64 VEGETABLE PHYSIOLOGY 
till it appears ag 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, 
Fite. 53,—Crtts or PARENCHYMA UNDERGOING PLASMOLYSIS. 
ud, 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 
