FERMENTATION IOI 
the solution of a portion of the cell-walls of a layer of cells 
(the abscission-layer) at the place where the leaf joins the 
branch. This greatly weakens the attachment, and the 
leaf falls, often merely from its own weight. The solu- 
tion of this tissue is caused by an enzyme secreted by 
the cells adjacent to the abscission layer. Many of the 
peculiar effects produced by molds and by bacteria are 
caused by enzymes secreted by these organisms. The 
ripening of cheese, the formation of alcohol in beer and 
wine, the changing of sweet to sour or "hard" cider, the 
turning brown of cut fruit, such as apples, when exposed 
to the air, and all processes of decay are caused by fer- 
mentations produced by enzymes secreted by bacteria, 
molds, yeast, or other living cells. 
103. The Significance of Alcoholic Fermentation. 
From what has just preceded, it is seen that the various 
processes of fermentation are useful or otherwise im- 
portant to mankind, but we must seek for the real sig- 
nificance of alcoholic fermentation in its use to the 
organism that secretes . the ferment. In this connection 
we must recall the fact that all the life-processes of plants 
and animals require energy. The continued release of 
this energy within the cells usually makes necessary a 
supply of free oxygen from the air; but some organisms 
can secure the necessary oxygen by decomposing com- 
pounds that contain it such as carbohydrates, proteins, 
and fats. Thus we must regard alcoholic fermentation 
(in part, at least) as a process by which yeast and other 
organisms or cells secure the necessary energy for their 
activities when deprived of the free oxygen of the air. 
Alcohol and other complex compounds result because 
the processes of oxidation are not completed, owing to 
