I] INTRODUCTION 19 



PLANT COMMUNITIES . 



A plant formation is the whole of the vegetation which 

 occurs on a definite and essentially uniform habitat. A plant 

 association is of lower rank than a formation, and is characterized 

 by minor differences within the generally uniform habitat. ' A 

 plant society is of lower rank than an association, and is marked 

 by still less fundamental differences of the habitat. These 

 grades of difference of the habitat are marked by corresponding 

 differences in floristic composition. The three vegetation units 

 may be compared respectively with the taxonomic units of the 

 genus, the species, and the variety ; and differences of opinion 

 arise, both among systematic and geographical botanists, with 

 regard to the precise limits of the respective units. Sub- 

 formations and sub-associations may be recognized; and these 

 would be comparable to subgenera and subspecies. Plant 

 community is a convenient and general term used for a vege- 

 tation unit of any rank. 



Examples of the above types of plant communities are 

 well illustrated in the present district. The acidic peat of the 

 non-calcareous plateaux of the district furnishes a definite and 

 generally uniform habitat; and the whole of the vegetation 

 of this habitat is the plant formation of the acidic peat moors. 

 The plant associations of Calluna vulgaris (i.e., heather moor), 

 of Eriophorum vaginatum (i.e., cotton-grass moor), and of 

 Vaccinium Myrtillus (i.e., bilberry moor) are subdivisions of 

 this plant formation, and are characterized by minor differences 

 within the generally uniform habitat and by corresponding 

 differences in floristic composition. Still less important 

 differences in the habitat may cause certain members of the 

 associations to become locally dominant; and thus, in the 

 heather moor, for example, plant societies, or mere local 

 aggregations of species, occur of Erica cinerea, of Juncus 

 squarrosus, of Carex Qoodenowii, and of other species. 



For the purpose of vegetation maps on a scale of one inch 

 to the mile (1 : 63,360), the most important vegetation unit 

 is the association, as it is, in general, possible to indicate the 

 more important associations on a map of this scale. To indicate 

 the smaller associations and the plant societies, maps on a 



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