138 PROTOPLASMIC ACTION AND NERVOUS ACTION 



Such fine- textured membranes must resist the passage 

 of water as well as of dissolved substance. In Morse's 

 experiments many days were often required to reach 

 osmotic equilibrium. There is also evidence that many 

 living plasma membranes offer a high resistance to the 

 passage of water; i.e., are relatively water-impermeable. 

 This statement may seem surprising in view of the fact 

 that the passage of water into and out of living cells in 

 anisotonic solutions is usually rapid; but the membranes 

 are extremely thin and the ratio of the surface to the 

 inclosed volume is very large, so that relatively rapid 

 entrance of water is quite consistent with a very low 

 specific permeability to the liquid. 



The loss of semi-permeability at death is associated 

 with an increase of permeabiHty to water as well as to 

 dissolve substances. This is shown in some experiments 

 of Bernstein who, with another problem in mind (the 

 conditions under which water is held in cells) , determined 

 the relative rates of evaporation of water from living 

 and dead tissues.' Bernstein used the method (intro- 

 duced by Liebig) of measuring the ''force of imbibition" 

 of membranes. A tube widened at one end (thistle tube) 

 is fixed vertically with its narrow end dipping into 

 mercury; the tube is completely filled with water and 

 the upper expanded end is closed by the membrane under 

 examination. As water evaporates through the mem- 

 brane the mercury rises in the tube, showing the develop- 

 ment of a pressure; the rate of evaporation through the 

 membrane is thus indicated. Bernstein used fine tubes 

 ending above in equally sized funnels which were closed 

 (A) with living membrane and (B) with dead membrane 



^ Bernstein, Electrohiologie, Braunschweig (1912), pp. 167 £f. 



