FACTORS AFFECTING PERMEABILITY 125 



ably varies not only with their concentration but also with different types of 

 cells, and with the same cell under different conditions. 



The effects of a mixture of two entirely different salts upon permeability 

 should also be considered. This is illustrated in an experiment described by 

 Bayliss ( 1924). A slice of freshly cut red beet was immersed in a solution of 

 0.31 ]M sodium chloride. It was observed that the red pigment gradually 

 diffused out into the solution. Since when similar slices were immersed in 

 water no such outward diffusion of red pigment occurred this showed the 

 effect of sodium chloride in increasing permeability. If, however, enough 

 calcium chloride was added to a portion of the sodium chloride solution before 

 the slice of beet was placed in it to bring the concentration of the former salt 

 to 0.17 M no such outward diffusion of red pigment could be detected. 

 Evidently the presence of Ca++ ions in some way offset the usual effect of 

 Na+ ions upon permeability. Such an effect of one ion upon another is 

 known as antagonism (see also Chap. XXV). 



3. Non-electrolytes. — Certain non-electrolytes are known to have effects 

 upon permeability. Among these are the anaesthetics. Exposure of the cyto- 

 plasmic membranes to extremely low concentrations of ether or chloroform 

 has no apparent effect upon their permeability; exposure to slightly higher 

 concentrations decreases their permeability. With exposure to still higher con- 

 centrations this decreasing effect on permeability is only temporary, and is 

 followed by a rapid increase in permeability which may continue until the 

 value characteristic for the death point of that tissue is reached. This sec- 

 ondary effect of anaesthetics is due to the toxic effect of high concentrations 

 upon the protoplasm resulting in its death after a relatively short period of 

 time. The initial decreasing effect of anaesthetics upon permeability is re- 

 versible, but the secondary increasing effect is usually non-reversible. 



4. Hydration. — Changes in the osmotic or imbibitional relations of the 

 water in other parts of the cell may induce changes in the hydration of the 

 cytoplasmic and cell wall membranes. It is inferred that variations in the 

 permeabilit}^ of the cytoplasmic membranes occur with shifts in their water 

 content. Such changes in permeability are often presumed to be related to 

 changes induced in the viscosity of the cytoplasm due to variations in its degree 

 of hydration. Extreme desiccation of most cells results in a coagulation of 

 the protoplasm, and a consequent complete loss of the property of differential 

 permeability by the cytoplasmic membranes. In some cells, however, particu- 

 larly those of drought resistant plants, the cytoplasm may regain its normal 

 condition of differential permeability when water again becomes available after 

 a period of practically complete desiccation. 



5. Light. — As shown by the work of Lepeschkin (1930) and others, ex- 



