Mason — Electrolytes in the Leaf-sap of Syringa vulgaris. 665 



on the concentration of sugars in the sap, but on the magnitude of the 

 osmotic pressure. 



Assuming, then, that the rate of carbon assimilation were for any reason 

 retarded, a decrease in the supply of sugars entering the cell would take 

 place, and ultimately a fall in the osmotic pressure. With this decline in the 

 osmotic pressure of the cell, the rate at which electrolytes could be removed 

 from solution must, by assumption, be diminished ; but as the supply of 

 electrolytes entering the cell would continue as long as the rate of. transpira- 

 tion was unchecked, a rise in the osmotic pressure would Lhus be brought 

 about. 



As a result of this, the rate of metabolism would be again accelerated, and 

 no further accumulation of electrolytes would occur. In this manner a 

 decrease in the rate of carbon assimilation might effect a large concentration 

 of electrolytes. The prominent part which electrolytes play in the produc- 

 tion of the osmotic pressure of plants growing in greenhouses, to which Dixon 

 and Atkins have drawn attention, may provisionally, the writer considers, 

 be explained from this point of view. The rate of carbon assimilation must 

 be often limited by the reduction in the intensity of the illumination in 

 greenhouses. It would not be profitable, however, to speculate further until 

 the nature of the ions responsible for the fluctuations in conductivity have 

 been established. 



Summary. 



1. The determination of the concentration of electrolytes of the cell by 

 means of conductivity observations has been found unsatisfactory, unless 

 allowance is made for the viscosity of the sap. Methods of making a correc- 

 tion for the viscosity have been described. 



2. The osmotic pressure of the cell is frequently mainly due to elec- 

 trolytes ; but the presence of solutes which are neither electrolytes nor 

 sugars is not excluded. 



3. Considerable fluctuations in the concentrations of electrolytes in the 

 leaf-sap of Syrin<ja vulgaris trees growing in different localities have been 

 indicated. 



4. A tendency for the concentration of electrolytes to vary inversely with 

 that of the non-electrolytes has been found. 



5. It is suggested that these fluctuations are associated with the rate of 

 carbon assimilation, which determines the rate at which electrolytes are 

 removed from solution in metabolism. 



