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 mUes 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. Societies. — Occasionally a patch of ground within 

 an association is tenanted by a plant-community which 

 differs in its main features from the main association. 

 Such a community is called a plant-society. It sometimes 

 owes its presence to a local peculiarity of the soU, as in 

 the case of the rush society found occasionally in damp 

 hollows within the grassland association of clay soils, 

 or it may be due to a sub-dominant plant becoming locally 

 dominant, through the plant spreading from one centre, 

 either by vegetative means or by a colony of seedlings 

 springing up round the parent plant. 



3. 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. 



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

 of different associations. The rainfall, temperature, 

 exposmre to wind and light are the same throughout ; but 

 the texture of the soil varies, and this determines the 



15 



