48 BOTANY: PRINCIPLES AND PROBLEMS 



evenly distributed throughout the whole space available to it 

 by moving from points of greater to points of lesser concentration. 

 Its operation is familiar in the diffusion of odors, for the minute 

 particles given off by any strongly scented substance will move 

 outward rapidly, even in perfectly still air, and will soon become 

 equally distinguishable in all directions from their point of 

 origin. Two gases liberated within a closed space soon diffuse 

 throughout its whole extent and become thoroughly mixed. 

 In the same way, a crystal of salt dissolved in a vessel of water 

 will in time have its molecules dispersed so uniformly, even though 

 the liquid is free from moving currents, that samples taken from 

 any part of the contents of the vessel will be salt solutions of 

 exactly the same strength (Fig. 29). This constant tendency 

 toward diffusion is explained by the fact that in gases and hquids 

 the molecules are in very active movement, continually striking 

 against one another and rebounding. There are obviously 

 fewest collisions, and thus most frequent opportunity for unob- 

 structed movement, in those directions where there are fewest 

 molecules, and in such directions a dispersal of the molecules 

 therefore necessarily takes place until they are present every- 

 where in uniform abundance. The principle of diffusion is 

 operative in so many of the physiological processes of plants 

 that it must be thoroughly grasped if these processes are to be 

 understood. 



When two liquids are separated by a membrane through which 

 they can pass, diffusion between them will still take place. Such 

 diffusion through a membrane is called osfnosis, and it tends to 

 continue (if the permeability of the membrane allows) until the 

 composition of the liquids on both sides of the membrane is the 

 same. If a solution of salt in water, for example, is present 

 on one side and pure water on the other, the salt will tend to 

 diffuse through the membrane until its concentration is the same 

 throughout. It is important to note that the concentration 

 (the amount of substance dissolved per unit of volume) rather 

 than the total amount of the substance or the bulk of the solution, 

 is the factor which determines the direction and rate of diffusion. 

 It is by diffusion through the cytoplasmic membranes of the 

 root-hair that mineral salts in the soil solution enter the plant. 

 This inward movement of any given salt will continue so long as 

 its concentration is greater in the soil water than in the sap of 

 the root-hair. 



