26 PLANT PHYSIOLOGY 



of them pass into the cell so easily and rapidly that no plasmolysis 

 is induced. To this group belong chiefly the alcohols, the ethers, 

 and some narcotics, such as chloroform. A careful investigation 

 of various substances in respect to their capacity to penetrate 

 into the cell drew Overton's attention to the fact that the proto- 

 plasm is readily permeable to those that are easily soluble in fats. 

 Substances insoluble in fats penetrate with greater difficulty or 

 not at all. And since the osmotic diffusion of a substance 

 through a septum is determined, in the first place, by its solu- 

 bifity in the material forming the septum, Overton draws the 

 conclusion that the protoplasmic membrane must be composed 

 chiefly of fats or fatlike compounds, such as lecithin and other 

 lipoids. 



It must be noted that, according to the view held by Pfeffer 

 and now shared by the majority of investigators, the properties 

 of the protoplasmic sac are not uniformly the same throughout 

 its extent. The external layer, the one nearest to the cell wall, 

 and the internal layer lining the vacuole show semipermeability 

 to a considerably higher degree than the intermediate layers. 

 Pfeffer has called the external layers the ''plasma membrane," 

 or ''Plasmahaut." In Overton's opinion, this surface layer 

 consists chiefly of fathke substances, which accumulate there 

 because fats decrease surface tension. According to a funda- 

 mental law of physical chemistry, substances decreasing surface 

 tension are bound to accumulate at the surface (Gibbs' law). 



The lipoid theory of Overton, therefore, explains well the 

 ready permeability of the cell to narcotics, alcohols, and esters; 

 but it gives no answer to the question why water, though insoluble 

 in fat, rapidly enters the cell, and why salts and sugars, which are 

 readily soluble in water, penetrate but very slowly or not at all. 

 Hence, this theory must be regarded as only a proximate idea. 



Another theory of permeabihty, first proposed by Traube 

 (1867) and again put forward by Ruhland, assumes that the 

 structure of the protoplasm is similar to a very fine molecular 

 sieve, or ultrafilter, letting pass small molecules but holding 

 back molecules or molecular aggregates. This theory was 

 developed by Ruhland chiefly on the basis of his experiments 

 with the penetration of dyes, which did not show any parallelism 

 between the solubility of dyes in lipoids and their permeabihty. 

 Ruhland also assumed that undissociated molecules of salts 



