368 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



are numerous tiny seedlings springing up after heavy rain. Fig. 

 156, B, shows a close-up of a near-by less gravelly area when the 

 ephemerals have developed. Fig. 107, A, shows an area of salt 

 desert near the oasis of Shithatha, in southern central Iraq, where 

 higher vegetation occupied from one-eighth to one-half of the sur- 

 face, bushy Chenopodiaceae affording the main dominants. Soluble 

 salts totalled 7-5 per cent, of air-dry soil at the surface where tested 

 but decreased markedly below ; the pH was 8-3 at the surface and 

 scarcely varied from this below. Some widish and flat, white- 

 encrusted areas tended to be barren, though in places where water 

 could collect in the rainy season and Blue-green Algae grew, loose 

 mud ' medallions ' covered the surface. 



Salt-Marshes 



Whereas various salts, such as nitrates, sulphates, and phosphates 

 of potassium, calcium, magnesium and iron are essential, at least 

 in the small concentrations that are usually present in soils, for the 

 normal development of land vegetation, excessive salinity {e.g. of 

 more than 05 per cent.) is harmful to the growth of most plants. 

 Such high salinity, particularly due to sodium chloride, is most 

 commonly found around sea-shores, and is the most constant factor 

 leading to the replacement there of normal land-vegetation by plants 

 which habitually grow in very salty soils (halophytes) or at least 

 can grow in such soils (facultative halophytes). Thus in maritime 

 salt-marshes, which are primarily determined by periodic immersion 

 in salt water and mostly lie between the levels reached by the higher 

 neap and ordinary spring tides, the components of the characteristic 

 vegetation are almost all peculiar to the habitat. And whereas, if 

 the tide is effectively excluded and there is adequate drainage, the 

 salt in time is washed out of the soil and non-halophilous species 

 colonize the area, ' There is no good evidence that salt marsh can 

 develop by the mere accumulation of silt or humus, without human 

 assistance, into a non-maritime vegetation ' (Tansley, Introduction 

 to Plant Ecology, p. 78). Consequently the salt-marshes of tem- 

 perate and allied regions seem best considered here among subclimax 

 or local climax types. 



Salt-marshes are chiefly developed on mud-flats about sheltered 

 tidal estuaries. In them such Green Algae as Rhizocloniiim and 

 Enteromorpha, or such halophilous vascular plants as succulent or 

 shrubby Glassworts or Saltworts {Salicornia spp.), are the normal 



