278 DETERMINATIONS OF PERMEABILITY 



is to suspend the cell by a pair of forceps attached to the upper end, 

 cut oflf the lower end and bring it in contact with a glass slide, and 

 then grasp the upper end gently with another pair of forceps which 

 is slowly moved downwards while a slight pressure is maintained. 

 The cell sap then flows out on to the glass slide. 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 approximate quantita- 

 tive results may be obtained. 



Since in previous investigations the writer had employed indirect 

 methods of testing permeability it was of considerable interest to com- 

 pare such results with those obtained by direct tests of the cell sap. 

 An investigation was therefore made in which the permeability of 

 Nitella was tested by determinations of plasmolysis and of electrical 

 conductivity as well as by the direct method. This may be illus- 

 trated by a series of experiments^^ with NaNOs 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. The experiments re- 

 quire continuous observation of individual cells, since (especially in 

 unbalanced solutions) recovery is promptly followed in many cases 

 by injury and false plasmolysis, ^^ which may be mistaken for true 

 plasm, oly sis. 



It is evident that plasmolysis may be injurious to many cells even 

 in a balanced solution;^* while in an unbalanced solution there may be 



^- All the experiments were performed at about 19°C. All the solutions were 

 approximately neutral. 



^^Osterhout, W. J. V., Bot. Gaz., 1908, xlvi, 53; 1913, Iv, 446; Science, 1911, 

 xxxiv, 187. 



^* For this reason penetration may be more rapid than would otherwise be the 

 case. In order to reduce toxicity 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. 



