PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 27 



penetrate into the cells more easily than ions. He explains this 

 by the fact that free ions are surrounded by a thick layer of water 

 molecules, firmly held by electrical charges. His opinion, 

 however, is contradicted by the fact that hydrogen and hydroxyl 

 ions penetrate very rapidly and easily into the cell. 



In general, the question of the permeability of protoplasm to 

 different substances is very complicated and is far from its com- 

 plete solution. A large number of substances essential for 

 nutrition, e.g., sugars and mineral salts, produce a permanent 

 plasmolysis, which seems to show that these substances do not 

 penetrate into the cell. This leads to the assumption that during 

 plasmolysis the properties of the plasmatic membrane alter in 

 the direction of a decrease of permeability. In this connection, 

 the experiments of Sabinin are illuminating. He found that 

 when the roots of seedlings are immersed in dilute solutions of 

 different salts, all these salts are very soon present in the exudate 

 that flows from the cut aerial parts of seedlings. Consequently, 

 the root cells are considerably more permeable to salts than 

 might be assumed on the basis of plasmolytic experiments. 



These data as well as the ready permeability of protoplasm to 

 water lead to the suggestion that the plasma membrane is built 

 up not only of Upoids but also of protein substances that swell 

 easily in water. Lepeschkin represents the plasmatic membrane 

 as a rather unstable chemical combination of proteins and lipoids. 

 Other authors suggest that these substances are not chemically 

 combined but form a sort of comphcated emulsion, as in the 

 mosaic theory of permeability suggested by Nathansohn. It is 

 of interest to note that the permeability of protoplasm to sub- 

 stances dissolved in water is not a constant magnitude but varies 

 considerably according to external and internal conditions. 

 Light, for instance, augments permeability of the plasma mem- 

 brane, and concentrated solutions decrease it considerably. By 

 means of certain stimuli, the permeability of protoplasm may 

 be greatly increased. In the case of extremely sensitive plants, 

 e.g., Mimosa, these changes lead to sharp movements that are 

 easily observed. This change of permeability perhaps is con- 

 nected with the variations in the relative amounts of protein 

 and lipoid particles in the surface layer. With the accumulation 

 of the first, salt permeability increases; with the predominance of 

 the latter, it decreases. 



