IMPORTANCE OF WATER 111 



who first investigated the phenomenon of plasmolysis, noted that 

 some substances, for instance, salt and sugar, induce persistent 

 plasmolysis lasting for hours; while others, as glycerin and urea, 

 cause only a temporary plasmolysis. In a few hours the shrunken 

 protoplasm regains its former shape and once more becomes closely 

 appressed to the cell wall. De Vries explains this phenomenon 

 quite correctly. The substances slowly penetrate into the cell, 

 their concentration outside and inside the cell becoming equal, and 

 the distending influence of the substances contained in the cell sap 

 again manifesting itself. 



Further experiments have established that the number of sub- 

 stances that enter readily into the cell is rather considerable. 

 Many of them pass into the cell so easily and rapidly that no plas- 

 molysis is induced. To this category belong chiefly the alcohols, 

 the ethers, and a series of narcotics, such as chloroform, etc. A 

 careful investigation of various substances in respect to their 

 capacity to penetrate into the cell has drawn Overton's attention 

 to the fact that the protoplasm is readily permeable to those sub- 

 stances which are easily soluble in fats. Those insoluble in fats 

 penetrate with greater difficulty, or not at all. And since the 

 osmotic diffusion of a substance through a septum is conditioned, 

 in the first place, by its solubility in the material forming the sep- 

 tum, Overton draws the conclusion that the protoplasmic mem- 

 brane 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 impermeability to a 

 considerably higher degree than the intermediate layers. Pfeffer 

 has called these layers the plasma membrane or " Plasmahaut." 



In Overton's opinion, this surface layer consists chiefly of fat- 

 like substances which accumulate there, owing to the fact that 

 fats decrease surface tension. And according to a fundamental 

 law of physical chemistry, substances decreasing surface tension 

 are bound to accumulate near the surface (Gibb's law). The 

 lipoid theory of Overton, therefore, explains well the ready per- 

 meability of the cell to narcotics, alcohols, and esters. But it 

 supplies no answer to the question why water, though insoluble in 



