58 VEGETABLE PHYSIOLOGY 
noticeable as soon as the surface of the protoplasm is 
brought into. contact with water. The membrane of the 
cell, therefore, through which osmosis must take place, 
is composed of four different layers. In the experiment 
we have assumed that the outer liquid was pure water ; 
this is not, however, the case with the fluid in which the 
plant is living, for all such water contains a large number 
of various inorganic salts dissolved in it, though of course 
the concentration of these salts is extremely small. While 
all the layers of the cell’s membrane are permeable to 
water, they are not at all equally so to the salts which it 
contains. In such a weak solution these can pass freely 
through the cell-wall, but the plasmatic layers of the proto- 
plasm offer a variable resistance to their passage further. 
Moreover, the protoplasm is living substance, and no 
doubt takes an active part in the transmission of solutions 
through it. A further experiment will show a very im- 
portant modification of the process depending on this 
property of the protoplasm, and demonstrating that the 
entry of both water and its dissolved saline contents into 
the cell is very largely under the regulation of the 
living substance, when what is practically a dilute saline 
solution is presented to it. 
Take a cell of the cortex of a plant and put it into 
contact with a liquid of higher osmotic power than that 
which is contained in its own vacuole; for instance, a 
solution of common salt of about 10 per cent. concentra- 
tion. Watch its action on a slide under the microscope, 
and let the salt solution be coloured with some vegetable 
dye which will not injure the living substance. As the 
salt solution reaches the cell, the protoplasm of the latter 
gradually retreats from the walls (fig. 53), at first at the 
corners and then all round the sides, till it appears as a 
rounded or irregular mass in the centre. The salt solution 
has abstracted the water from the vacuole, and the proto- 
plasm, relieved of the pressure outwards caused by the 
liquid in the latter, has shrunk away from the walls. The 
