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-association, and 
finally reach the cotton-grass association; but the 
