i8o TRANSPIRATION AND ASCENT OF SAP ch. 



Even in the 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 the former application 

 of pressure. Hence, later samples must be more con- 

 centrated. 



Necessity of rendering the protoplasm perme- 

 able. 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 application of 

 pressure will force out solvent and solutes alike. It need 

 scarcely be said that the method adopted for rendering 

 the membranes permeable must not itself alter the con- 

 centration. 



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. 



Effect of toluene vapour on protoplasmic per- 

 meability. To test the efficiency of toluene vapour in 

 making the protoplasm permeable, a sample of leaves of 

 Hedi ra helix was gathered ; each leaf was halved, and two 

 lots (A and B) were made, each containing a half of every 

 leaf These two lots were then kept under the same con- 

 ditions 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. After 48 hours the freezing-point and the electrical 

 conductivity of the sap pressed from the two lots were 



