248 GENERAL BOTANY 



At the end of a fermenting process, therefore, the same amount 

 of ferment is left over as there was present at the beginning. 

 This is indicated in the above equation for the fermentation of 

 grape sugar, in which the zymase appears on both sides of the 

 equation. It is estimated that zymase can convert as much as 

 100,000 times its own volume of sugar into alcohol and carbon 

 dioxide without being diminished in volume or in the power to 

 cause renewed fermentation in a sugary solution. Other fer- 

 ments behave similarly, -acting like the so-called catalytic agents 

 among inorganic substances. 



Digestion has already been defined as the transformation of 

 solid foods into soluble food substances which can diffuse through 

 cell membranes and be used in the making of new living sub- 

 stance. Some cases are known, however, in which soluble sub- 

 stances need to be changed chemically before they are of use 

 for assimilation by plant and animal cells. This appears to be 

 the case with cane sugar, which is first changed to grape sugar 

 in plants and is then capable of being used in assimilation. 



If yeast cells are placed in cane-sugar solutions, they secrete 

 cane-sugar ferments (maltase or invertase), which change the cane 

 into a grape sugar. Alcoholic fermentation yields energy which is 

 available to the yeast cell, while digestive fermentation prepares 

 a food for assimilation by the cell protoplasm. In both cases the 

 active agent in the chemical reactions involved is a secretion of 

 the yeast cell, which is called an enzyme or a ferment. 



Cane sugar + water -f- cane-sugar ferment 



= grape sugar + cane-sugar ferment. 



Grape sugar + zymase ferment 



= alcohol + carbon dioxide -f- zymase. 



Ferments may be classified, therefore, into energy -forming and 

 digestive ferments, according to the results of their activity. 



All cases of digestion in animals and plants are caused by the 

 presence of active ferments, which are secretions of plant or 

 animal cells. In the higher animals these secreting cells usually 

 occur in the form of glands, but in higher plants they may be 



