20 VEGETATION OF THE PEAK DISTRICT [CH. 
scale of six inches to the mile (1: 10,560) are desirable; and, 
on maps whose scale is a quarter of an inch to the mile 
(1 : 253,440), plant formations could, in general, only be shown. 
It is extremely doubtful if vegetation maps of limited areas on 
a scale much smaller than this can be constructed on a strictly 
scientific basis until some method of classifying formations into 
larger but natural vegetation units has been devised. 
A plant association is a closed association when the ground 
is fully occupied by plants, and when it is dominated either by 
a single species, as in the case of a heather moor, or by two 
or more species all belonging to the same plant form, as in the 
case of some reed swamps. A plant association is an open 
association when the ground is only partially covered with 
vegetation, as in the case of denuding peat moors. Other 
associations are intermediate in character between open 
associations and closed associations. In an intermedzate associ- 
ation, the ground may be more or less fully covered with 
plants; but there is no single dominant plant or plant form: 
there are, in fact, several plants which compete with each other 
for dominance, as on an East Anglian fen, where Cladiwm 
Mariscus, Phragmites communis, Molinia caerulea, Calam- 
agrostis canescens, Juncus subnodulosus, and other plants compete 
in this way. Differences in a single association caused by the 
varying abundance of the constituent species may be spoken of 
as the facies of an association. When, in a plant association, the 
more abundant species become very conspicuous at different 
times of the year, seasonal aspects of associations are produced 
(cf. Clements, 1905: 296 and 315). 
If the succession of associations within a single formation 
is studied, it is found that the initial stages are marked by 
open and unstable associations, that these are followed by inter- 
mediate associations, and these again by stable associations (ef. 
Clements, 1904: 135; Moss, 1907 a: 12). The stable associations, 
however, may degenerate, and give rise to other intermediate 
associations. In the present account of the vegetation of the 
Peak District, the associations will be considered from this point 
of view; and accordingly the following terminology will be used. 
Open and intermediate associations leading up to a stable asso- 
ciation are termed progressive associations: intermediate and 
open associations resulting from the decay of a stable association 
