The Structures and Processes of Roots 171 



water supply of plants, therefore, depends upon the presence 

 of living cells in the roots. 



Three physical processes are involved in absorption in 

 plants. These processes are diffusion, imbibition, and osmosis. 

 Their action may be demonstrated to a large extent by the 

 use of physical apparatus. 



Diffusion. If a small dish of ether is exposed in a room, in 

 a few minutes the odor of the ether may be noticed in all 

 parts of the room. Even if there were no air currents, the 

 ether would evaporate ; that is, particles of ether would rise 

 from the surface of the liquid, pass out of the dish, and move 

 through the room in every direction. This is an example of 

 the diffusion of a vapor. The vapor is concentrated in the 

 dish and the particles move outward into the room where 

 there is none ; that is, the particles move from the place 

 where the concentration is greatest to where it is less. After 

 the ether has evaporated, the vapor tends to become evenly 

 distributed throughout the room. 



Similarly, if a few crystals of copper sulfate are placed in 

 the bottom of a vessel of water, particles of the copper sulfate 

 diffuse through the water. The crystals are blue in color, 

 and as diffusion proceeds, the water in the vessel gradually 

 becomes blue. The direction of the movement is again from 

 the place where the diffusing substance is most concentrated 

 to where it is less concentrated. The particles pass from the 

 place where they are most abundant to where there are fewer 

 of them, and this process is continued until they are evenly 

 distributed throughout the water. 



Diffusion of a gas or vapor is very rapid. Diffusion of a 

 dissolved substance is slow, but the distances that substances 

 must travel in plant cells are very small. Oxygen and carbon 



