132 PLANT-LIFE ON LAND [CH. 



peatedly checked by the high tides and heavy winter 

 seas washing away their bases. The sand then falls 

 in steep avalanches from above, and may be heaped 

 up in various ways by the prevailing winds. But it 

 is hardly possible to give more than a very general 

 description of their form; it is so various. The 

 factors that determine it are primarily the growth 

 of the plants which serve to collect the sand and to 

 bind it together ; and secondarily, the effect of the 

 wind in again undoing what the plants have done, 

 by carving out and redistributing the sand collected 

 under their influence ; for there is in the White 

 Dune formation a constant construction and as con- 

 stant a destruction going on so long as the surface 

 is not covered in by plant-growth. This absence of 

 permanence suggested the designation of "Shifting 

 Dunes." 



Before we consider the plants which take a 

 leading hand in the formation and the shaping of 

 Dunes, it may be well to enquire what are the 

 qualifications for their success. In the first place, 

 Dune-forming plants must be perennials. Annual 

 Dunes are always small and temporary, for when the 

 plants die at the end of the season the sand collected 

 round them is dispersed : this is the case with such 

 plants as Salsola (Fig. 21). Secondly, the plants 

 should be capable of spreading by means of runners, 

 or by adventitious buds upon their roots. By this 



