58 VEGETABLE PHYSIOLOGY 



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 mem- 

 branes of the protoplasm offer a variable resistance to their 

 passage further. A further experiment will show a very 

 important 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 latter, 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 

 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 



