[Chop. XXII THE MOVEMENT OF MATERIALS IN PLANTS 201 



systems. To explain why a particular process occurs we must be able to 

 show that it is a consequence of preceding processes. 



Diffusion under special conditions. When the basic facts of diffusion 

 are clearly understood thev may be used in interpreting complicated 

 processes in which diffusion plays a part. Among these processes are 

 imbibition, osmosis, and transpiration. 



Imbibition. We have already seen that a piece of wood is composed 

 of cell walls. Even more familiar is the fact that when a piece of dry 

 wood is placed in water or in a moist atmosphere it swells, or increases in 

 volume and weight. Before the discovery of explosives stone was quar- 

 ried by pouring water upon pieces of drv wood that had been wedged 

 into holes drilled in the rock. Even at the present time this method is 

 used in quarrying marble and granite for special purposes. The swelling 

 of wood is the result of the entrance of water between the particles of 

 which the cell walls are composed. The entering water pushes these 

 particles farther apart and increases the size of each cell wall. This fact 

 is easily demonstrated by placing a flat piece of dry gelatin, or agar, in 

 water for a few minutes. Any of the solid or colloidal parts of a plant 

 may increase in size in the same manner, but the swelling of cell walls, 

 protoplasm, and mucilages are the most conspicuous examples. The en- 

 trance of water into solids or colloids and the resultant swelling have 

 long been referred to as imbibition. 



Evidently the energy of diffusion is involved in imbibition. The con- 

 centration of water is greater outside than inside the piece of dry 

 gelatin or the dry cell walls of the piece of wood. But the water also 

 adheres to the smaller particles of the wall and fomis thin films around 

 them. The cohesion of the wall particles is overcome by the pressure of 

 diffusion and by the adhesion of the water to them. They are forced 

 farther apart by the entering water and the walls swell. When a piece of 

 dry wood is placed in water the water first enters the intercellular spaces, 

 pores, and open vessels, forcing out the contained air. This movement is 

 the result of surface tension ( capillarity ) . It rapidly distributes the water 

 into some of the xylem tubes and intercellular spaces in the wood block, 

 but it does not result in increasing (swelling) the volume of the block. 



Osmosis. The term osmosis is sometimes used synonymously with 

 diffusion. It has been used also to refer to the diffusion of anything 

 through any membrane. These uses are not acceptable, because there is 

 no need for the word if it is a synonym of diffusion, and because the 



