196 



PLANT SOCIOLOGY 



is almost completely leached out (0.15 per cent of NaCl). The 

 sodium chloride content of soils which are not continuously overflowed 

 with sea water depends upon the amount of atmospheric precipitation. 

 The effect of this is greatest in the uppermost layers of the soil, that is, 

 in the region of the roots of the halophytes. Investigations on the 

 seasonal fluctuations of salt concentration in the soils of southern 

 France were made by Lagatu and Sicard (1911). They have reference 

 to dry and moist soil as well as to the aqueous soil solution. 



The Salicornia vegetation of these soils can stand sodium chloride 

 concentrations of 8 to 10 per cent; the seasonal fluctuations in the top- 

 most soil layers approximate 8 to 9 per cent. Below depths of 50 cm. 



6. 

 June 



26. 



9. 

 Sept. 



8. 



Oct 



Nov. Dec. 



May June July Aug. 



Fig. 104. — Annual fluctuation in the NaCl content of the soil solution (percentage 

 by weight) at (.4) to 15 cm., {B) 15 to 25 cm., and (C) 25 to 100 cm. depth in a lagoon 

 soil, with stands of Salicornia, near Montpellier, France. {After Lagatu and Sicard.) 



they are rather insignificant (Fig. 104). The sodium chloride con- 

 centration of the ground water at 1 to 2 m. shows only small changes 

 in the course of a year. This soil water is the permanent storage 

 reservoir which constantly gives off salt in solution to the upper soil 

 layers as water evaporates. 



In the classification of floating and submerged plant communities of 

 the sea which live in the constant salt optimum, physicochemical 

 factors other than the sodium chloride content, such as light, wave 

 motion, and temperature, become decisive habitat factors. This 

 seems to be the case also on steep rocky coasts where the nature of the 

 rock, the exposure, Avave motion, wind, and light often become of 

 greater importance to vegetation and to its zonation than the salt 

 content. 



It is quite different in the case of low shores, which are subjected to 

 periodic flooding and are therefore characterized temporarily and 

 locally by great fluctuations of the sodium chloride content of the soil. 

 The researches of numerous investigators show the sodium chloride 



