64 VEGETABLE PHYSIOLOGY 



till it appears as a rounded or irregular mass in the centre, 

 still coloured red. The salt solution has abstracted the 

 water from the vacuole, and the protoplasm, relieved of 

 the pressure outwards caused by the liquid in the latter, 

 has shrunk away from the walls. The membrane shows 

 its complex character, for the living protoplasm on each 

 side of the non-living cell-wall becomes separated from it. 

 But it will be seen that the colour will not penetrate the 

 protoplasm, which in fact retreats before the salt solution, 



a b 



FIG. 63. CELLS OF PARENCHYMA UNDERGOING PLASMOLYSIS. 



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



represents the protoplasm. 



holding the colouring matter of the vacuole in its own 

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

 the latter gradually passes 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. Not only did the protoplasm prevent the 

 colouring matter from escaping, but it also retained the 

 osmotic substances of the sap. 



The protoplasm thus can oppose the passage through it 

 of various soluble 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 



