110 PLANT PHYSIOLOGY 



wS{ be movements of the salt as well as of the water mole- 

 cules through the membrane if, in the other cases, we sepa- 

 rate the two volumes of liquid by a permeable membrane. 

 As we have already seen ( p. 106 ) , this form of movement, 

 of diffusion, is called osmosis. The rate of osmotic transfer 

 will vary for every salt according to the difference in the 

 proportions of the salt in the two adjacent liquids. This 

 difference is known as the osmotic pressure. Furthermore, 

 the rate of movement will differ with the salt, with the 

 composition, thickness, etc., of the membrane, and with 

 other factors (e. g: the relations of the salts to one another, 

 with the dissociation, etc. ) which find their natural place 

 for discussion in a text-book on physics.* 



Turning back now to our vegetable cell — an alga, a root- 

 hair, a parenchyma cell, etc. — a body consisting of aqueous 

 solutions enclosed in and permeating concentric membranes 

 of different physical and chemical properties, we see that we 

 have precisely the conditions imagined in our discussion of 

 the purely physical phenomena. The cell-sap is a solution 

 greater in density than the water outside the cell and dif- 

 fering from it in composition. In consequence, molecular 

 movements wiU take place into and from the cell. These 

 movements will tend to reduce the density and modify the 

 composition of the cell-sap. Because of the greater density 

 of the cell-sap — in other words, because of the smaller pro- 

 portion of water in the cell-sap to substances dissolved in 

 it — water molecules will pass into the cell through the cellu- 

 lose wall, the cytoplasmic membranes, and the protoplasm, 

 diffusing- throughout the cell as well as in the vacuoles. 

 Thus the density of the cell-sap will be lowered, the propor- 

 tion of water to substances dissolved in it will be raised, 

 and the volume of the cell-sap will be increased. But if the 

 cellulose-wall resist any increase in the volume of the cell- 

 sap and of the cell, the cell-sap will be subjected to pressure, 

 the protoplasm will be forced by this means against the 

 cell-wall, the cell-wall itself will be stretched, the whole cell 

 will be in a state of tension, will be plump, will be turges- 



* See, for example, Ostwald's Solutions, translated by M. M. P. Muir, 

 London, 1891. 



