CHAPTER IX 

 RESPIRATION 



105. Anaerobes and Aerobes. — In the preceding chapter 

 we learned that all plants require energy for their ac- 

 tivities, and that this energy is derived by the oxidation 

 of substances within the cell. In the case of yeast and 

 other organisms, when hving in an atmosphere devoid of 

 free oxygen, the necessary oxygen is obtained from com- 

 pounds which contain it, by the process of fermentation, 

 brought about by enzymes. As is well known, many organ- 

 isms, such as man and most other animals, and most 

 plants, cannot live in the absence of free oxygen; such 

 organisms, called aerobes,^ continually take in oxygen from 

 the air. The using up of oxygen by the cells creates the 

 need for more, and in the case of man and other mammals, 

 the supply is obtained through the lungs by breathing. 

 Plants, however, have no lungs, nor any organs that 

 correspond to lungs. ^ 



When oxygen is consumed by plant tissues, its pressure 

 within the plant becomes less than its pressure outside 

 the plant, and therefore more passes in through the 

 stomata and epidermis by the simple physical process of 

 diffusion. 



'■ aero (air) + be (bios, life) living in air. 



2 Leaves have sometimes been called "the lungs of plants." From our 

 study of nutrition, it will be readily recognized that leaves may much more 

 appropriately be called the stomachs of plants. 



103 



