THE RELATION OF THE PLANT TO WATER 161 



up water by imbibition tend to swell or increase in bulk. This 

 is an important factor in plant growth, for it must be remem- 

 bered that not all parts of the living plants are necessarily alive. 

 The heart wood of the living tree is composed of dead cells 

 through which a great deal of water is passing by means of im- 

 bibition. The coats of seeds are composed of dead cells which 

 imbibe the water very readily. 



Diffusion. This is the gradual movement or separation of 

 the molecules of a gas or liquid (or in some cases of a solid) to a 

 condition of less concentration. When two gases or liquids are 

 in contact with each other there is a tendency to form a mixture 

 of equal density throughout. This occurs even though the 

 lighter of the two may be uppermost. It may be readily demon- 

 strated by placing a crystal of some readily soluble colored ma- 

 terial in a glass of water. The color will gradually diffuse 

 throughout the liquid independent of the convection currents. 

 This is equally true of any of the soluble materials, although 

 there may be no color by which the movement may be observed. 



The soil water contains many different salts in solution which 

 become thoroughly mixed as a result of diffusion and other 

 movements. The atmospheric gases become thoroughly mixed 

 as a result of the wind currents, but there is also a continuous 

 exchange of gases through the stomata which is due to diffusion 

 (see pages 172 and 193). 



Osmosis. Osmosis is a purely physical process. When two 

 solutions of different density are separated by a membrane 

 which is permeable to the solvent and not to the solute (i.e., 

 semi-permeable membrane), there is always a movement of solv- 

 ent molecules from the less dense to the denser solution. Such 

 movement continues until the equilibrium is established. Tf the 

 11 



