334 J. DAVIDSON 



the vacuole. It is these membranes which are, according to 

 him, responsible for the osmotic properties of the cell and for 

 its selective permeability. He does not agree with DeVries 

 that the plasma-membranes originate from specific tissue, but 

 thinks that any part of the cytoplasm may be transformed into 

 a membrane when exposed to the surrounding medium, as it is 

 shown by the formation of vacuoles in the protoplasm which 

 is pressed out from a young root hair of Hydrocharis morsus- 

 ranae when it comes in contact with colored water. It is the 

 contact with the surrounding medium and the cell sap, which, 

 according to Pfeffer, causes the respective membranes to be 

 formed. He is inclined to think that the molecular forces in- 

 volved in the phenomenon of surface tension play an important 

 role in the formation of the plasma-membranes, but he does 

 not think that they are the only cause. The contact with the 

 surrounding medium may precipitate substances in the periph- 

 eral film of protoplasm thus transforming it into a plasma- 

 membrane. He believes that the nature of the surrounding 

 medium exerts some influence upon the formation of the plasma- 

 membrane, but the nature of that influence and the extent to 

 which it modifies the osmotic properties of the cell are not as 

 yet known. The properties acquired by the plasma-membrane 

 are not permanent, according to Pfeffer, and it is transformed 

 into ordinary protoplasm when it happens to be removed from 

 the periphery into the inner layers. It is evident that, according 

 to Pfeffer's conception, the differences between the plasma- 

 membranes and the rest of the cytoplasm are rather of a physical 

 than of a chemical nature. 



The following evidence is offered in favor of the existence of 

 special plasma-membranes. 



(1) When cells of a root hair of Hydrocharis are placed in an 

 isotonic solution of cane sugar to which a trace of hydrochloric 

 acid is added, the protoplasm becomes turbid, its streaming 

 stops — it is evidently killed. When the solution is then colored 

 with eosin the coloring matter does not enter the cells, notwith- 

 standing the fact that the protoplasm in them is dead. When, 

 however, the cells are transferred into a colored hypotonic 



