SALT TOLERANCE 155 



absorption seemed to be related particularly to metabolism of 

 pyruvate, whereas potassium uptake was identified more closely 

 with phosphorylation. The possible location of the absorption 

 mechanisms in Uha has already been discussed (Chapter 6, p. 103). 



Seaweeds are obligate halophytes, that is, they not only tolerate 

 a high concentration of salt but they appear to require it. Attempts 

 to grow seaweeds in artificial sea-water, only a little different in 

 composition from normal, have proved unsuccessful. Among 

 submerged spermatophytes, Zostera maritima requires normal sea 

 water for maximum growth whereas some species of Myriophyllum 

 growing in brackish water have a small tolerance on either side of 

 the optimum salt concentration, and Ruppia spiralis and Ruppia 

 maritima can be adapted to growth in either salt or fresh water. 

 Limits to the salt concentration that different submerged aquatic 

 plants can tolerate largely controls the distribution of vegetation in 

 river estuaries. The factors which determine whether an organism 

 can live over a wide or narrow range of salt concentrations have 

 been much less studied in plants than in animals, and further 

 investigations would be welcome. 



Kniep (1907) showed that the early growth of fertilized Fucus 

 eggs depends on the presence of salt. The minimum salt requirement 

 below which no development occurred was found to differ for 

 different species, being a 0-8 per cent solution for F. vesiculosus, 

 1-0 per cent for F. serratus, and 1-5 per cent for F. spiralis which 

 tends to grow more completely submerged than the other two 

 species. Maximum germination of zygotes was observed in normal 

 sea-water, and solutions containing higher concentrations of salt 

 were inhibitory. 



C. Terrestrial Halophytes 



The inhabitants of sah marshes are an interesting group of plants 

 which have intrigued ecologists and physiologists for many years. 

 At least some of these plants will grow successfully and complete 

 their life cycles only in the presence of high concentrations of salt 

 (true halophytes). An example of this is the marsh samphire, 

 Salicornia striata (herbacea), which reaches maturity only in the 

 presence of sodium chloride at about the same concentration as in 

 sea-water. On the other hand, Aster tripolium and Plantago maritima 



