6a
SAND DUNKS
   As a rule, in passing landwards from the shore, dunes become less mobile, and with the decline of mobility of surface many additional species of plant are able to establish, till the surface becomes closed over by vegetation. In this condition dunes become relatively stable, though, of course, the constant action of rabbits tends to reopen them.
   Mobile dunes are mainly characterized by Psamma, though as an alternative, especially on the East Coast, the Lyme Grass (Elymus arenarius) is frequently met with. Other plants proper to mobile dunes are the Sea Holly (Eryngium maritimum), the Sea Spurge (Euphorbia Paralias), the Sea Sedge (Carex are-naria, Plate V), and, along the drift line on the seaward face, the Saltwort (Salsola Kali) and Sea Rocket (Cakile maritima). These two plants of the strand edge being annuals, have little value as dune builders. They are, however, in the absence of other debris, a reliable guide to the position of the highest drift lines.
   On the more stable dunes other plants come in and complete the fixing process. These are very numerous, and include the Storksbill (Erodium cicularium), Stonecrop (Sedum acre), Bird’s Foot Trefoil (Lotus comiculatus), Ragwort (Senecio Jacobcza), Lady’s Bedstraw (Galium verum), Thyme (Thymus Serpyllum), and many others. (See Appendix I, p. 263.)
    Very important as sand fixers are the lichens Peltigera canina and species of Cladonia, which include the Reindeer “ Moss”, Cup “Moss”, &c. It is the presence of this element that has gained for the fixed dune the name of “grey dune” in contradistinction to the “white dune”.
   Some mosses, such as Tortula ruraliformis and Ceratodon purpureus, sometimes form continuous sheets on old dunes and have a corresponding mechanical value.
   A certain number of bushes are to be found on dunes. The following is a list of shrubs occurring on the Scolt Head system (Norfolk). It is interesting to note that the fruit in each case is fleshy, thus indicating introduction by birds.
Elder,	Gooseberry,	Blackberry,
Privet,	Dop Rose.	Hawthorn.