384 ECOLOGY 



little ridges ; the windward slope of each ripple is gradual, 

 but the lee side is steep (Fig. 246). If we watch the ripples 

 during high winds we can see them shift their position ; the 

 sand is blown up the gentler slope to the crest of the ridge, 

 then rolls down the leeward slope where it is protected 

 against the wind and is likely to lodge. Thus, while the 

 windward slope is wearing away, accumulation is going on 

 on the leeward side. In this way the little ripples move 

 steadily forwards in the direction in which the wind is 

 travelling. If in its progress a plant (or even a pebble) is 

 encountered, a little heap of sand is built up around it, and 

 it may eventually be buried. If, however, the plant by 

 continued growth is able to keep its tip above the surface, 

 the mound may increase in size and form a miniature sand- 

 dune. It is in this way that sand-dunes are built up, and 

 they are thus the result of two causes : (1) the action of 

 wind on mobile sand, and (2) the binding action of plants 

 which establish themselves on the sand so transported. 



The plant which is best able to overcome the difficulties 

 of shifting sand is the Marram-grass (Psamma arenaria), 

 which, by virtue of its long perennial rhizomes and deep- 

 growing fibrous roots, helps to bind the sand together, 

 while its shoots, by being able to grow and keep above the 

 surface, not only maintain the existence of the plant, but 

 aid considerably in building up the dune. By the decay 

 of the older parts of the plants, humus is added to the soil, 

 so that the Marram-grass becomes a valuable pioneer of 

 coast- vegetation. As we have seen (p. 127), the vegetative 

 mode of reproduction of this plant is put to practical use 

 along miles of our coasts as a ' land-winner ' (Figs. 247, 

 80, and 81). 



The windward side of a dune is usually bare (Fig. 244), 

 but on the crest are the Marram-grass and Sea Lyme-grass, 

 which not only serve as sand-binders but provide protection 

 and a suitable soil for other species. On the more sheltered 



