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 
areed-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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