SALTS 435 



Is one element replaceable by another in the nutrition of a plant? 

 We shall later see that it is not. Were it so, then barium and 

 strontium, both closely related to calcium, should replace the 

 latter in plant nutrition. Both are far more toxic than calcium, 

 particularly barium; it is therefore obvious that they cannot 

 replace calcium. They do, however, occur in plants and animals, 

 though their function is unknown. 



Manganese. — McHargue has found that when plants are 

 grown in purified sand cultures from which manganese has been 

 carefully excluded and in which pure compounds of the so-called 

 10 essential elements are present in available form, the plants 

 make no further growth after the food material stored in the 

 seeds has been exhausted. The addition of a small amount of 

 manganese produces normal growth. 



As for animals, McCoUum makes it quite clear that manganese 

 is necessary. Rats restricted to a diet free from manganese grow 

 well, but while they produce normal litters, they do not care for 

 their young, and the latter die from neglect. 



Copper. — Copper is another metal that has come to the front 

 as a necessary constituent of diet. Its chief value to human 

 health is its effect on the availability of iron as a cure for anemia. 

 In fact, copper dissolved in milk is alone effective in the regenera- 

 tion of hemoglobin, though more so when accompanied by iron. 

 In plants, copper is necessary (or at least beneficial) to chloro- 

 phyll formation. Thus do both iron and copper stimulate the 

 production of the chief pigments of plants and animals, which 

 brings further evidence to bear on the analogy between chloro-; 

 phyll and hemoglobin. Other experiments, such as those of 

 Nageli, emphasize the high toxic effect of copper. Small aquatic 

 plants are not harmed by concentrations of 1:100,000,000 of 

 copper but suffer a pronounced reduction in growth by a con- 

 centration of 1:50,000,000. 



Zinc. — A. L. Sommer has found zinc necessary for the con- 

 tinued life of barley, wheat, etc. Zinc appears also to be bene- 

 ficial to microorganisms living in the soil. It may be necessary 

 for higher animals, as it occurs in calves' liver. 



Tin. — Tin is said to occur in some body organs. Its use is not 

 known. 



Aluminum. — Uncertainty has arisen over the presence and 

 function of aluminum in plant and animal matter. The quantity 



