TOXICITY OF HEAVY METALS 89 



stimulating substances which bring them about. Copper 4s the most toxic 

 metal here dealt with, whether we consider its toxicity as bringing about 

 changes that result in death or as bringing about the less final changes 

 that lead to abnormal growth. In the same way, magnesium, calcium and 

 potassium are the least toxic metals of the list, on whatever physiological 

 criterion we may base our judgment. 



GENERAL DISCUSSION AND CONCLUSION. 



The results of the experiments described in the foregoing pages show 

 that, in certain cases at least, the effect of a toxic salt on the germination 

 of the conidia of Glomerella cingulata may be influenced by the addition, 

 to the medium, of calcium, magnesium or potassium nitrate. That this 

 effect is not due to depression of ionization of the toxic salt has been 

 demonstrated. That it cannot be due to the formation of undissociated 

 double salts has been shown for the combination of Cu(NO 3 ) 2 with 

 Ca( NO 3 ) 2 and of Zn(NO 3 ) 2 with calcium or magnesium nitrate. The influ- 

 ence of calcium upon the toxicity of the salts of the heavy metals here 

 employed must then be related to an effect of the Ca(NO 3 ) 2 on the spore, 

 or to an effect on the contained protoplasm, as is made otherwise probable 

 from the fact that copper, lead, aluminum and nickel nitrates sensibly effect 

 the protoplasm in various ways, without producing any apparent changes 

 in the spore wall. 



There are, of course, a number of logically possible explanations for the 

 antagonistic actions of salts upon the germination of fungus spores as 

 here recorded, but any such possible hypothesis must necessarily be yery 

 complex if it be able to account generally for even the small array of 

 facts so far at our disposal. 



Various theories have been suggested from time to time to explain similar 

 salt antagonisms. Loeb 28 has advanced the theory of ion-proteid formation, 

 suggesting that one or both ions of a salt may combine with proteids of 

 the protoplasm to form ion-proteid compounds. This writer found, for 

 example, that a solution of NaCl is poisonous unless some calcium and 

 potassium be also present in the medium, in which case the toxicity of 

 NaCl is greatly decreased or inhibited altogether. He considers that cal- 

 cium and potassium ions may to some extent take the place of the sodium 

 ion in the ion-proteid compounds above mentioned, and suggests that the 

 organism cannot live without some of these proteid compounds containing 

 calcium and potassium as well as those containing sodium. The work of 

 Osterhout 29 on balanced solutions supports this theory of Loeb. 



28 Loeb, J., On ion-proteid compounds and their role in the mechanics of life phenomena. I. The 

 poisonous character of a pure NaCl solution. Amer. Jour. Physiol. 7: 327-138. 1900. 



29 Osterhout, W. J. V. [06], On the importance of physiologically balanced solutions for plants. II. 

 Fresh water and terrestrial plants. Bot. Gaz. 44: 259-272. 1907. 



PHYSIOLOGICAL RESEARCHES VOL. I, NO. 2, SERIAL XO. 2 



AUGUST, 191^. 



