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 



