664 Scientific Proceedings, Royal Dublin Society. 



It is clear, however, that the conductivities of the ascending saps bear no 

 relationship to those recorded for the leaves in Table IV. 



Discussion. 



In what follows it has been necessary to assume, since nothing is known 

 of the nature or concentration of the organic ions, that the fluctuations in 

 the conductivities recorded in Table IV are mainly due to changes in the 

 concentration of the inorganic salts dissolved in the leaf-sap ; it is probable, 

 however, that the organic constituents of the leaf-sap play only a minor part 

 in the conductivities recorded. 



The concentration of electrolytes in the leaf is mainly a function of the 

 rate at which salts have entered with the transpiration current, and been 

 removed in the course of metabolism. The factors on which the supply of 

 salts entering the leaf depend are, firstly, the concentration of salts in the 

 ascending sap, and secondly, the rate of transpiration. It has already been 

 seen that the former is not responsible for the fluctuations encountered in 

 electrolytes of the leaf. On the other hand, that the rate of transpiration is 

 not the sole factor is demonstrated by the fact that in the sap pressed from 

 leaves with a southern exposure a smaller conductivity is often met than in 

 that of leaves from the north side of the tree, where transpiration is normally 

 less rapid. 



The factors tending to limit the concentration of electrolytes may be now 

 considered. An ion, which takes part in the synthesis of organic molecules, 

 will presumably be removed from solution provided the necessary concentra- 

 tion of sugars is present ; Schimper's (7) and Zaleski's (8) investigations on 

 protein synthesis clearly show that this is so in the case of nitrates. If, on 

 the other hand, the ion takes no part in metabolism, its concentration in the 

 leaf will continue to rise whether the sugar supply is large or small. 



If the distribution of plants showing low conductivities and high non- 

 electrolyte contents in the leaf were purely fortuitous, then the nature of the 

 ions in the sap might provide the explanation of the fluctuations in the 

 concentration of electrolytes; but, as it is in the locality where carbon 

 assimilation is limited that a high conductivity and low non-electrolyte 

 content is encountered, it would appear that it is in the rate of carbon 

 assimilation that the explanation must be sought. It is significant that it is 

 in leaves where the sugar-content is relatively low that the electrolytes 

 assume a very prominent part in the maintenance of the osmotic pressure ; 

 in some cases even the major part. 



It may be of interest to consider what might be expected to occur if the 

 rate at which inorganic salts were removed from solution was dependent, not 



