CH. IV, 8] 



RESPIRATION OF PLANTS 



171 



and alcohol, though it is ultimately overcome by the alcohol 

 it produces. As to the significance of fermentation, all evi- 

 dence unites to indicate that it represents primarily the 

 Yeast's form of respiration, exaggerated usually by unnatural 

 experimental conditions. Shut away from the air, the Yeast 

 is unable to take oxygen from that source, and has to resort 

 to the supply contained in the 

 grape sugar, into which this plant 

 has the power to convert cane 

 sugar or starch. By action of an 

 enzyme, the sugar is broken up, 

 whereupon its carbon unites with 

 oxygen from the same molecules 

 into carbon dioxide, with the usual 

 release of energy at the moment 

 of union. The remainder of the 

 molecule falls as a natural chemi- 

 cal incident into alcohol, accord- 

 ing to the following equation, 



e = 2 CO 2 + 2 C 2 H 6 O. 



FIG. 118. Arrangement 

 for demonstration of the re- 

 lease of carbon dioxide in fer- 

 mentation ; X . Explanation 

 in text. 



Thus in the formation of the 

 carbon dioxide, a process essen- 

 tial to the Yeast, the alcohol is 

 incidentally and, so to speak, in- 

 evitably formed. 



That fermentation is fundamentally the Yeast's respiration 

 is attested by many lines of evidence, including the existence 

 of intermediate steps between fermentation and ordinary 

 respiration. Thus Peas, which ordinarily respire in the usual 

 way, can also form carbon dioxide, with incidental production 

 of alcohol, when derived of all free oxygen ; and this power 

 is probably rather widespread, though in limited degree, 

 among plant tissues which have imperfect access to air. 



The production of carbon dioxide and alcohol by fermen- 

 tation renders the process of great importance to man. Thus 



