SELECTIVE PERMEABILITY 333 



not true, or unless there are some other factors involved which 

 cause plasmolysis or prevent its reversibility in case of electro- 

 lytes and sugars, we are confronted with the fact that the cell 

 is impermeable to substances of the highest importance in plant 

 nutrition, while it is permeable to substances which are non- 

 essential or are of no value at all. It is possible that the imper- 

 meability of the cell to the essential food elements is a provision 

 on the part of the cell against exosmosis while it is provided 

 with some means unknown to us for absorbing these elements 

 and translocating them from cell to cell. 



As it is, the plasmolytic phenomena are considered as one of 

 the means to demonstrate the property of selective permeability 

 of living cells. It is further demonstrated by the behavior of 

 the living cell toward aniline dyes which was first studied by 

 Pfeffer.2 jj^ -tj^jg p^se the permeability can be studied more 

 directly as the entrance of the dye is observed by the coloring 

 of the cell sap. It is, however, possible that observations of 

 this kind refer only to the accumulation of the dyes.^ The 

 property of selective permeability of the cell is further shown 

 by the facts that colored cell sap never enters the surrounding 

 living protoplasm and that cherries and red beets color water 

 only after the cells have been killed by heating.^ 



The question arises where is the seat of this property of selec- 

 tive permeability. Is it the entire mass of cytoplasm or only 

 its limiting surfaces. This brings up the question of the existence 

 of special plasma-membranes, i.e., whether the limiting layers 

 are of a different chemical or physical nature than the rest of 

 the cytoplasm or not. 



Physiologists are inclined to answer the question in the affirma- 

 tive. Pfeffer^ distinguishes between the ^'ectoplasmic" mem- 

 branes which separate the cytoplasm from the cell wall and the 

 endosplasmic membranes which are in immediate contact with 



* Pfeffer, W., tjber Aufnahme von Anilinfarl.ien in lebenden Zellen. Unter. 

 a. d. Bot. Inst. Tubingen 2: 179-332. 18S6. 



* Ruhland, W., Beitrage zur Kenntnis der Permeabilitat der Plasmahaut. 

 Jahrb. Wiss. Bot. 46: 29. 1908. 



< Hober R., Physik. Chemio der Zelle und der Gewebe, 53. 1906. 

 « Pfeffer, W., Pflanzenphysiologie, 8&-94. 1897. 



