VEGETATION OF BLAKENEY POINT. 41 



As we pass inwards to the slightly older and larger dunes, 

 the process of their formation by the floating action of the 

 Psamma can be seen in all the phases and vicissitudes of 

 accumulation and erosion. By this term "floating" is implied 

 the well-known character of Psamma which it bears in common 

 with many other dune plants, viz., that the burying of its aerial 

 parts beneath a fresh accumulation of sand serves as a stimulus 

 to renewed growth, so that a new crop of leaves is produced at 

 the surface. The Psamma thus keeps pace with the growth of 

 the dune, and so binds each fresh layer to those beneath. It 

 not infrequently happens that strong winds obtain for some 

 days in a direction different from those which normally prevail. 

 The sand which before accumulated is thus in large part 

 removed and there become exposed the long stems of Psamma, 

 bearing at intervals the tufts of leaf bases that coincided with 

 the surfaces of successive increments in the building up of the 

 dune unit. By the time that considerable fusion and increase 

 in size of the primary units has taken place, the pure association 

 of Marram grass becomes invaded by other species. 



Of these by far the commonest and probably the first of the 

 perennials to appear is Festuca rubra var. ctrenaria, whilst 

 Senecio Jacobcea is of almost equal prevalence and likewise a 

 pioneer. These, together with Triticum junceum and two 

 thistles which are found as occasional weeds (viz., C. arvensis 

 and C. lanceolatus) , constitute the bulk of the perennial flora 

 of the seaward face, the majority of the species found 

 on the primary dunes being confined to the more sheltered 

 slope of the landward face. An exceedingly common plant on 

 this slope is Tortula ruraliformis, which, though frequent 

 amongst the larger plants, attains its maximum luxuriance on 

 the stretches of sheltered sand which are otherwise bare. The 

 carpet that it forms often provides foothold for the small 

 ephemerals, of which Cerastium semidecandrum is its com- 

 monest associate. 



The Ephemerals. 



The extreme porosity of a sandy soil renders the water 

 problem of prime importance to the dune flora, and this is 



