INTRODUCTION 17 



both sodium and calcium combine with a constituent of 

 the protoplasm, forming a compound which inhibits the 

 death process. This enables us to formulate an equa- 

 tion by means of which the death curve in any mixture 

 of sodium and calcium can be predicted with consider- 

 able accuracy. 



The changes in electrical conductivity which occur 

 under the influence of reagents run parallel to changes 

 in the permeability of the protoplasm. This is to be 

 expected, since it is evident that when a current passes 

 from a salt solution into living protoplasm, ions must 

 enter the protoplasm, and if there is an increase in the 

 permeability of the protoplasm to these ions its electri- 

 cal conductivity must increase, and vice versa. The 

 electrical conductivity of the protoplasm may therefore 

 be regarded as a measure of its permeability to ions. 



The resistance of the tissue does not depend upon 

 the protoplasm alone, but also upon the cell wall and the 

 cell sap. But we find, as a matter of fact, that the re- 

 sistance of the protoplasm rises and falls with that of 

 the tissue as a whole. Hence when we observe that the 

 conductivity of the tissue increases in a solution of 

 NaCl and decreases in a solution of CaCl 2 , we may con- 

 clude that the permeability is increased by NaCl and 

 decreased by CaCl 2 . This is in harmony with experiments 

 in which permeability is measured by other methods 

 (such as plasmolysis, specific gravity, exosmosis, tissue 

 tension, and the diffusion of salts through living tissue). 

 It is likewise confirmed by direct determinations, in which 

 the penetration of various substances is ascertained by 

 testing for their presence in the cell sap. 



It has been observed in the course of these investi- 

 gations that plants which have developed in a normal 

 2 



