CHAPTER VII 

 DIFFUSION 



The chemical elements which constitute the bulk of the body of any 

 plant are among the commonest ones on the surface of the earth. They occur 

 in the environment of plants as relatively simple inorganic compounds, and 

 enter plants in the form of such compounds. From the relatively small num- 

 ber of compounds which enter it from its environment the green plant fabri- 

 cates the numerous complex organic compounds which are essential to its 

 continued existence as a living system. 



The movement of substances into a plant from its surroundings is ac- 

 complished principally through the agency of one form or another of the 

 process known as diffusion. Substances enter a plant partly through its aerial 

 organs, and partly by way of its root system. From the atmosphere carbon 

 dioxide and oxygen gases diffuse into plants, principally through the stomates. 

 From the soil, water and the cations and anions of simple inorganic salts pass 

 into plants by processes which are basically diffusion phenomena, although the 

 entrance of both water and mineral salts is complicated by other factors. 



Similarly the loss of substances from a plant into its environment is accom- 

 plished principally by diffusion. Large quantities of water-vapor pass out of 

 leaves and other aerial organs of plants into the atmosphere. At times oxygen 

 gas and at times carbon dioxide gas diffuse into the atmosphere. Certam 

 volatile compounds also escape from the aerial organs of many plants by 

 diffusion. Similarly the roots lose carbon dioxide and other compounds into 

 the soil by diffusion. 



Likewise some of the movement of substances from one part of a plant 

 to another is accomplished by diffusion. This is true both of the gases which 

 move through the intercellular spaces and the water and solutes which move 

 within the cells. However, much of the translocation of materials from one 

 plant organ to another is accomplished by more complicated mechanisms than 

 diffusion. Within any living plant cell diffusion of substances from one part 

 of a cell to another is also continually in progress. 



In brief, there are few if any of the physiological processes occurring in 

 plants which do not directly or indirectly involve diffusion phenomena. 



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