Chapter XV 



VEGETATIONAL TYPES OF FRESH 

 AND INLAND SALINE WATERS 



Reed-swamp and other semi-aquatic types of vegetation in which 

 at least half of the plant body is aerial have already been treated 

 in Chapters XII-XIV, dealing with the terrestrial vegetation of dif- 

 ferent climatic belts. This leaves the more fully aquatic freshwater 

 communities, with some others, to be dealt with in the present 

 chapter, followed by the marine ones in Chapter XVI. Such a 

 separation of aquatic from terrestrial habitats and attendant vegetation 

 seems the more proper when we reflect that whereas on land it is 

 the climatic factors which are of primary importance in determining 

 the distribution of particular vegetation-types, in aquatic media it 

 is rather the chemical composition that is fundamental in this respect. 

 This is particularly the case with salinity, which gives us our primary 

 division into fresh and salt waters. Yet the ecological diff"erences 

 between terrestrial and aquatic habitats are largely matters of degree, 

 chemical and physical conditions in the soil being still extremely 

 important in the former, for example, and light and temperature in 

 the latter. Even the distinction between fresh and salt waters is 

 incomplete, as there are various intermediate ' brackish ' waters of 

 varying degrees of salinity and, in addition, inland salt-lakes ; these, 

 however, except in such instances as the Caspian Sea which are of 

 marine origin, seem best dealt with in the present chapter, leaving 

 only marine types to be considered in the next. 



Some Features of the Freshwater Aquatic Environment 



The temperature and some other conditions vary less in aquatic 

 than in terrestrial habitats, water acting as an efl^ective ' damper ' 

 on changes concerning heat. Moreover, major bodies of water 

 exercise an equalizing influence on the temperature of adjacent air. 

 As a result of the minimization of variation in aquatic environments 

 and of the fact that, obviously, they offer no problem of water-supply, 

 aquatic plants and vegetation-types tend to be more widespread than 



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