Dixon and Atkins — Osmotic Pressures in Plants. 



375 



In connexion with our investigations on the osmotic pressures and 

 conductivity of solutions of vegetable origin, it seemed to us desirable to make 

 observations on the sap drawn from the conducting tracts of trees. 



With this endiu view we subjected wood taken from freshly cut branches 

 and roots of Acer pseudoplatcnms in the montli of August to such pressure 

 that sufficient sap was yielded for our determinations. The results are given 

 in Table I. 



Table I. 

 Sap pressed from wood of Acer psendoplatamts, August, 1918. 



In this table under A are given the depressions of freezing-point due to 

 all dissolved substances ; under the electrical conductivity, or reciprocals of 

 the resistance, measured in ohms. This, of course, depends upon the coucen- 

 tration of the electrolytes only. Under Ac is given tlie depression whicli 

 would be caused by a concentration of- potassium chloride having tlie 

 observed conductivity. When this depression is subtracted from the total 

 observed A, we obtain an approximate estimate of the depression caused by 

 the non-electrolytes — mostly carbohydrates, as recorded under A-Ae. Under 

 P are given the osmotic pressures in atmospheres calculated from the total 

 depressions observed of freezing-point. 



It may be mentioned here for the sake of comparison that a — 

 1 per cent, solution of glucose gives a depression of 0"106°, and a 

 1 per cent. „ „ sucrose ,, „ „ „ 0'054°. 



Accordingly the sap must contain I'o - 3 per cent, of sugar at least, on the 

 assumption that the non-electrolytes are all glucose : or from 3-6 per cent, 

 approximately if sucrose forms the preponderating part. 



A little consideration made it clear that the liquid issuing from tlie crushed 

 wood must contain some sap pressed from the cells of the medullary rays and 

 of the wood parenchyma. If these cells were burst by tlie pressure, a more 

 or less concentrated solution liberated from their vacuoles would be set free 

 to contaminate the sap in the tracheae. If, on the other hand, the cells are 

 unbroken, their semipermeable membranes will filter the solutions of the 

 vacuoles, and the escaping liquid will dilute the wood sap with nearly pure 



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