212 THE SCOTTISH BOTANICAL REVIEW 



present, especially in damper stations. On the embryonic dunes 

 formed by this species few associated plants occur, principally on 

 account of the more exposed situation. These hummocks are sub- 

 jected to frequent denudation by the wind, and their bases commonly 

 present a weathered appearance. In the fixed dune association 

 where Pteris is the dominant plant, the amount of shade cast by its 

 fronds is relatively greater, and the dependent species are mostly 

 grasses, Agrostis sp., etc. These three dominant species are rhizomic 

 in habit, and are specially adapted to hold their own under apparently 

 unsuitable edaphic conditions. 



Extent and Distrihutioti of these Associations. 



The Marram grass association extends inland as far as Kenfig 

 Pool, where its outlying stations dovetail with those of the Salix 

 association. This latter covers a broad area on the alluvium on 

 the seaward side of the pool, and extends laterally on its flanks. It 

 gradually loses its identity in the bracken association of the fixed 

 dunes. Distributed throughout these chief associations are minor 

 associations which are of local occurrence. Ruhis ccesius, the dew- 

 berry, for instance, forms a community of this description. This 

 species occurs in certain situations on the dunes, and represents, 

 with Pteris and Salix, the last stages in succes.sion. Sometimes 

 associated with it is a hybrid form of Ruhus which gives rise to 

 hummocks. In addition to these principal associations many sub- 

 associations and plant societies exist. These will be described at 

 a later stage. 



The term " plant association " implies a plant community, and it 

 therefore follows that the dominant types, mentioned above, have 

 associated with them species which may be regarded as subdominant, 

 abundant, or occasional, according to the frequency of their occurrence. 

 Some species are apparently always associated with a particular 

 dominant, but on the dunes there are many cosmopolitan types 

 which are not limited in their distribution to any one association. 

 A list of subordinate species occurring in anyone plant community 

 does not therefore imply that they are characteristic of that associa- 

 tion alone, or that they do not occur as frequently in any other 

 association. The majority of associated species colonise the sheltered 

 "flats" and dune valleys which wind in all directions among the 

 high dunes. The configuration of these valleys is varied, and some 

 are characterised bv a flijra quite distinct in composition from that of 

 a neighbouring depression. Some are marsh-like in character, while 

 others are undulating plains of blown sand, with embryonic dunes in 

 various stages of formation. The species which occupy the dry 

 hollows are psammophilous, although some, like Cnicus arvensis, 

 which frequently occurs, are as characteristic of other formations. 

 On the fixed dunes sward-forming plants and others are associated 

 with Pteris and Ammophila, while many species of moss form patches 



