EELATION OF WATEE TO THE PEOTOPLASM 59 



membrane, even in the condition of dilution which must 

 result from its mixing with water which has been with- 

 drawn from the vacuole. If now the salt solution is 

 replaced by water, the latter is gradually attracted again, 

 of course osmotically, into the cell. It passes through 

 the whole thickness of the protoplasm, the vacuole is re- 

 established, and the protoplasm again comes to line the 

 cell-wall, pressed against it by the water. 



The protoplasm by means of its plasmatic membranes 

 thus can oppose the passage through it of various osmotic 



/, 



FIG. 53. CELLS OF PARENCHYMA UNDEBGOING PLASMOLYSIS 



a, b, c, d represent successive stages. The dotted area in each cell 

 represents the protoplasm. 



bodies with which it may be brought into contact, though 

 it allows the water in which they are dissolved to permeate 

 it freely. In the experiment just described the strong salt 

 solution failed to pass through the external plasmatic 

 membrane ; the re-entry of the water into the vacuole 

 showed that the internal one prevented the osmotic sub- 

 stances, originally present in the water which the cell 

 contained, from escaping in the issuing osmotic stream. 

 These substances must have been left behind, or there 

 would have been no osmotically active material to draw the 

 water back, when it was allowed to replace the salt solution 

 outside the cell. 



