I 6 CELLULAR 



I proximately as much water as before the sub- 

 j stance was added. The cell in consequence 

 recovers its normal appearance. The time re- 

 quired for such recovery is commonly regarded 

 as an approximate index of the rate of penetra- 

 tion of the substance in question. 



Even under the most favorable conditions 

 this method is more or less injurious. When the 

 protoplasm is torn away from the wall of a plant 

 cell its surface is usually altered and is often 

 torn so that detached bits of protoplasm remain 

 adhering to the wall. Even if the cell subse- 

 quently recovers after such treatment we can- 



/ not be sure that during the experiment it was 



( entirely normal. 



Another method is by measuring electrical re- 

 sistance. In earlier experiments it appeared pos- 

 sible to measure the permeability of normal cells 

 to ions by this method. In measuring the electri- 

 cal resistance of the marine plant Laminaria by 

 means of an alternating current it was assumed 

 that the current passed in part through the pro- 

 toplasm and in part through the spaces between 

 the protoplasmic masses. Hence by making al- 



I 



