RELATION OF WATER TO THE PROTOPLASM 59 



outward stream has been accompanied to a certain extent 

 by an inward one, as in the first experiment. The coloured 

 salt solution will be visible inside the cell-wall, between it 

 and the protoplasm. There has been an osmotic stream 

 therefore through the cell-wall inwards. But it will be 

 seen that the colour will not penetrate the protoplasm, 

 which in fact retreats before the coloured salt solution. 

 The latter has no power to pass the external plasmatic 

 layer, even in the condition of dilution which must result 

 from its mixing with water which has been withdrawn from 



FIG. 53. CELLS OF PARENCHYMA UNDERGOING PLASMOLYSIS. 



a, 6, c, d represent successive stages. The dotted area in each cell 



represents the protoplasm. 



the vacuole. If now the salt solution is replaced by water, 

 the latter is gradually attracted again, of course osmoti- 

 cally, 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 thus can oppose the passage through it 

 of various osmotic 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 layer ; the re-entry of the water into 



