OSMOTIC EQUILIBRIUM 85 



pressure of the cell sap.* The majority of vegetable cell 

 walls are permeable to dissolved substances as well as to 

 water, but some peculiar exceptions to this have recently 

 been investigated. 



Since the cellulose wall is usually in a distended con- 

 dition, it follows that when attempts are made to ascertain 

 osmotic pressure by the plasmolytic method, a very 

 appreciable volume change occurs in many instances, 

 before there is any perceptible forcing back of the 

 protoplasm from the cellulose. This is owing to the 

 diminished turgor of the cells and contraction of the 

 cellulose walls if they belong to young tissue. 



In addition to the source of error arising from the dis- 

 tension of the walls, there is always uncertainty as to 

 how far the agents employed for plasmolysis penetrate 

 the cells, for in various groups of plants marked differ- 

 ences are shown in this respect. 



CRYOSCOPY OF SAP. 



For the reasons just mentioned, it is desirable, whenever 

 possible, to investigate the relationship between the cell 

 sap and the medium with which it is in equilibrium, by 

 determining osmotic pressures by cryoscopy, combined 

 with the equation as given by Nernst : 



P = 12-03 A atmospheres at 0, where A is the depres- 

 sion of freezing-point of the liquid under examination. 



To obtain the cell sap required, it has been shown by 

 Dixon and Atkins (1913, 1) that it is not sufficient merely 

 to apply pressure to plant tissues, for this results at first 

 in the forcing out of water through the protoplasm, which 

 is later on mixed with the contents of many burst cells. 



* Determinations of the breaking stress of cellulose carried out by 

 Dixon (1897) show that an osmotic pressure of 100 atmospheres can be 

 withstood by cslltilose walls of the dimensions usually encountered. 



