CHAPTER IX 
RESPIRATION 
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106. 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 living 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, 1 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. 2 
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. 
1 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. 
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