Dixon and Atkins — Osmotic Pressures in Plants. 435 



It is needless to point out tliat there is no necessary connection between 

 the osmotic pressure and the specific electrical conductivity, as the former 

 depends on the tolal amount of solutes in the sap, and the latter on the 

 amount of electrolytes alone. 



Determinations of the mean molecular weight of the sap solutes liave been 

 omitted, since, not only is the desiccation of the sap, which is necessary 

 according to tlie usu.al method, very tedious, but also the presence of 

 undetermined quantities of colloids leaves these estimates open to an error, 

 which is probably only constant for each organ dealt with. Hence the 

 electrical conductivities, although, as we shall see presently, open to another 

 error, seemed more suitable for our present purpose. 



Measurements of the specific conductivity of plant juices have been made 

 previously by other investigators, e.g., Heald (8) and Nicolosi-Roncati (9); 

 the apparatus used in the present work differs in no essential respect from 

 that used by Heald, viz., a Hamburger conductivity tube of 2-3 c.c. capacity, 

 a resistance box, a metre bridge with sliding contact, inductorium, and 

 telephone. The whole was supplied to us by Messrs. F. Kohler, Leipzig. 



The constant of the tube was found by means of N/25 solution of potassium 

 chloride. All measurements were made at 0° 0, and according to the usual 

 metliod described in Oswald's Physico-Chemical Measurements, or Findlay's 

 Practical Physical Chemistry. The results are expressed as reciprocals of the 

 resistances measured in ohms, not in Siemen's units according to Hamburger's 

 custom (7). 



With regard to the values obtained for the conductivities of the saps, it 

 must be pointed out that only those carried out on tlie same organ of each 

 species are strictly comparable. Even here errors may be present. Such 

 uncertainties are due to variations in the viscosity of the saps, owing to the 

 various solutes and colloids which they contain. This is clearly brought out 

 when comparative experiments are made on the conductivity of crude sap and 

 of the same sap after filtering or centrifuging. In our determinations the 

 saps used were first brought to conditions as uniform as possible, by filtering 

 or centrifuging to remove the suspended matter. It may be noted that many 

 saps gave a clear fluid when centrifuged, though they could not be easilj' 

 filtered ; but, except in the case of very viscid saps, the difference in the 

 conductivities after centrifuging and after filtering was found to be negligible. 

 The centrifuge employed attained a speed of 9000 revolutions per minute, 

 but even prolonged treatment at tliis speed failed to clear some pulpy juices. 

 It was found that a speed of 3000-4000 revolutions per minute was quite 

 ineffective where leaf-saps were dealt with. 



Heald's observation of tlie close agreement of the conductivity of the sap 



