164 University of California Publications in Agricultural Sciences [Vol. 3 



is changed by the nature and balance of the solution surrounding the 

 roots there can be no doubt from the data already given. That a 

 number of ions are capable of acting in a very similar manner to one 

 another as regards permeability is also evident from the present work. 

 Further, the same salt may act differently at different concentrations, 

 preserving nearly normal permeability at some and allowing the pene- 

 tration of large numbers of ions at others. As previously stated, the 

 total balance of the solution is of vital importance in the preservation 

 of normal permeability, which is in turn correlated with normal 

 growth. 



In connection with the salts of the heavy metals, the amounts of 

 the kation of cupric and ferric salts which had penetrated the plant 

 tissue were determined in a number of instances. The percentages 

 found were low. Further, whenever these salts proved toxic, the 

 amounts of calcium and magnesium found in the plants were high; 

 high enough in fact to account for the toxic effect alone. In many 

 instances the percentages of those two elements found were as high in 

 toxic solutions of copper, iron, or zinc salts as when toxic concentra- 

 tions of calcium or magnesium chlorides were used. We might, 

 therefore, in the light of our present knowledge, be justified in attrib- 

 uting the decreased growth of the plants to the abnormally high ab- 

 sorption of calcium and magnesium and the consequent reactions 

 taking place within the plant . cells. The permeability of the mem- 

 brane must be altered to allow of the presence of these ions in large 

 numbers. The toxic effects due to the presence of large amounts of 

 calcium or magnesium salts might be evident if we could inject solu- 

 tions of these salts into the plant without altering the permeability 

 of the plasma membrane. But from the present data it seems that 

 the alteration in the permeability of the membrane is the essential 

 consideration. 



It is probable also that the toxicity of any solution is accompanied 

 by the increased permeability of the plant tissue to all inorganic salts 

 which are normally found in plants. There may be exceptions as 

 noted already for iron and calcium, but in general this relation holds 

 from the data now at hand. 



Ruprecht 61 has localized the effects of aluminum salts in the few 

 Layers of cells surrounding the root hairs and attributes the death of 

 the plants grown in solutions of aluminum salts to starvation incident 

 upon the inability of the plant to obtain nutrient salts for normal 



« Mass. Exp. Sta. Bull. 161 (1915), p. 125. 



