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XXIX. 



OSMOTIC PRESSURES IN PLANTS. 



II. — Ckyoscopic and CoNDuoriviTY Measurements on some 

 Vegetabi.e Saps. 



By henry H. DIXON, Sc. D., F.R.S., 

 University Professor of Botany, Trinity College, Dublin ; 



AND 



W. R. G. ATKINS, M.A., A.I.C., 

 Assistant to tlie Professor of Botany, Trinity College, Dublin. 



[Read December 17, 1912. Published Feuruauy 8, 1913]. 



In the paper (6) preceding this one we have shown tliat the sap pressed from 

 living untreated tissues does not give a true estimate of tlie concentration of 

 that in the vacuoles of the cells of the organ before the application of pressure. 

 In order to extract the sap from the cells without altering its concentration, it 

 is necessary to render the protoplasmic membranes permeable. This we found 

 might be effected by the application of liquid air. The discovery makes it clear 

 that our former work and that of others who employed sap expressed from the 

 living tissues for cryoscoj)ic or electrical conductivity determinations require 

 revision, and it becomes necessary to repeat our measurements of osmotic 

 pressure, making use of sap pressed immediately after thawing from tissues 

 frozen solid in liquid air. 



The first of these corrected observations are recorded in this and the 

 preceding paper. The osmotic pressures tabulated were calculated from 

 freezing-points found by the thermo-electric method of cryoscopy described 

 in earlier papers (1, 2). 



Specific electrical conductivities of the same saps were also measured. 

 The object of this was to trace out what part of the total osmotic pressure of 

 the sap was due to electrolytes, and how far such variations as were met with 

 are due to changes in the electrolyte content. For the sake of comparison 

 with the osmotic pressures, the conductivity measurements were made at 

 0° C also. 



