VEGETATION OF BLAKENEY POINT. 
525 
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. arenaria, 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 
