MARITIME 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), Huphorbia Paralias, Cakile maritima, Convolvulus 
Soldanella (sea-convolvulus), and various species of 
Atriplex. Psamma arenaria (marram-grass) occurs spar- 
ingly in this zone, but once it gets a footing the sand 
collects very rapidly, for the grass is very quick-growing, 
and the incipient dune is converted into 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-holly, and horned 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 
