126 PLANT GEOGRAPHY 



and halophytes, the plants of saline soils. Reference 

 has before been made to the dwarf, tufted growth, 

 evergreen leathery and hairy leaves, either cylindric 

 (" juncoid "), adpressed (" cupressoid "), or inrolled, and 

 the large bright flowers of alpines; and to the long 

 roots, fleshy texture, and glaucous (blue-grey) bloom on 

 the surface of many halophytes. 



If the presence or absence of lime in the soil be a 

 master factor, we may recognise a calcifuge class, in- 

 cluding such a marked xerophyte as the Broom, as well 

 as a mesophyte like the Foxglove, in opposition to the 

 calcicole class above mentioned, which includes the Man-, 

 Bee-, and other terrestrial Orchids, in addition to the 

 species then named. 



THE PLANT-ASSOCIATION. In the study of vegetation 

 a convenient and often most obvious unit is the plant- 

 association, a community or aggregation of plants having 

 a definite specific composition and a definite habitat, 

 either with one social dominant species, or with several 

 prominent ones. The former is termed a pure, the latter 

 a mixed association. Some of our heaths consisting 

 almost entirely of Calluna are good examples of pure 

 associations, whilst in many copse associations two or 

 three species are equally prominent (Fig. 13). For conveni- 

 ence of reference associations are named by the addition 

 of the suffix -etum to the stem of the generic name of the 

 dominant species, followed by its specific name in the 

 possessive case. Thus a Pine-forest may be a Pinetum 

 sylvestris, an Oak-wood a Quercetum sessiliflorce. 



PLANT-SOCIETIES. Small aggregations of a non- 

 dominant species within an association, due not to any 

 noticeable differences in the habitat, but apparently to 

 the gregarious habit of the species, such as spreading 

 from one rhizome or self-sown from one parent, are 

 known as plant-societies. Thus in a Callunetum, or 

 heath consisting mainly of Ling (Calluna Erica L.), 

 there may be patches or " societies " of Erica Tetralix L. 



PLANT-FORMATION. A group of associations on 

 identical habitats, that is practically, the various com- 

 munities in one region which inhabit the same soil, 

 constitute a formation. On the sand-dune formation of 

 a region, for instance, there may be Cariceta arenaricz 

 and Ammophileta arundinacece, i.e. associations in which 

 either Car ex arenaria L. or Ammophila arundinacea 



