SALT TOLERANCE 



153 



more. Normal sea water contains about 3 per cent of salts, and its 

 approximate composition is shown in Fig. 47. 



A characteristic feature of marine halophytes is their capacity 

 to accumulate potassium in preference to sodium from sea water 

 (Table 17). Steward and Martin (1937) studied the effect of environ- 



Table 17. Salt Composition of Some Seaweeds. 



A. Ratios of potassium/sodium in sea-water and in some marine halophytes. 



(From Bertrand and Perietzeanu, 1927). 



B. 



The sap composition of Valonia ventricosa and Valonia macrophysa at 



Dry Tortugas, Florida. 

 (From Steward and Martin, 1937). 



mental conditions on the potassium and sodium content of species of 

 Valonia growing at the Dry Tortugas, in Florida. They found that in 

 general V. macrophysa exhibits a lower potassium to sodium ratio 

 than V. ventricosa (Table 17b), but plants of V. macrophysa exposed 

 to long daily periods of full sunlight exhibited a higher ratio than 

 those growing in more shaded situations. The quantity of illumin- 

 ation does not seem to affect the cation ratio in V. ventricosa which 



