184 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



alkaline habitat, and, as a result, at or near the limits of their ranges 

 they often appear as obligate calciphiles. 



Salinity.— Under conditions of poor drainage and high tempera- 

 ture, much of the water deposited in low places evaporates and 

 leaves behind the salts it has carried from the soil of surrounding 

 slopes. If precipitation is seasonal and alternates with extreme 

 drought, there is insufficient leaching to prevent accumulation of 

 these soluble salts, which then form alkali soils, so called regard- 

 less of the salt involved. Alkali soils of various kinds occur in all 

 parts of the world and are common in the arid portions of western 

 North America. Lowlands bordering the oceans are subject to 

 periodic inundation with sea water and, consequently, contain 

 relatively high concentrations of salts. 



Plants that can tolerate the concentrations of salts found in 

 saline soils are termed halophytes. How they survive where ordi- 

 nary plants have little chance has been the subject of much debate. 

 If not actually dry, these saline habitats may be termed "physi- 

 ologically dry" because of the high concentrations of salts, which 

 would limit osmotic activity and, consequently, absorption of 

 water by the ordinary plant. The morphological and anatomical 

 characteristics usually appearing in plants of arid regions are com- 

 mon in plants of saline habitats. Succulence is particularly general. 

 Yet these xeromorphic characters have been shown to be relatively 

 ineffectual in maintaining low transpiration rates in halophytes. 

 They must then be able to absorb water in spite of the high salt 

 concentrations, and this is possible because of their own high salt 

 contents. 



Not all species are equally tolerant, and, therefore, they will 

 often be found in zones adjusted to the concentrations of salts in 

 the soil and the plant. Flat areas with uniform salt concentration 

 may support a constant group of species over their entire extent. 

 The number of species tolerant to salinity is not great and many 

 of the same genera are found in all parts of the world where similar 

 conditions occur (e.g., several Chenopodiaceae). Because certain 

 species in alkali areas are tolerant to definite ranges of salt concen- 

 tration and, in addition, to particular salts, they may be rather 

 positive indicators of soil conditions. There are other species that 

 are not so limited. In some, the concentration of the cell sap ad- 



