292 



Scientific Proceedings, Royal Dublin Society. 



Discussion of Results. 

 At the outset it was necessary to find out if there really existed any 

 regularity in the osmotic pressures of leaves, and to examine the possibility 

 that the exact pressures were of no consequence, and that measurements of 

 them might show them to vary in quite an erratic manner. 



Experiments 62, 63, 64, 65 were made with a view to test this possibility. 



Table II. 



Syringa vulgaris : Leaves. 



Three samples of leaves were gathered from three similar adjacent 

 branches, which, so far as could be seen, were, and had been previously, under 

 precisely similar conditions of transpiration, assimilation, &c. The freezing- 

 point of the sap pressed from these three samples was observed ; and the three 

 readings in the first column show how closely the concentrations of the sap in 

 the leaves of the three branches agree. This result is what would a priori be 

 expected ; and the concordance of the experiments may be taken as a measure 

 of the accuracy of the results obtained by this method. It is to be noted 

 that these figures were obtained by the first arrangement before the complete 

 jacketing and the reversing-key were introduced. It appears that, even with 

 the less perfect arrangement, we may hope that the errors do not amount to 

 more than 1 per cent. 



A series of experiments was designed to test the possibility that the 

 osmotic pressure of the sap of the leaves at any region in the brandies is 

 defined by the resistance which has to be overcome in drawing the trans- 

 piration current from the roots to that part. Ewart' had previously looked 

 for such a difference by means of the plasmolytio method, but seems to have 



1 A. J. Ewart, Ascent of Water in Trees. PhO. Tr.\ns. Roy. Soc., B., vol., cxcviii. (1905) and 

 idem. vol. cxcix. (1908). 



