118 STRUCTtTRE AND UNITS OF VEGETATION. 



independent chief association which finds a natural end in itself and which 

 consists of similar or related life-forms in a habitat with the same conditions for 

 existence (altitude, exposure, soil, water) is a vegetation formation. It is 

 assumed that an essential change can not occur on the site of such a com- 

 mimity without external changes: the community is 'closed.' " 



Clements's concept. — Clements (1905 : 292) placed particular emphasis on 

 the habitat as determining the formation concept and as affording a more 

 accurate basis for recognizing and delimiting formations. 



"In vegetation, the connection between formation and habitat is so close 

 that any application of the term to a division greater or smaller than the habi- 

 tat is both illogical and unfortunate. As effect and cause, it is inevitable that 

 the unit of the vegetative covering, the formation, should correspond to the 

 unit of the earth's surface, the habitat. This places the formation upon a 

 basis which can be accurately determined. It is imperative, however, to have 

 a clear understanding of what constitutes the difference between habitats. 

 A society is in entire correspondence with the physical factors of its area, and 

 the same is true of the vegetation of a province. Nevertheless, many societies 

 usually occur in the same habitat, and a province contains many habitats. 

 The final test of a habitat is an efficient difference in one or more of the direct 

 factors, water-content, humidity, and light, by virtue of which the plant cover- 

 ing differs in structure and in species from the areas contiguous to it. This 

 test of a formation is superfluous in many cases where the physiognomy of the 

 contiguous areas is conclusive evidence of their difference. It is also evident 

 that remote regions which are floristically distinct, such as the prairies and the 

 steppes, may possess areas physically almost identical, and yet be covered by 

 different formations." 



This concept of the formation recognized both the physiognomic and floristic 

 sides, but assigned the chief value to the habitat because of its fundamentally 

 causative character. The habitat was regarded as something to be measured 

 and studied exactly, with the object of determining the causes of the develop- 

 ment and structure of communities, and hence arriving at the real limits of 

 the formation and its divisions. 



Moss's concept. — ^Moss (1907 : 12) was the first to take development into 

 account in determining formations: 



"A plant association in which the ground is carpeted in this sparse manner, 

 with patches of bare soil here and there, is spoken of as an open association. 

 An open association represents an early stage in the succession of associations 

 which finally lead to a closed association, when the groimd is fully occupied by 

 one or a very few dominant plants, and a period of stability of vegetation has 

 been reached. The series of plant associations which begins its history as 

 an open or unstable association, passes through intermediate associations, and 

 eventually becomes a closed or stable association, is, in this paper, termed a 

 plant formation." 



Moss (1910 : 35, 36) states in a later paper that he: 



"Followed many previous authors in delimiting formations primarily by 

 habitat, and then subdividing the formations into associations. This writer 

 laid stress upon the succession of plant associations, especially on the succes- 

 sion of associations within the same formation. It is necessary to distinguish 

 the series of associations within a whole succession, that is, the succession 



