PROPERTIES OF PROTOPLASMIC MEMBRANES 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) 



23 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 Hving.^ Bernstein experi- 

 mented also with thin sheets of muscles (abdominal 

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

 and found that the rate of evaporation through the living 

 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 explana- 



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



