226 INJUEY, EECOVERY, AND DEATH 



has a higher resistance than the interior. It should be 

 noted, however, that in these methods the experimental 

 errors are so great that the results must be accepted 

 with caution. 



The view that the surface layer of the protoplasm is 

 less permeable than the interior has long been current. 

 Such a layer need not be a visible membrane:*" on the 

 contrary, it need only have the thickness of a single layer" 

 of molecules. This surface layer is commonly spoken of as 

 the plasma membrane, but the writer prefers the term cell 

 surface (since a morphologically distinct membrane is not 

 necessary in order to ensure selective permeability). If 

 it is not necessarily a visible structure, we may ask what 

 evidence there is for its existence and whether it is any- 

 thing more than a convenient fiction. 



It is easy to understand how the idea of the plasma 

 membrane was accepted by botanists. In many cases 

 the interior of the plant cell is filled with cell sap around 

 which the protoplasm forms a layer, so thin as to be almost 

 invisible under the microscope, except under the most 

 favorable conditions. In such a case the whole of the 

 protoplasm might be looked upon as constituting the 

 plasma membrane. 



"When the layer of protoplasm is thicker, it may be 

 shown that there are differences between the permeability 

 of its inner and outer surfaces, de Vries found that 



"A layer of liquid may serve as in the experiment of Nernst (1904) 

 where a layer of water is interposed between pure ether and benzene dis- 

 solved in ether; such a layer is permeable to ether, but not to benzene. 

 In the same way a layer of air may be employed, e.g., the layer of air 

 over an aqueous solution of cane sugar is permeable to water molecules, 

 but not to sugar. If we place under a bell jar two beakers, one con- 

 taining pure water and the other sugar solution, the water will pass 

 over, in the form of vapor, into the sugar solution. 



"Of. Langmuir, I. (1917). 



