation. It is the dimax 

 community of a natural area in which the essential climatic 

 relations are similar or identical." It is an organic entity 

 covering a definite area marked by a climatic climax. It con- 

 sists of associations which are actual parts of the area with distinct 

 spatial relations. The author distinguishes in North America 

 ten forest climaxes or formations, six scrub formations, a prairie- 

 plains climax and the tundra formation. 



The subordinate vegetational units are either climax or de- 

 velopmental; the relation of these units to the formation, the 

 relation of the units of each series to each other and the corre- 

 spondence of the units in the two series is shown in the following 

 table. 



Formation 

 Climax Units: Serai Units: 



Association Associes 



Consociation Consocies 



Society Socles 



Clan Colony 



Family 



The need of a set of units to distinguish serai from climax 

 phases of vegetation is felt by ecologists who are interested in 

 the study of the development of vegetation, while to others, as 

 the author points out, this may seem an unnecessary refinement. 

 That the system of vegetation outlined is not only logically 

 complete but that it corresponds well with actual phenomena of 

 vegetation is shown in addition to the author's own illustrations 

 by its application to vegetation in Minnesota by Bergman & 

 Stallard* and to three plant formations in the Pacific Northwest 

 by the reviewer. + 



The term association is restricted "to those climax communities 

 which are associated regionally to constitute the formation." 

 Associations agree with their formation in physiognomy and 

 development, and are recognized chiefly by floristic differences. 

 Thus the prairie-plains formation consists of three associations; 



* Bergman, H. and Stallard, H., Plant Succession in Northern Minnesota, 

 Minnesota Botanical Studies, Vol. 4, No. 4, 1916. 



t Weaver, J. E., Ecological Studies in Southeastern Washington and Adjacent 

 Idaho, Univ. Nebr. Studies 17: i, Jan. 1917. 



