264 PSAMMOPHYTES 



FLORA 



In the north of Europe the sea-marram, Psamma arenaria, is the 

 most important dune-grass, and far excels all others in the densely 

 caespitose arrangement of the leaves and in its faculty of collecting sand 

 and growing up through this. In addition we may mention Elymus 

 arenarius, Carex arenaria, and Lathyrus maritimus. Added to these 

 are also Hippophae rhamnoides and Sonchus oleraceus, whose long 

 creeping roots emit numerous suckers, and some others. 



ADAPTATIONS 



Plants fitted for life on dune-sand assume characteristic growth- 

 forms, which are encountered on shifting dunes all the world over. The 

 dune is almost entirely constituted by a very loose aggregation of particles 

 and is easily traversed by roots and rhizomes. Plants strictly limited 

 to a confined spot in the soil are scarcely fitted to exist on dune, as the 

 wind is incessantly bringing new supplies of sand, which cause constant 

 change in the conformation of the dune and in its soil. Consequently, 

 the characteristic features of the vegetation are that it is very open, and 

 consists of xerophytic perennial species possessing long and richly branched 

 rhizomes ; while exceedingly long roots (often many metres in length) are 

 found permeating the sand. 



There are certain ' sand-binding ' species of plants that first arrest 

 the sand and produce dune-embryos ; in addition, plants promote the 

 growth in height of sand-dunes. Triticum junceum in the north of 

 Europe is one of the halophilous psammophytes that begin the production 

 of dune both on the shore and at the base of the white sand-dune ; another 

 plant like it, is Honckenya peploides : but these can only give rise to 

 low dunes on the beach. Psamma arenaria and Elymus arenarius expel 

 them and produce high dunes (psammeta). This is due to their faculty 

 of enduring burial by the sand and of growing up through this ; it is 

 evident that, inasmuch as new shoots accumulate fresh stores of sand, 

 the dune must constantly increase in height. In the fine sand composing 

 the interior of the dune one finds a great number of old rhizomes and 

 roots ; and if the wind breaks down an old dune these enclosures are 

 revealed. 



ASSOCIATIONS 



In northern Europe we find various associations including 



Elymetum, composed of Elymus arenarius ; usually occupying only 

 a narrow zone nearest to the sea-beach ; this species may also give rise 

 to inland associations. 



Psammetum, consisting of Psamma arenaria, sea-marram ; in the vast 

 majority of cases white dunes are produced by Psamma ; for binding 

 the shifting sand this is the most important species, and is often artificially 

 utilized for this purpose. 



In white dunes here and there woody species, particularly Hippophae 

 rhamnoides, Salix repens, and Empetrum nigrum, appear. The first 

 two may give rise to bushland. 



In northern Europe the next developmental stage to the white sand- 

 dune is the grey dune. 



