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 aleohol. 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 
