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from its mixing with water which has been withdrawn from 
Fic, 538.—CELLS or PaRENCHYMA UNDERGOING PLAsMoLysIs. 
a, b, e, 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 
