224 BRITISH PLANTS 



take into consideration not only the soil in which the 

 plant is growing, but the amount of rainfall, the tempera- 

 ture, intensity of light, and wind. 



The Ecological Units of Vegetation. 



1. Plant -Associations. — By " plant - association " we 

 mean an assemblage of plants of definite floristic composition 

 associated with a definite biological habitat. Each asso- 

 ciation may be recognized, as a rule, by the presence of 

 one most abundant or dominant plant, which gives the 

 name to the association — e.g., the cotton-grass association 

 of wet moors (p. 254) ; or several plants may be equally 

 abundant — e.g., the Festuca-Agrostis association of dry 

 grassland (p. 250). The cotton-grass association is found 

 only on moorlands where the rainfall is very high and 

 the conditions favourable for accumulation of deep peat. 

 It is true the cotton-grass grows in other places, but only 

 in this habitat does it become so abundant as to con- 

 stitute a definite association. 



Occasionally, however, the association is not distin- 

 guished merely by the dominant plant, but the plants 

 growing with it must be taken into consideration. For 

 example, Calluna vulgaris (heather) may be dominant 

 on damp moorlands and also on dry stony heaths, but 

 the association is not the same in the two cases. The 

 habitat is different, and this is expressed, not in the 

 dominant plant, but in the other plants present — moisture- 

 loving plants being found in the one and dry-loving 

 plants in the other. 



The same difficulty is experienced, but to a much 

 greater extent, in ecological as in systematic botany, 

 when we attempt to define the exact limits of the units. 

 The systematic unit — the species — however, is much more 

 constant ; only in comparatively few cases do they over- 

 lap or run into one another ; whilst in the ecological unit — 

 the association — this is always so. 



One habitat seldom passes suddenly into another — there 

 is a more or less gradual transition ; and, in the same 

 way, associations are not sharply marked off from each 

 other. Thus, in walking up a moorland slope, one may 

 pass a heather-moor, an Erica Tetralix-a,ssocia.tion, and 

 finally reach the cotton-grass association ; but the 



