1914-15.] BOTANICAL SOCIETY OF EDINBURGH 359 



but it is only in this area that it occurs on the Culbins in 

 any quantity. Juniper was not found by the authors. 



B. Culbin Sandhills Proper. (See Map II.) 

 1. Shifting Dunes. 



The highest dunes, where the fine sand moves about 

 incessantly under the prevailing westerly winds, are almost 

 completely devoid of vegetation. These are adjoined, how- 

 ever, by lower dunes, fairly well covered with vegetation, 

 especially Ammophila arenaria, Link., by ridges carrying 

 Ammophila alone on their tops, and by dunes and plains 

 among which rise individual hillocks clad with and built 

 up by means of Ammophila. These formations are par- 

 ticularly clear on the eastern side of the highest dunes. 



Marram Grass Association. — The characteristic plant 

 of the " white " dunes, the plant which grows among the 

 shifting sand where no other vegetation is visible as far 

 as the eye can reach, is Ammophila arenaria. In certain 

 parts it has been planted ; but most of it is obviously of 

 natural origin. The mode of formation of the isolated 

 tufts or hillocks or ridges of it is frequently apparent. 



At first one or two steins take firm root and grow in the 

 comparatively stable sand. The persistently westerly 

 winds heap up a little slope of sand against the western 

 edge of the tuft, throwing a heap of sand among the bases 

 of the stems (see fig. 1, a). The sloping sand on the west 

 increases in bulk, retaining its steepness, the Marram grass 

 becomes more and more lifted up on a mound and grows 

 upward and keeps its leaves above the surface of the sand. 

 The sand blown over the hillock and off its top and along 

 its sides accumulates in a tail on the eastern side. In form 

 the mound thus takes on the crag-and-tail appearance, 

 which is also possessed, in the reverse direction, by the 

 dunes themselves (see fig. 1, b). In the course of time the 

 hillock becomes more and more flat-topped and elongated, 

 with Marram grass growing along the summit. Many 

 irregularities in shape, of course, occur, as the joint-effects 

 of the situation, the sand, the wind, and the plant-growth. 

 Another general effect is the hollowing-out action of the 

 wind, which, coming against the mound, is forced down- 



