400 True AND Gres: Heavy METALS 
In the cases of our mixtures of salts having different anions, 
chemical reactions might be regarded as possible, with a conse- 
quent change in the forms of molecules. Here again, however, 
the great dilution of the salt of the heavy metal in our most im- 
portant mixtures produced complete or nearly complete ionization, 
the heavy metal acting practically as free ions. We can then 
hardly regard changes of an ordinary chemical nature as being — 
responsible for the differences in the physiological results. We : 
think that interior physiological modifications are responsible for 
the observed differences in growth rate. This belief implies that 
the simple salt of the heavy metal and the mixture of this salt with 
that of a lighter metal, after penetration into the cell, affect the 
processes there being carried on in such a way as to bring about 
different results on cellular growth. In studying the effect on 
growth of simple solutions of copper and calcium salts, for example, 
we see that at the concentrations employed copper retards growth 
whereas the calcium salts greatly stimulate it. With each we 
have, in all probability, to do with antagonistic phases of physio- 
logical action. When we examine the results in cases like the 
above, it seems highly probable that the so-called antitoxic action 
of ions is due to different interior physiological modifications, and 
that the growth-rate observed in such experiments as these repre- 
sents the physiological sum of oppositely acting stimuli, or of 
antagonistic protoplasmic changes. * 
It has been shown that when salts of heavy metals are suffi- 
ciently dilute they exert a stimulating effect on growth, and when 
solutions of calcium and similar salts are concentrated enough, 
they hinder or entirely prevent growth, and may, in the case of 
the more soluble chloride and nitrate, prove fatal. Coupin + has 
shown that at different dilutions compounds exhibit three distinct 
phases of physiological action. When the solution is sufficiently 
dilute it seems too attenuated to produce any effect on growth. 
As the concentration increases, a stimulating phase is seen, which, 
on further concentration, passes over into the retardation phasé — 
pronounced in proportion to the concentration. 
Os ae 
* Loeb and Gies, /. c., 267. 
ft Coupin, H. Sur la toxicité du chlorure de sodium et de l’eau de mer € Végard 
des végétaux. Rev. Gén. Bot. 10: 177. 1898. 
