PERMEABILITY OF PLANT CELLS 141 



sea water in solid form salts of calcium, lanthanum, etc. This 

 causes a very marked increase in resistance which is completely 

 reversible. The experiment may be repeated on the same piece 

 of tissue for several successive days without injury. 



The addition of these salts in soUd form increases the conduc- 

 tivity of the solution and at the same time decreases the con- 

 ducti\'ity of the tissue. This affords the most convincing proof 

 that the change in the conductivity of the tissue in these experi- 

 ments can not be due to any cause other than a change in per- 

 meability; for the concentration of the ions of the sea water re- 

 mains unchanged, and if they were able to penetrate as freely as 

 they did before the addition of the salt the resistance would not 

 increase. It would, in fact, diminish on account of the increased 

 conductivity of the solution held in the cell walls, as is clearly 

 shown by experiments on dead tissue. 



It ma}' be remarked incidentally that these experiments effec- 

 tualh- dispose of the possible objection that the current passes 

 between the cells but not through them. Were this objection well 

 founded the decrease in conductivitj' could be explained only as 

 the result of a decrease in the size of the spaces between the 

 cells. This decrease could not be brought about except hy 

 greatly reducing the thickness of the cell walls. Both macroscopic 

 and microscopic measurements show most conclusively that this 

 does not occur. The contrary effect would be produced b}' the 

 addition of salts in solid form, for they would tend to produce 

 plasmolysis and thereby to increase the space between the cells. 



We are therefore justified in drawing the following conclusions: 



1. It is possible to cause rapid and very large changes in per- 

 meability by means of electrolytes. 



2. These changes may consist in either an increase or a 

 decrease in permeability. 



3. Within wade limits these changes may be completely reversi- 

 ble and entirely devoid of injurious effects. 



4. The plasma membrane is readily altered by a variety of sub- 

 stances in a fashion which indicates that it is protein rather than 

 lipoid. 



