MATILDA M. BROOKS 349 



penetration of Sr from a balanced solution containing 10 parts SrCla 

 (0.075 m), 9 parts tap water, and 1 part sea water, was still slower. 

 These experiments demonstrate that Li, Cs, and Sr penetrate into 

 the interior of the living cell and that this occurs more rapidly in an 

 unbalanced than in a balanced solution. That they may penetrate 

 in the form of ions is evident from the fact that in salts of these metals 

 the electrical conductivity'' of the living cell is such as to show that 

 the cations Li, Cs, and Sr readily penetrate the protoplasm. Since 

 this occurs at the very start of the experiment, before any appreciable 

 injury has occurred, it is evident that the cell in its normal condition 

 is permeable to these cations. 



SUMMARY. 



Direct tests of the cell sap of Nitella show that the protoplasm is 

 normally permeable to Li, Cs, and Sr, and that penetration is more 

 rapid in an unbalanced than in a balanced solution. 



^ These results were obtained by the method described by Osterhout (Osterhout, 

 W. J. v., J. Gen. Physiol, 1921-22, iv, 275). 



