CH. Vl] MARSH AND AQUATIC ASSOCIATIONS 145 



liverworts, and Algae, but by few characteristic aquatic flowering 

 plants. 



In a sense, some of the moorland associations described 

 in the next chapter are aquatic, and were so classified by 

 Warming (1895) in his earlier work on plant communities. 

 The cotton-grass moor (see page 183), for example, possesses a 

 soil which, during very considerable periods of most years, 

 is supersaturated with moisture ; and many members of 

 the cotton-grass association possess such aquatic structural 

 adaptations as aeration channels in their stems and leaves. It 

 is now, however, very generally held (see Schimper, 1903 — 4; 

 and Warming, 1909) that it is the physiological and not the 

 physical wetness of the soil that determines whether or not 

 plants are really hydrophilous ; and, as peaty soils are now 

 frequently regarded as being physiologically dry, moorland 

 plants are not now usually placed among hydrophytes. On 

 the other hand, Clements (1907 : 170) maintains that the 

 aquatic adaptations found in many moorland species are normal, 

 that the xerophytic adaptations which they possess were 

 acquired during some past period when the plants in question 

 inhabited dry habitats, and that the xerophytic structures have 

 persisted. It is indeed necessary to remember that very little 

 experimental investigation has yet been performed on the 

 physiological water-contents of soils, most ecologists and plant 

 geographers being apparently content with general impressions 

 and general statements. 



As Schimper has pointed out (1904 : 781), " every classifica- 

 tion of the aquatic flora commences with the separation of 

 salt-water forms from fresh-water forms." All the aquatic 

 plants of this district belong to the latter class; and a 

 classification of fresh-water aquatics may be based on the 

 richness or poverty of the water in soluble mineral salts. 

 From this point of view, the aquatic species of the calcareous 

 streams of the district may be placed in one association, and 

 those of the non-calcareous streams in another. 



The rivers of lowland districts are probably always rich or 

 fairly rich in soluble mineral salts, as the streams have in their 

 earlier courses dissolved much material from the rocks through 

 which they have cut their way; but this statement does not 

 apply to the small streams of non-calcareous hill-slopes. In 



M. 10 



