82 TRANSACTIONS OF THE [Sess. lxxvi. 



rhizomic habit of growth makes it likewise a successful 

 sand-binder, and it forms the last stage in succession. 

 According to Massart, Pteris is entirely absent from, or 

 very rare on, the Belgian dunes. It has here undoubtedly 

 adapted itself to life on the dunes and grows luxuriantly. 

 The fronds are regularly harvested for cattle - bedding. 

 Although it is such an abundant type on the " grey " dunes 

 near Kenfig pool, it is sparsely represented on the adjoining 

 boulder clay. This may be due, in part, to the fact that 

 the latter is under cultivation, or perhaps its absence may 

 be accounted for by the more or less calcareous nature of 

 the clay ; the bracken being regarded by some authorities 

 as a calcifuge. 



In these three chief associations the subordinate species 

 vary according to the character of the association. The 

 Marram grass association is essentially an open one, and 

 on the seaward side of the first range of " mobile " dunes it 

 is an almost pure association. As a result of this open 

 character, competition among the associated species is re- 

 duced to a minimum. The Salix repens association is of 

 a less open nature. The habit of growth of the Salix 

 affords protection from blowing sand, and excessive insola- 

 tion, to the types which it shelters. At the same time, a 

 certain amount of humus is 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 subjected to 

 frequent denudation by the wind, and their bases commonly 

 present a weathered appearance. In the fixed dune asso- 

 ciation 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 Distribution 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 



