6 Proceedings of the Ohio State Academy of Science. 



tween the concepts of formation, association, and society; but 

 as to the value of the respective terms. Griesebach^^ was the 

 first to employ the term formation in connection with phyto- 

 geography. To him a "phytogeographical formation" is a group 

 of plants which has a fixed physiognomy, such as a meadow or 

 forest". Warming^^ writes : — "A formation may then be defined 

 as a community of species, all belonging to definite growth 

 forms, which have become associated together by definite ex- 

 ternal (climatic or edaphic) characters of the habitat to which 

 they are adapted. Consequently, so long as the external condi- 

 tions remain the same, or nearly so, a formation appears with a 

 certain determined uniformity and physiognomy, even in dif- 

 ferent parts of the world, and even when the constituent species 

 are very dififerent and possibly belong to difi:'erent genera or 

 families." 



Hence Warming's concept agrees with that of Griesebach in 

 making physiognomy the controlling factor in a formation and 

 uses this as the basis of classification. Clements" also sees the 

 formation as an organic unit conditioned by the habitat and it 

 must therefore be co-extensive with the latter. Consequently 

 his classification of formations is based on habitat. Adams* 

 says : — -"A formation or climax society is composed of a rela- 

 tively limited number of species which are dominant in a given 

 environment of geographic extent. Such dominance implies ex- 

 tensive range, relative abundance and ability to indefinitely suc- 

 ceed or perpetuate itself under given conditions." And 

 Schimper''- recognizes two ecological groups of formations, 

 climatic or district, composed of three chief types, woodland, 

 grassland and desert; and edaphic or local formations, which 

 are conditioned by the soil ; as swamp, rock, sand dune, etc. 

 Graebner-'- advocates making the percentage and nature of in- 

 organic salts dissolved in the soil water, the controlling factor 

 in the character of vegetation. He therefore recognizes the 

 groups of formations; i, where the water contains a high per 

 cent of mineral salts ; 2, where the per cent of mineral salts 

 (supposedly available to plants) is low; and 3, saline water. 



