1911-12.] BOTANICAL SOCIETY OF EDINBURGH 87 



on the "shifting 7 ' dunes, usually in somewhat exposed 

 situations. HippophaS rhamnoides, which forms a dense 

 scrub on the Norfolk dunes, and is also reported by Mobs 

 as frequent on the Somerset sand hills, does not occur. 



tation of the Fixed Dv 



The transition from the associations of the " mobile " 

 dunes to those of the " fixed " dunes is gradual. Fixation 

 by Salix, Rubns, and sward-forming plants, like Festuca 

 rubra, begins in the more sheltered hollows, near the inner 

 limit of the shifting dunes. In the more exposed stations 

 the Marram grass still holds its own, and on the dune 

 grassland, where Pteris is absent, it remains a dominant 

 type. Where dune grassland gives place to cultivated 

 land, bush vegetation and trees form a final barrier to the 

 invasion of the sand. 



Pteris aquilina Association. 



In the more open stations of this association the follow- 

 ing species occur, in addition to many of those already 

 mentioned : — 



E • ■"■•'• ■ verna, E. Meyer. v rifragatridactylite8,lAim. 



Cerastium semidecand/rium, Carduus nutrias. Linn. 



Linn. s lum acre, Linn. 



Cerastiwm tetrandrum., Lycopsis arvensis, Linn. 



Curt. Myosotis coUina, Hofim. 



SteUa/ria apetala, Ueria. Koderia gracilis, Pers. 



Sagina nodosa, PrenzL Festuca rubra, Linn, and its 

 V I angustifolia, Linn. different maritime forms. 



On those fixed dunes, from which Pteris is absent. 

 AmmophUa is almost a dominant type, and the association 

 is a closed one. Many of the included plants are those 

 of cultivated land. In addition to some of the species 

 already mentioned as occurring in the bracken association, 

 the following are abundantly represented : — 



Geranium moUe, Linn. L mtodon nudicaule,Ii.&&S. 



Galium saxatHe, Linn. Rumex AcetoseUa, Linn. 



Bettis perennis, Linn. Urtica dioica, Linn. 



Cnicus lamceolatus, Willd. Luzula campestris, DC. 

 Cnicus arvensis, Hofim. 



