PERMEABILITY OF PLANT CELLS 



131 



The analysis of the solution before and after living tissue has 

 been placed in contact with it (as well as the analysis of the tissue 

 itself) usually proves to be disappointing on account of adsorp- 

 tion and of errors introduced by dead cells or by intercellular 

 structures or spaces. 



Another way of testing penetration is by plasmolysis, accord- 

 ing to the method of Pfeffer and DeVries. If a solution plas- 

 molyzes a cell without injuring it and of the protoplast does not 

 subsequently expand if left in the solution it is clear that the 

 dissoh'ed substance does not penetrate. If it penetrated it would 

 gradually raise the osmotic pressure within the cell until the latter 

 was equal to the external osmotic pressure. The protoplast 

 would consequently expand and return to its original condition. 



By means of this method Overton tested the behavior of a 

 great variety of substances. He drew the general conclusion 

 that the rate at which they penetrate is a function of their lipoid 

 solubility, and assumed that the outermost layer of the li\dng cell 

 is composed of lipoid. 



A consideration of all these results shows that future progress 

 must depend very largely on the introduction of quantitative 

 methods. To the writer it seemed that the plasmolytic method, 

 as employed by Overton, Lepeschkin and others, needed impor- 

 tant improvements in order to develop it into a reliable quantita- 

 tive method. These may be stated as follows: 



a. The first requisite was to find a solution which could be 

 used as a general standard of comparison, the chief requirement 

 being that it should have a purely osmotic action, involving no 

 alteration of the normal permeabiUty of the plasma membrane. 

 It had previously been assumed that sugar, NaCl, KNO3 and 

 various other substances fulfilled this requirement, but the writer 

 found this assumption to be erroneous.^ It was found, however, 

 that a properly balanced solution fulfils this requirement. In 

 most cases sea water (suitably concentrated or diluted) proves 

 to be an entirely satisfactory balanced solution and solves per- 



" The proof of this will be given when describing the electrical method of meas- 

 uring permeability. 



