THE FUNCTIONS OF THE WOOD 



189 



wood parenchyma. If these cells were burst by the 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 semi-permeable membranes will filter the solutions 

 of the vacuoles, and the escaping liquid will dilute the 

 wood sap with nearly pure water, as previously shown by 

 the authors (1913, 1). Evidently, then, the true concen- 

 tration of the sap in the tracheae may be very different from 

 that pressed from the wood. 



The possibility of centrifuging the sap from the tracheae 

 of pieces of freshly cut wood subsequently suggested itself, 

 and this method was found very successful. 



The solution obtained thus was found to be much less 

 concentrated than that obtained by pressure. In the 

 following table are given measurements made on sap 

 derived by centrifuging pieces of the same branches and 

 roots as those which supplied the sap for the determinations 

 recorded in Table XLVII. 



TABLE XLVIII. 



SAP FROM Acer pseudoplatanus, AUGUST, 1913. 



Comparison of the depressions of freezing-point and the 

 conductivities of the liquids obtained from the wood by 

 the two methods shows conclusively that pressure, by 



