114 PROTOPLASMIC ACTION AND NERVOUS ACTION 



penetrates gradually, its water-abstracting effect is only 

 temporary, since its concentration, at first greater 

 outside than inside the cell, is eventually equalized by 

 diffusion; the cell then tends to resume its original 

 water-content. In the case of a solute which penetrates 

 rapidly (with a readiness like that of water) no effective 

 inwardly directed pressure can be exerted against the 

 membrane and no osmotic eft'ect is produced. 



Using plant cells (Spirogyra and others) and moder- 

 ately hypertonic solutions of various substances, Overton 

 found that in solutions of sugars, amino-acids, and neutral 

 salts the plasmolysis was permanent; in solutions of 

 glycerine, glycol, urea, and similar compounds the 

 degree of plasmolysis was less than in sugar solutions, 

 and after the initial shrinkage, water gradually re- 

 entered the cell; while in solutions of alcohols and many 

 other organic substances (of the same osmotic pressure 

 as the effective sugar solutions) plasmolysis was entirely 

 absent. Similar differences were typical of a large 

 number of other compounds. He therefore divided 

 soluble substances into three groups, according to their 

 ability to penetrate the living plasma membrane: 

 (i) Those to which the plasma membrane is completely 

 or nearly impermeable, including sugars, polyatomic 

 alcohols (from erythrite up), soluble amino-acids, neutral 

 salts of alkali and alkali earth metals; (2) those which 

 penetrate the membrane, but slowly and with varying 

 degrees of resistance, including glycol, glycerol, and 

 certain amides such as urea; and (3) those which enter 

 without encountering any evident resistance; here 

 belong a variety of organic compounds of the groups 

 cited above. These general conditions were found by 



