Mason — Electrolytes in the Leaf-sap of Syringa vulgaris. 663 



of salts in the respective soils. It is tolerably clear, however, that the salt- 

 content of the soil of the Botanical Gardens is uniformly great, whereas that 

 of the Milltown area is consistently the smallest. 



These experiments do not, however, afford any explanation of the con- 

 centration of electrolytes which have been encountered in the leaf-saps. 



On the Concentration of Electrolytes in the transpiration current as a factor in 

 the determination of the electrolyte contents of the leaves. 



Dixon and Atkins' (1) work on the composition of the sap in the conduct- 

 ing tracts of trees suggested that some information might be obtained from 

 observations carried out on the sap of the transpiration current. 



The electrical conductivities and freezing-points of the sap obtained by 



centrifuging samples of wood from the base of the stem are shown in 



Table XI. 



Table XI (August 28th, 1919). 



The conductivities recorded in the table indicate the relative concentra- 

 tions of electrolytes in the sap ascending the stem ; they do not, however, 

 show the quantity of electrolytes entering the leaves with the transpiration 

 current, for this is a function of not only the concentration of electrolytes in 

 the ascending sap, but also of the rate of transpiration. 



The saps obtained from the xylem were all slightly acid to litmus; 

 whether this is due to the rupture of the living elements of the wood in the 

 preparation of the material for the centrifuge buckets, or is to be regarded as 

 the normal condition of the transpiration current in this tree, is not clear. 



It will be seen from an inspection of tables X and XI that the freezing- 

 points, and therefore the osmotic pressures of the soil-solution may be much 

 greater than that of the ascending sap ; should these conditions prevail while 

 root-pressure is active, it is evident that the part played by the living cells 

 of the root in producing this pressure cannot be a passive one. 



