I02 THE VEGETATIVE EUNCTIONS OE PLANTS 



the restricted supply of oxygen that can be obtained in 

 the absence of air. If the oxidation processes could he 

 completed the end products would be carbon dioxide and 

 water. Organisms that may continue to live without a 

 supply of free oxygen are called anaerobes.^ Either the 

 entire organism may live anaerobically, or this condition 

 may be confined to one or more of its organs, or even to 

 a single cell or group of cells. It is possible for yeast to 

 live in contact with free oxygen, but when this is not 

 present it can secure sufficient oxygen for its needs by 

 alcoholic fermentation alone. ^ 



104. Relation of Fermentation to Otir Daily Lives. — 

 Reference has already been made to the fact that such 

 diverse operations as the manufacture of all alcohohc 

 drinks and the making of bread are dependent upon 

 fermentations, and we have seen that the process funda- 

 mental to all Hfe — the formation of carbohydrates by 

 photosynthesis — probably involves the action of at least 

 six different ferments. The active principles of all the 

 digestive juices of our own bodies are also enzymes — 

 diastase in the saliva, pepsin in the stomach, trypsin in the 

 intestine, and so on. We shall learn, in the next chapter, 

 that all muscular and mental activity, even life itself, is 

 dependent on the fermentative activity of enzymes. 



'■ a -\- aer (air) + bios (life) = living without air. 



2 From a reading of Chapters VII to IX the student will learn that 

 the transformations wrought in other substances by the catalytic agents, 

 called enzymes, are not all of the same nature; some (as alcoholic fer- 

 mentation, and digestive processes) are destructive, others (as the poly- 

 merizations and other transformations in photosynthesis, mentioned on 

 pages 79 and 8o) are constructive. The author believes that it con- 

 tributes to simplicity, without sacrificing clear and accurate thinking, to 

 consider " enzyme " and " ferment " as synonymous terms, and to call all 

 activities of enzymes fermentations, whether they are analytic or synthetic. 



