PLANT ASSOCIATIONS 225 



boundary between them is ill-defined, just as the amount 

 of moisture present in the soil gradually increases, and 

 the peat becomes deeper, yet the associations themselves 

 are quite distinct from one another. 



The moorland-associations are of wide extent, some- 

 times covering miles of country, for the habitat is the 

 same over extensive areas. Where the habitat varies 

 rapidly, however, one association passes quickly into 

 the next. On the shelving bank of a river or lake, for 

 example, the plants which constitute the reed-swamp 

 (p. 241) are arranged in zones running parallel to the 

 bank. Each zone constitutes a separate association, 

 dominated by a single plant and associated with a definite 

 habitat, determined by the depth of the water. Yet each 

 association may be only a few feet in width. 



2. Formations. We have already drawn attention to a 

 unit of a higher order than the association in the case of 

 a reed-swamp. This exists in a definite biological habitat, 

 or, more strictly, a series of very closely related habitats, 

 which only differ from each other in one factor the depth 

 of the water. To a unit of this kind we give the name 

 formation. If the water of the reed-swamp is uniformly 

 deep, the plants are not zoned, but intermingled with one 

 another, in which case we have a very mixed association 

 which is equivalent to the formation. It is an association 

 because of its definite floristic composition, and a forma- 

 tion because the environment constitutes a definite bio- 

 logical habitat. In the same way a " moss "-moor is a 

 formation comprising a single association the cotton- 

 grass association. A formation, then, may include one 

 or many associations, just as a genus may possess one or 

 many species. 



A sand-dune is a plant-formation containing a number 

 of different associations. The rainfall, temperature, 

 exposure to wind and light, is the same throughout ; but 

 the texture of the soil varies, and this determines the 

 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, 



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