118 TEXTBOOK OF PLANT PHYSIOLOGY 



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 only as a proximate idea, 

 requiring elaboration and correction. 



The question of the permeability of the protoplasm to different 

 substances is, on the whole, rather complicated, and still far from 

 a definite solution. A large number of substances absolutely 

 necessary for cell nutrition, such as sugars and mineral salts, move 

 freely from cell to cell in the body of a living plant. The same 

 substances, however, induce persistent plasmolysis lasting for 

 hours, and, consequently, under these conditions do not penetrate 

 into the cell. This contradiction leads to the supposition that in 

 plasmolysis the properties of the protoplasmic membrane have 

 changed in the direction of an increased impermeability. The 

 recent experiments of Sabinin are especially illustrative in this 

 regard. This investigator has shown that by immersing the roots 

 of seedlings in very weak solutions of different salts, the same salts 

 will be found in the fluid extruded from the cut surfaces of the 

 above ground parts of the plant under the influence of exudation 

 pressure. These experiments have shown that the root cells at 

 least are much more permeable to salts than one might think on 

 the basis of plasmolytic experiments. 



These results, as well as the ready permeability of protoplasm 

 to water, lead to the supposition that the structure of the proto- 

 plasmic membrane is made up not only of lipoids but also of pro- 

 tein substances, which swell easily in water. Lepeschkin conceives 

 of the protoplasmic membrane as an unstable chemical compound 

 of protein and lipoids. Other authors conclude that these sub- 

 stances do not combine chemically, but form a sort of complex 

 emulsion. It is interesting to note that the permeability of the 

 protoplasm to substances dissolved in water is not a constant 

 phenomenon. It varies considerably, according to changes in 

 external and internal conditions. Thus, for instance, narcotics 

 and light increase the permeability of the protoplasm, while strong 

 solutions, as has been shown, considerably decrease it. By means 

 of certain irritations, the permeability of protoplasm may be 

 greatly augmented. Thus, in case of extremely sensitive plants, 

 as for instance, Mimosa, these changes lead to sharp movements 

 which are perceptible to the eye. This variation of permeability 

 perhaps is connected with the variations in the relative amounts of 



