CH. VI] 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 aération 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 
