47 



then three : (1) shortage of water, (2) 'burial by sand, (3) mobility 

 of soil. The last named is met by rapid fixation due to the growth 

 of the plants themselves, of which there is a constant succession, 

 each type carrying on the work of the preceding forms. The 

 shortage of water is met by (a) an extensive root system ; {b) by 

 storage of water in the plant tissues to bide over periods of drought ; 

 (c) by a reduced vegetative period corresponding to the wet season 

 (ephemerals); (</) most commonly by a more or less marked reduction 

 in water loss by various devices such as sunken stomates, thick 

 cuticle, hairs, rosette habit, etc., tending to check transpiration. 

 Burial by the sand is a regular feature in the early phases of dune 

 formation, and the plants which thrive under these conditions are 

 species in which burial only stimulates further growth, so that with 

 every fresh layer of sand a constantly enlarging branch system is 

 produced, each accretion thus bringing about its own fixation. 



Dunes originate as small heaps of sand around a seedling of 

 Psaiiiiiia or 'rn'ticiini ; these, by their repeated growth, as the dune 

 increases, form a skeletal system that knits the whole together. As 

 the young dunes grow in size lateral fusion takes place between 

 them and the isolated units become merged into a dune system. 

 Further stabilization of the surface is brought about by the sand 

 binders, such as Carex arenaria, Convolndus sohianella, and Fcstiica 

 arenaria. These are capable of extensive growth in the horizontal 

 direction, and thus in a single season can ramify over a considerable 

 area. At this stage the arrival of ephemerals (<'..'/., lamb's-lettuce, 

 whitlow grass, ( 'erastiian spp., Plileioii arenon'a) not only more 

 completely protects the surface by the close carpet of leaves which 

 they form, but also, by their deca}', add considerably to the organic 

 content of the soil and therefore to its water-retaining capacity. 

 By this time the dune has become sufficiently stable to afford a 

 foothold for lichens and mosses. These complete the clothing of 

 the surface and add still more rapidly to the increase of humus, so 

 that eventually these grey dunes become colonized by shrubs 

 (elder, blackthorn, etc.), and become incorporated with the main- 

 land. Throughout the period of stabilization, however, dunes are 

 always liable to erosion by winds which blow out the sand from 

 unprotected surfaces. In this way retogression phases are brought 

 about that can only be checked by the use of artificial protective 

 devices. Preventive measures can also be adopted, such, for 

 example, as the destruction of rabbits whose burrows, by exposing 

 the surface, render dunes vulnerable to wind action. 



We find a similar phenomenon of stabilization by the plants of 

 the shingle beach. Here the chief role is played by the sea- 

 campion and sea purslane, though many. other species contribute 

 to the vegetable complex which knits the surface layers. There is 

 abundant evidence to show that shingle banks, such as that at 

 Westward Ho, Aldeburgh, and the great Chesil Bank, tend to travel 

 landwards. The richer the vegetation the slower will be this process ; 



