TUBGIDITY 85 



analyses of the sap itself and his results show that different 

 plants vary much as to the nature of their osmotically 

 active substances. 



Heald 1 has recently determined the electrical conductivity 

 of sap expressed from the roots, stems, and leaves of various 

 plants. The amount of electrolytes thus indicated is in 

 reasonably close agreement with the amount of ash found by 

 incineration. This only goes to show that most of the 

 electrolyte molecules are dissociated in the sap, and are 

 therefore active in conducting the current. Any conclusions 

 with regard to the osmotic pressure of the sap which are 

 based on conductivity methods must be absolutely unreliable, 

 unless it is first ascertained that there are no non-conductors 

 present, and also that the electrolytes present are in the ionic 

 condition. But it is probably impossible to find a natural 

 plant juice whose solutes are all electrolytes. Therefore 

 Heald' s method cannot be of use in determining osmotic 

 pressures. The freezing-point, boiling-point, and vapor ten- 

 sion methods are applicable to this problem, however. 



Maquenne's work on the freezing-points of plant juices 2 

 has recently been added to by Sutherst. 3 The latter author 

 has merely given the freezing-points, without determining 

 the weight per liter, so that his results will not be available 

 for any determination of the nature of the solutes. In the 

 following table, taken from his paper, I have calculated the 

 osmotic pressures to facilitate comparison: 



Vegetable marrow: Fr. Pt. 



Leaf and stalk - - -0.75 6,880 



Fruit -0.75 6,880 



Swede turnip: 



Entire plant - -1.0 9,173.2 



IF. DE F. HEALD, "The Electrical Conductivity of Plant Juices," Science, N. S., 

 Vol. XV (1902), p. 457; IDEM, same title, Bot. Gaz., Vol. XXXIV (1902), pp. 81-92. 



2 See above, p. 84. 



3 W. T. SUTHEKST, "The Freezing Point of Vegetable Saps and Juices," Chem. 

 News, Vol. LXXXIV (1901), p. 234. 



