108 PLANT PHYSIOLOGY 



The living plant-cell is then a series of concentric perme- 

 able (or partially permeable) membranes of different compo- 

 sition, properties, and needs, surrounding and enclosing 

 one or more bodies of water holding various substances 

 in solution. Under ordinary conditions the density of this 

 aqueous solution is greater than that of the solutions out- 

 side the cell, and its composition is different. The difference 

 in density is maintained in land plants, as we shall see from 

 another section of this chapter, by the loss of water from 

 the leaves ; the difference in composition is due to the activi- 

 ties of the protoplasm. In constantly submerged aquatics 

 the means of maintaining the density of the cell-sap is less 

 obvious, for from these plants no water is lost by evapora- 

 tion, and no concentration by this means takes place. Al- 

 though the water in the cells of algae may not be lost or 

 changed, the same result as regards the density of the cell- 

 sap may be attained in another way. If one or more solu- 

 ble substances are formed by the protoplasm and trans- 

 ferred to the cell-sap faster than they can pass out into the 

 surrounding water, greater density will be maintained. It 

 is upon the density and composition of the cell-sap, whether 

 this is accumulated in. larger volumes in vacuoles or is 

 uniformly distributed throughout the living protoplasm, 

 that absorption depends. 



DIFFUSION AND OSMOSIS 



Let us turn aside for a moment from the living cell to 

 consider some of the purely physical phenomena and princi- 

 ples underlying the physiological process we are studying. 

 If two equal volumes of liquid of exactly the same compo- 

 sition — say, two volumes of pure water — are brought into 

 contact with each other, there will be molecular movements 

 in and between them, but there can be no change in the 

 composition or pressure or any other quality of either. 

 Suppose five grammes of common salt to have been perfectly 

 and uniformly dissolved in one of these volumes of water 

 before the two were brought into contact. As a result of 

 bringing the two volumes of water into contact the molecu- 

 lar movements in and between the two volumes will produce 



