[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 199 



diffusion. As we shall see later, some substances do not pass through 

 the layer of protoplasm in the cells as readily as others. 



The accumulation of water in the vacuole by the process of osmosis 

 and the swelling of cell walls or of a piece of wood by the process of 

 imbibition, are merely processes of diffusion under special conditions 

 with special results. Even the mass movement of a stream of water up 

 the conductive tissues of a plant is indirectly dependent upon the energy 

 of diffusion. 



An understanding of all these processes evidently depends upon a fairly 

 clear concept of the process of diffusion. One may begin by thinking 

 of all solids, colloids, solutions, liquids, and gases as composed of mole- 

 cules always in motion. Anyone who has used a microscope has probably 

 observed that the smallest visible particles mounted in a drop of water — 

 for example, chalk dust — are constantly in motion as the result of being 

 bombarded from all sides by the molecules of the water. This is called 

 Brownian movement after Robert Brown, an English botanist who first 

 described it. The smaller the particles of a given substance the more 

 rapidly they move. While watching them one needs only to imagine 

 more and more rapid movement of smaller and still smaller particles 

 to form a mental image of molecular motion. 



Diffusion is the result of molecular motion. Consequently the energy 

 involved in all diffusion processes can be traced to the energy of molec- 

 ular motion of the diffusing substances themselves. Each substance 

 diffuses independently of the diffusion of other substances, except for 

 the interference caused bv one molecule colliding with another. A sub- 

 stance will diffuse more rapidly into a vacuum than into air because of 

 the fewer collisions with other molecules. Likewise, a gas, such as 

 oxvgen or carbon dioxide, will diffuse more rapidly in air than in water. 



The energy of molecular motion is correlated with heat and is directly 

 proportional to the absolute temperature. Apparently at minus 273° C. 

 there would be no molecular motion and no diffusion. One may therefore 

 think of the heat energy of the environment as being the source of 

 energy of the movement of materials into, within, and out of plants by 

 diffusion. The heat energy liberated in the plant by respiration would 

 be an additional source of energy in this process, but a rather insignifi- 

 cant one. The radiation of energy from the sun is the principal source 

 of the heat energy of the earth's surface, and this in turn becomes the 

 source of the energy of molecular motion in plants and all other objects 

 on the surface of the earth. 



