4 BOTANICAL GAZETTE [SEPTEMBER 
unit characterized by an essentially definite physiognomy and 
ecological structure, and by an essentially definite floristic composi- 
tion as regards dominant species; (3) that plant association in the 
concrete be defined somewhat as follows: a plant community of 
essentially uniform (or homogeneous) physiognomy and ecological 
structure and of essentially uniform (or homogeneous) floristic 
composition as regards dominant species. This simply formulates 
the more or less unconscious practice of most ecologists, who, when 
speaking of “‘a Scirpus-Typha association” have a concrete com- 
munity in mind, while “the Scirpus-Typha association” of a certain 
region is plainly an abstract concept. 
The next higher unit is also generally recognized as the forma- 
tion, but there is not yet the same agreement in regard to its content 
as there seems to be for the association. The following brief survey 
of the progress of opinion in regard to the formation is summarized 
from TANSLEY (9). According to the definition adopted by the 
Brussels Congress, the formation is composed of associations which 
differ in their floristic composition, but are in agreement (1) with 
the conditions of the habitat, and (2) as regards their growth forms. 
TANSLEY says, “‘ Though this concept is apparently accepted by most 
European phytogeographers, it has little real hold on actual concrete 
research because it is abstract and one sided.” In 1907 Moss (5) 
proposed a unit, later embodied by Tanstey (8), in which all 
associations developed on the same habitat or on one of essentially 
constant character were considered as belonging to one formation. 
Not all the stages of a succession were necessarily included in one 
formation. If the habitat obviously changed its character com- 
pletely, it was recognized that a new formation had been initiated. 
This conception was widely criticized, and TaNsLEy admitted the 
validity of criticisms of the habitat element in the definition. 
CLEMENTS (1) refused to recognize any formations except those de- 
termined by climate, regarding all communities in a region where 
forests are climatically possible only as stages in the development 
of forest formations. ANSLEY believes that this view has not been 
generally accepted in Europe or in America, and feels that the uni- 
versal dominance of climatic factors as determinants of climax 
vegetation has not been proved. 
