CONDUCTIVITY AND PEEMEABILITY 213 



gently with another pair of forceps, which is slowly moved 

 downwards. The cell sap then flows out on to the glass 

 slide in contact with the drop. By uniting the drops from 

 a number of cells it is possible to get a sufficient amount 

 for qualitative chemical tests, and in many cases approx- 

 imate quantitative results may be obtained. 



Since in previous investigations the writer had 

 employed indirect methods of testing permeability, it 

 was of considerable interest to compare these results with 

 those obtained by direct tests of the cell sap. An investi- 

 gation was therefore made in which the permeability of 

 Nitella was tested by the direct method, as well as by 

 determinations of plasmolysis and of electrical conduc- 

 tivity. This may be illustrated by a series of experi- 

 ments^e with NaNOg and Ca(N03)2. 



Experiments on plasmolysis were carried out by 

 placing the cells in a hypertonic solution and observing 

 the time required to recover from plasmolysis (without 

 removing the cells from the solution) on the assumption 

 that the more rapid the recovery, the more rapid is the 

 penetration of the salt. 



In these experiments the smaller cells near the tip 

 of the plant were largely employed. They were observed 

 in Syracuse watch glasses or placed on glass slides and 

 covered with large cover glasses, the edges of which were 

 sealed with vaseline. 



Plasmolysis may be harmful to many cells, even in a 

 balanced solution,^'' while in an unbalanced solution there 



"All the experimentB were performed at about 19° C. All the 

 solutions were approximately neutral. Cf. Osterhout (1922). 



=" For this reason penetration may be more rapid than would otherwise 

 be the case. In order to reduce toxicity chemically pure salts should be 

 used and the water should be distilled from quartz (or from glass which 

 has been in use for some time), using cotton plugs in place of rubber 

 or cork stoppers and rejecting the first and last parts of the distillate. 



