1899] ON THE STUDY OF PLANT ASSOCIATIONS 115 



(2) Secondary social forms struggling for dominance. 



(3) Dependent species — simply protected by the presence 



of the dominant species, or living upon the humus 

 they form, or parasitic upon them, etc. 



(e) The influence of animals and of man upon c and d. 



(/) The general conditions of climate and of vegetation of the 

 region compared with other regions. 



A concrete example of a brief survey of a type region will illustrate 

 the application of these methods. 1 For this purpose I have chosen 

 such an area as may be found in many places round the coast of 

 Britain, namely, a stretch of sand dunes and links, the present example 

 being from the Ayrshire coast, between Prestwick and Troon. 



The small associations are numerous, but I select only the more 

 important for the purposes of illustration. These are the strand plants, 

 the dune bents, the grassy links, the heathy knolls, and the pine woods. 

 If expressed in terms of the most important species of each, these 

 would be respectively the associations dominated by Atriplex, Ammo- 

 phila, Agrostis, Calluna, and Pinus. 



The strand plants {Atriplex, Cakile, Salsola) are on loose shifting- 

 sand, occasionally submerged by spring-tides, and beaten upon by the 

 wind. They are all annuals (cp. the great proportion of annuals to be 

 found on the regularly overturned earth of cultivated land) ; recumbent 

 in habit, thus spreading over the loose sand, holding it together and 

 escaping the full force of the wind ; succulent, an adaptation connected 

 with presence of much salt ; and their surfaces are reduced in extent 

 or otherwise modified to prevent excessive transpiration. On the 

 strand competition between the species is almost absent, for so few can 

 live at all in these conditions that each plant has a free space of 

 ground to itself. Human influence has not appreciably modified the 

 conditions. 



The dunes are beyond the reach of the waves, but are exposed to 

 the fury of the wind, to drought, and to the moving of the sand. The 

 dune plants are adapted in various ways to withstand excessive 

 transpiration. Some are also salt-loving species, such as Ammophila, 

 Elymus, Agropyron junceum, Eryngium maritimum, Arenaria peploides, 

 Volvulus Soldanella, etc. The dominant and by far the most abundant 

 species is Ammophila arenaria. which grows in great tussocks, holding 

 the sand together, and sheltering from the full force of the wind 

 numerous more delicate species that occur sparsely distributed on the 

 dunes. The conditions are still too hard to allow a large flora, and 

 competition between the species is almost at its minimum. Except 



1 For the present purpose, as will be clear, we are not concerned with how the plants 

 came into this region, but with how they live^there. The study of the immigrations of the 

 British Flora has such an interest from its complex and striking nature that it has long 

 attracted students. It is partly due to this that the oecological standpoint we are now 

 assuming has received comparatively little attention. 



