MAEITIME ASSOCIATIONS 281 



consolidated to some extent, other plants colonize the 

 ground. Chief among these are Arenaria peploides, 

 Eryngium maritimum (sea-holly), Glaucium luteum (horned 

 poppy), Euphorbia Paralias, Cakile maritima, ConvolvvXus 

 Soldandla (sea-convolvulus), and various species of 

 Atriplex. Psamma arenaria (marram-grass) occurs spar- 

 ingly in this zone, but once it gets a footiag the sand 

 collects very rapidly, for the grass is very quick-growing, 

 and the incipient dune is converted iato a high one. The 

 Agropyron disappears, for it cannot keep pace with the 

 growing dune. 



Shifting Dunes. — The first line of high dunes is very 

 characteristic in appearance. The seaward side is bare 

 of vegetation, and rises at a sharp angle to the crest, where 

 Psamma is abundant, and then slopes more gradually 

 down on the landward side. The sand is being con- 

 tinually blown along, and for this reason the dunes are 

 often called " shifting dunes." The term white dune is 

 also used, referring to the fact that so much bare sand is 

 exposed. The binding power of Psamma is much greater 

 than the plants of the embryonic dune, and for this 

 reason it is often planted in places where it does not grow 

 naturally, in order to prevent the dunes from travelling 

 inland. It is also frequently planted on sand-bunkers on 

 golf-links. 



The plants of the shifting dune are subjected to a 

 number of factors which increase transpiration. The 

 wind is strong, the light very intense, and the white sand 

 reflects nearly all the heat of the sun, rendering the air 

 extremely hot and dry. The surface-layer of sand is 

 heated rapidly, and the water quickly driven off, so that 

 both in regard to the soil and the air above it the dune is a 

 true desert. Yet beneath this dry layer there may be an 

 abundance of water, as is shown by the presence of fresh- 

 water marshes in many of the hollows. The adaptations 

 to these xerophytic conditions take the form of erect 

 cylindrical rolled leaves (Fig. 11), as in all the grasses ex- 

 cept the lyme-grass ; the development of a surface-coating 

 of wax, as in the latter plant, sea-hoUy, and homed poppy ; 

 succulent leaves, as in most of the plants other than 

 grasses ; the formation of thorns or spines, as in rest- 

 harrow and sea-buckthorn ; and in all cases the presence 

 of very long roots which can explore the moister soil 



