276 Scientific Proceedings, Royal Dublin Socicfi/. 



C. Records of determinations of the mean molecular weights of the dis- 

 solved substances in the saps upon which the experiments were 

 made. 



Since tlie epocli-makiiig work of Pfeffer and De Vries tiie osmotic pressures 

 of plants have been investigated by many from various points of view. The 

 present investigation was undertaken primarily to determine if the osmotic 

 pressures in the leaf-cells of plants are sufficient to resist the tensions in 

 the sap indicated by tlie considerations on which the Cohesion Theory of the 

 Ascent of Sap is based.^ It was also proposed to determine if any relation 

 existed between the osmotic pressure in the leaves and the height above the 

 roots, or between it and tlie resistance of the stem connecting the water- 

 supply and the leaves. Other problems also presented themselves during 

 tlie work which liave not as yet been solved, and indeed afford a large field for 

 further investigation. 



Determinations of osmotic pressures in plants have usually been carried 

 out by the plasmolytic method ; and a number of determinations by this 

 method are available. These observations have been mostly made on the 

 tissues of stems, and, as a rule, liave not been applied to transpiring organs. 



Quite recently E. Pringsheim^ has applied the plasmolytic method to 

 determining the turgor of leaves, dealing mostly witli the leaves of succulents. 

 He states that tlie turgor of the plants he examined was greater in the apical 

 leaves than in tlie basal leaves, except in those of Phaseolus vulgaris. In 

 winter turgor is less than in summer. Bwart^ attempted to establish a rise 

 in osmotic pressure according to the height above ground. In one ease he 

 found tliat the leaves of an elm at a level of 1350 cm. above tlie ground had 

 an osmotic pressure two to three atmospheres greater than those at a level of 

 250 cm. He found high pressures, e.^., equivalent to the osmotic pressures 

 of a solution of potassium nitrate having a concentration of 6 per cent, or 

 more, and records that the " plasmolytic concentrations " for leaf-cells of 

 Acacia, Eucalyptus, and Grevillea vary in the same plant and at the same 

 level between wide limits. At the same time he comments strongly on the 

 want of precision inherent in the plasmolytic method. 



' H. H. Dixon, Transpii'iiuoii and the Ascent of Sap, Progressus Eei Botnuicte, Bd.iii. Hit. 1, 

 1909, p. 60 et seq. 



*E. Pringslieim, Wasserbewegnng und Turgorregulation in welkenden Pflanzen. Jahrb. f. wiss. 

 Bot. Bd. xliii. 1896. 



3 A. T. Ewart, On the Ascent of "Water in Trees (First Paper). Phil. Trans. Roy. Soe. 

 vol. cxcviii, 189.5, B, p. 77, and idem (Second Paper), vol. cxcix, 190S, B. p. 311. 



