PROPERTIES OF PROTOPLASMIC MEIMBRANES 139 



(e.g., killed by heat). Results of the following kind 

 were obtained with frog skin: 



(A) Living (B) Dead 



After 4 h ht- 23 (mm.) 52 (mm.) 



6h 85 160 



8h 180 285 



10 h 256 347 



(both dead) 



22, h 401 400 



It is clear that the rate of evaporation is much greater 

 through the dead membrane. Similar experiments with 

 plant tissues (leaves) gave similar results; von ]Mohl 

 (in 1847) and Naegeli (in 1861) had already called 

 attention to the fact that frozen plant tissues (leaves, 

 fruits) dried more rapidly than living.'' Bernstein experi- 

 mented also w^ith thin sheets of muscles (abdominal 

 oblique of frog), both living and killed with chloroform, 

 and found that the rate of evaporation through the Uving 

 muscle was about half of that through the dead; the 

 difference was greatest during the first hour, when the 

 living tissue was relatively normal. 



Bernstein's conclusion was that living tissue has 

 greater water-binding power than dead, and he regarded 

 the electrically polarized condition of the plasma mem- 

 branes as the chief factor hindering the outward passage 

 of water. Apparently a potential difference of the value 

 of ca. 0.1 volt exists normally between the outer and 

 the inner surfaces of the plasma membrane; this gradient 

 is positive externally and negative internally. Resistance 

 is thus offered to the passage of the positively charged 

 water outwardly across the membrane. This exphma- 



^ Cf. Bernstein, op. cit., p. 170. 



