FERMENTATION 301 



the more economical, not involving the consumption of 

 much energy in reconstructive processes. This cannot, 

 however, be regarded as finally established. 



The substance which seems most readily available for 

 this purpose is sugar. Under the conditions mentioned it 

 becomes decomposed or broken up entirely, the resulting 

 products being carbon dioxide and alcohol. The process of 

 alcohol-formation which was for so long a time associated 

 exclusively with the word fermentation, was first observed 

 in connection with the life of the yeast-plant. It has, how- 

 ever, since been ascertained to be much more widespread, 

 and to be indeed the most common of the anaerobic 

 respiratory processes. In cases where the metabolic 

 activities are very great, as in germinating peas, we find 

 this process supplements the ordinary respiration, for 

 alcohol can be detected in their cells in small quantities. 

 The same thing has been noticed in the leaves of the vine. 

 We must suppose here that the amount of oxygen absorbed 

 is insufficient for their requirements, and that partial 

 asphyxiation results. 



Till quite recently it was held that alcoholic fermenta- 

 tion was conducted exclusively by the activity of the proto- 

 plasm of the cells in which it was observed. It has been 

 ascertained, however, that it may also be caused by the 

 action of an enzyme, which is secreted under conditions of 

 incipient asphyxiation by many cells, and which is formed 

 in the yeast-plant even in the absence of such stimulus. 



Though the term < fermentation ' was originally applied 

 and confined to the formation of alcohol, it is now usual to 

 extend it far more widely. Many other processes of similar 

 nature have been discovered, nearly all of which at first 

 were found to be carried out through the agency of microbes 

 or higher fungi. Hence the meaning of the term has been 

 extended to include them, and the organisms themselves 

 have been called ferments. As, however, these processes have 

 come to be recognised as normal in many of the higher 

 plants, and to be carried out in them by the protoplasm of 



