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 intermediate 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 Cladium 

 Mariscus, Phragmites communis, Molinia caerulea, Calam- 

 agrostis canescens, Juncus subnodidosus, 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 fades 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 (cf. 

 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 



