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 (1.e., heather moor), 
of Eriophorum vaginatum (2.e. cotton-grass moor), and of 
Vaccinium Myrtillus (v.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 Hrica cinerea, of Juncus 
squarrosus, of Carex Goodenowi, 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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