PERMEABILITY OF SURFACE LAYER OF CELLS. 189 



cell and disappears at death, which is always associated with a 

 loss of turgor and a diffusion of dissolved materials (coloring 

 matters, etc.) from the cell. The increase in permeability may be 

 the consequence, or it may be the cause, of the death-process ; 

 at all events it is the invariable concomitant of the latter a fact 

 apparently signifying that the living condition is incompatible 

 with more than a temporary loss of impermeability. 



The dependence of certain fundamental life processes (as 

 growth, which is a manifestation of osmotic energy) on the im- 

 permeability of the plasma-membrane is more clearly apparent in 

 plants than in animals. The proof that a similar impermeability 

 characterizes animal cells is due mainly to the use of the plasmo- 

 lytic method, which was first accurately applied to plants by de 

 Vries, and has later been extended to animal cells by Ham- 

 burger, Overtoil, Koeppe and others. The principle of the 

 method is well known ; the turgor-pressure of plant cells, due to 

 the osmotic pressure of the cell-contents, may be compensated by 

 the application of an external osmotic pressure, provided that the 

 boundary surface of the protoplast is impermeable also to the 

 external solute ; over-compensation, with compression of water 

 from the cell and a shrinkage of the protoplast from the cellulose 

 cell-wall, results if the external pressure distinctly exceeds that 

 of the cell-contents ; under-compensation leads to absorption of 

 water ; while with equality of external and internal osmotic pres- 

 sures no osmotic change is seen (cases of hypertonic, hypotonic 

 and isotonic solutions respectively). A means is thus afforded 

 of testing the permeability of the plasma-membrane to different 

 substances. If, using a moderately hypertonic solution of a given 

 substance, a loss of water results which remains permanent during 

 the period of immersion in the solution, impermeability relatively 

 to that substance is indicated (the case of sugars, polyhydric 

 alcohols, neutral salts). If the cell loses water only temporarily, 

 afterwards regaining its original proportions, or even swelling still 

 further, gradual entrance of the substance into the cell is indi- 

 cated ; such effects usually appear with solutions of urea, glyc- 

 erine, or glycol, to which most cells are slowly permeable. If 

 no plasmolysis results, even with strongly hypertonic solutions, 

 a free permeability to the dissolved substance must exist ; hence 



