CHAPTER XXII 



SALTS 



Continued life is dependent upon the presence of salts in proto- 

 plasm and in the surrounding fluid. Salts serve organisms in 

 their metabolic activities both as constituents of substances 

 formed and as catalysts or activators of reactions. They serve 

 also as physical agents in establishing and maintaining osmotic 

 pressure, concentration equilibriums, and electric balance. No 

 cell or organism will live long in an environment the salt concen- 

 tration of which differs greatly from that of the natural sur- 

 roundings. Marine amoebae can be made to grow in fresh 

 water by gradually reducing the salt content of the sea water, 

 and fresh-water amoebae can be grown for a time in distilled 

 water. The small marine fish Fundulus will likewise live for a 

 while in distilled water. But life in such instances is temporary. 



The percentage of salts in protoplasm is about 12. The form 

 in which they occur is not known. On analysis, potassium may 

 be found as the ion, but this does not mean that it exists as such 

 in protoplasm. It may occur so, but it may also be bound, e.g., 

 to a protein. Analysis reveals only the presence of potassium. 



Until a few years ago, it was taught that there were 10 elements 

 necessary for plant life — carbon, hydrogen, oxygen, nitrogen, 

 sulphur, phosphorus, potassium, calcium, magnesium, and iron 

 — and that some few others, such as iodine, bromine, and silicon, 

 are found in certain plants for the growth of which they may be 

 necessary. But we now know that this number is wholly 

 inadequate. 



Sodium is not given in the list of the first 10 elements necessary 

 for plant life. The absence of this element in plant nutrition 

 and its necessary presence in animal food are distinguishing 

 features of the salt needs of these two groups of organisms. 

 Animals, especially herbivorous ones, need common salt, while 

 plants grow well in a solution wholly lacking in it; on the other 

 hand, the element sodium is not infrequently found as a con- 



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