388 ECOLOGY 



and covered at high tides by the waves, it is devoid of 

 vegetation, and, like the sand between tide-marks, the 

 shingle is moved to and fro freely by the advancing and 

 retreating tides. On such a shifting surface, plants cannot 

 grow. Even in larger banks, and when the crest is beyond 

 the reach of the highest tides, the shingle is more or less 

 mobile by reason of the heavy impact of the waves and the 

 readiness with which the sea-water percolates and buoys 

 up the loose materials of which the bank is composed. 



The shingle bank has a steep seaward slope, and a more 

 gradual landward slope. On the seaward slope very few 

 plants occur, and the general appearance is that of a bare 

 bank of rounded stones. The bank is apparently dry, but 

 if a few of the stones are examined they will be found 

 to be wet even in very dry seasons, and often covered 

 with lichens. The water, too, is fresh, not salt. Drifted 

 materials, such as seaweed and animal-remains carried up 

 by the waves, especially during storms, accumulate between 

 the stones and form a black soil on which flowering plants 

 from neighbouring ground can establish themselves and 

 form an open association. 



Shingle-binding plants. Before a plant-covering is pos- 

 sible, the mobile beach has to be rendered more stable, 

 and several species of plants do great work as pioneers 

 and shingle-binders, e. g. the shrubby Sea Blite (Suaeda 

 fruticosa), Sea Campion (Silene maritima), and the Sea 

 Purslane (Arenaria peploides). As with sand-binders, the 

 essential character required is the power of the plant to 

 grow through the shingle and regain the surface when 

 buried, and the shrubby Sea Blite possesses this power in 

 a remarkable degree. The plant, when grown on stable 

 ground, is much branched, three to four feet high, and has 

 a stem an inch or more in thickness ; the leaves are small 

 and fleshy and covered with a waxy bloom. On mobile 

 shingle, however, the young plants quickly anchor them- 



