PERMEABILITY OF PLANT CELLS 



143 



associated with an increase of permeability which is practically 

 irreversible.'" 



According to Czapek a variety of organic substances produce 

 an irreversible increase of permeability when their concentration 

 is such as to diminish the surface tension of the solution (against 

 air) by a definite amount. He concludes that this affords a 

 measure of the surface tension of protoplasm, and that the plasma 

 membrane is of the nature of lipoid, oil or soap. 



Another investigation relates to the sugars. It has been gen- 

 erally assumed that sugars act only osmotically {i.e. without 

 altering the permeability of the plasma membrane), and for this 

 reason they have been widely employed to determine osmotic 

 pressure by means of plasmolysis. In view of this it is of inter- 

 est to find that the electrical method shows that saccharose, glu- 

 cose and glycerine cause a marked increase of permeability, which 

 is reversible. 



One of the most important results of these studies is the proof 

 that the permeability of the plasma membrane is variable and 

 that it depends on the nature of the substances with which the 

 membrane is in contact. From this it follows that the internal 

 membranes of the cell (membranes of nuclei, vacuoles and plas- 

 tids, etc.) may have a different permeability from that of the 

 outermost plasma membrane, since they are in contact with 

 substances different from those which surround the cell externally. 

 The writer has been able to secure what he regards as con\'incing 

 experimental evidence of the truth of this deduction. 



The electrical method has an especial advantage in that it 

 enables us to detect the effects of injurious conditions or sub- 

 stances long before there is any visible indication of them. It 

 is thus preeminently adapted to studying the effects of toxic 

 substances and following their action from moment to moment. 



This method also gives us what may be called a quantitative 

 measure of \dtality. By this is meant simply that it is possible 

 to examine a plant in its natural environment and state whether 



i»Cf. Science N. S. 37: 111, 1913. 



