12] VEGETATIONAL TYPES OF TEMPERATE LANDS 369 



pioneers, followed typically by Alkali-grasses {Puccinellia spp.) or 

 other halophytic Grasses. Any such growth tends to impede the 

 flow of water and increase the deposition of silt, helping to raise 

 the level of the bed. Higher up, on the ' flats ' covered by most of 

 the spring tides, a mixed vegetation is usually developed. This is 

 the general salt-marsh community which, for example on both sides 

 of the North Atlantic, characteristically includes such types as Sea 

 Plantain {Plantago maritima s.l.). Sea Arrow-grass {Triglochin 

 maritima), Sea-blite iSuaeda maritima), and Sea-pink (Armeria 

 maritima s.l.), with shrubby Chenopodiaceae especially in warm- 

 temperate regions. Various Brown and other Algae, often of peculiar 

 habit and squalid mien, are commonly associated. Still higher up, 

 where the saline flats are covered only by the higher spring tides, 

 a more or less close turf usually develops with the help of grazing, 

 composed of halophilous Grasses with associated forbs. At the 

 uppermost levels reached only by the very highest spring tides or 

 storm-waves, less markedly halophilous types such as Red Fescue 

 [Festuca rubra s.l., a facultative halophyte) tends to predominate, 

 with often an admixture of ordinary land species that can tolerate 

 small amounts of sea-salt in the soil. At the higher levels in river 

 estuaries the water is alternately fresh and brackish with the ebb and 

 flow of the tide, so inhabiting plants must be physiologically special- 

 ized to withstand rather sudden and extreme changes in the osm.otic 

 value of the inundating water. 



Whereas such geodynamic factors as tidal action may cause the 

 maintenance of a condition of equilibrium between accretion and 

 erosion, and thus of a lasting type of vegetation in maritime salt- 

 marshes, it seems probable that in some instances at least there is 

 gradual continuation of accumulation and advance towards normal 

 land vegetation — even without human interference — as in the case 

 of sand-dunes and shingle beaches, and in spite of what was quoted 

 above. But at all events the types just described are commonly 

 both long-lasting and distinctive, and consequently had to be 

 treated as special entities. 



In manv arid regions, for example of interior Asia and western 

 North America, there occur salt lakes or smaller basins or seasonally 

 dry ' alkali pans ' exhibiting a similar range of conditions of varying 

 salinitv which is, however, often more extreme than on sea coasts. 

 Inland, these saline areas result from excessive and prolonged 

 evaporation, changes in level and salinity in individual cases being 

 seasonal instead of tidal, though they may vary rapidly with the 



