Dixon and Atkins — Osmotic Pressures in Plants. 425 



These figures show very plaiuly the increase of concentration in the later 

 samples, and by inference the still higher concentration of the sap remaining 

 behind in the pressed leaves. Hence, the concentration of the expressed sap 

 may be expected, in all eases, to be less than the average concentration in 

 tlie vacuoles of tlie tissues before tlie application of pressure. 



The explanation of the increasing concentration is not liard to find. When 

 tlie pressure is first applied, almost pure water is extruded from the intact 

 cells, for the protoplasmic membranes are sensibly semipermeable, permitting 

 water to pass out under pressure, but resisting completely, or in part, the 

 passage of dissolved substances. Even in tlie first pressing many of the cells 

 are usually burst, and their sap passes out with, and is diluted by, the much 

 more dilute sap coming from the uninjured cells. Subsequent pressings 

 contain the sap of a larger proportion of burst cells, and those which are now 

 burst have had their sap concentrated by tlie former application of pressure. 

 Hence, later samples must be more concentrated.' 



From this consideration it appears that the problem of obtaining an 

 average sample of the sap of a plant-tissue by pressure resolves itself into 

 the problem of rendering the cell-membranes permeable, so that the applica- 

 tioji of pressure will force out solvent and solutes alike. It goes without 

 saying that the method adopted for rendering the membranes permeable 

 must not itself alter the concentration of the sap. 



Exposure to toluene vapour first suggested itself as a means for rendering 

 the protoplasm permeable. Owing to its extremely small solubility in water, 

 it was hoped that it would not appreciably alter the freezing-point. By 

 experiment it was found that A for water saturated with toluene is 

 approximately 0-024°, so that the correction for its vapour going into solution 

 would not be a serious one. 



To test the efficiency of toluene vapour in making the protoplasm permeable, 

 a sample of leaves of Hedera Helix was gathered ; each leaf was halved, and 

 two lots (A and B) were made, eacli containing a half of every leaf. These 

 two lots were then kept under the same conditions of moisture and darkness 

 in closed glass vessels, the only difference being that in the vessel enclosing 

 lot A an open capsule containing cotton wool soaked in toluene was placed. 



' The pressing out of sap by mechanical pressiue from uninjured cells may be observed in an 

 apparatus used by one of us (8) some years ago for the determination of the osmotic pressure of the 

 cells of leaves. In this series of experiments leaf-bearing stems were sealed into a stout glass vessel, 

 ■wUle their ends projected beyond its end and dipped into a capsule of water. By a suitable 

 contrivance air was forced into the vessel, and when the gas-pressure within it attained a certain 

 magnitude the wilting of the leaves was observed and fluid was pressed into the capsule from the 

 branch. It was then possible, by relieving the pressure, to allow the leaves to recover theii' 

 turgescence and reabsorb water from below. 



