PLANT HABITATS AND COMMUNITIES 319 



Plant-communities. The whole of the vegetation of 

 a habitat composed of plant-communities which are deter- 

 mined by the generally constant soil and climatic conditions 

 in that habitat is called a plant-formation. The vegeta- 

 tion of sand-dunes, also that of salt-marshes (Fig. 211), 

 furnishes good examples of plant-formations ; other exam- 

 ples are aquatic, marsh, fen, and moor formations. There 

 are also extensive and complex formations like those on 

 siliceous soils and those on calcareous soils. 



Minor variations of the habitat within the formation 

 give rise to well-marked plant-communities, such as the 

 Heather moor, the Cotton-grass moor, the Grass heath, 

 the Sessile Oak wood, the Pine wood, and the Limestone 

 Ash wood, which are easily recognizable during a country 

 walk as characteristic features in the landscape. 



These plant-communities within a plant-formation are 

 called pi ant- associations, and are usually dominated by 

 one or a few species of plants, often with a characteristic 

 form and habit, e. g. the small, rolled-leaved evergreen 

 shrubs of the Heather moor, the tussocks of Cotton-grass 

 on the Cotton-grass moor, or the dominance of true grasses 

 on the Grass heath ; and, in the case of woodlands, the 

 prevailing tree, Oak, Pine, Ash, or Beech, influencing 

 and being accompanied by a peculiar undergrowth. 



Compounds of the names of the dominant species are 

 used in naming the several associations, e.g. Heath associa- 

 tion, Ash wood association, Oak-Birch-Heath association, 

 Alder-Willow association, &c. 



Plant-associations contain within them a number of 

 smaller communities, the plant-societies. These consist 

 of species more or less related to each other as regards 

 periods of active growth, shoot-systems of varying heights 

 and shade-requirements, underground parts of different 

 kinds and at different depths, drawing upon different 

 constituents or tapping different layers in the soil. A good 



