226 BRITISH PLANTS 



distribution of the associations. On the seaward side 

 we find loose drifting sand covered with an association 

 of sea couch-grass; an association of marram-grass 

 occupies the dune-crest; and, farther back, on the more 

 consolidated sand, an association of low-growing herbs; 

 and, finally, either an association of pasture-grasses or 

 heather. These associations merge into one another, 

 and in the cotirse of time, as the sand consolidates, one 

 association is replaced by another. The formation thus 

 has a fast history, and, while it is still open, a future. 

 When once closed — i.e., when the soil is entirely occupied 

 by close-growing vegetation— it can be further altered 

 only by changing climatic or soil-conditions modifying 

 the biological nature of the habitat. In many cases we 

 do not know the past history of a formation; but on the 

 moorland a study of the plant-remains in the peat gives 

 us a clue, not only to the past vegetation, but also the 

 climate and other factors of the habitat (see p. 249); 

 whilst in a sand-dune pasture the past is being written 

 for us in the vegetation of the younger parts. 



Close by the sand-dune we may find an area of wet, salt 

 soil occupied by a salt-marsh formation. This differs 

 considerably from the plant-formation of the sand-dune, 

 and the differences are due almost entirely to the nature 

 of the soil, for the elimatic conditions are practically 

 identical in the two cases. Generally speaking, the soil 

 is always the chief factor in determining the plant-forma- 

 tion; climate plays a relatively subordinate role. 



To sum up, then, the larger differences in habitat serve 

 to differentiate the formations ; the minor differences in each 

 habitat, the associations. No hard-and-fast rule can be 

 laid down as to what constitutes a larger or a minor 

 difference, any more than we can say what constitutes 

 a generic or a specific character in systematic botany. 



The Study of the Association. 



In England we rarely have to deal with natural asso- 

 ciations — i.e., with associations undisturbed by man or 

 his domesticated animals. Traces only of the great 

 forests that once covered the country now remain, and 

 much of the woodland we see has been planted. Within 

 the cultivated area nearly all our grassland is artificial ; 



