212 LECTURE XL 



tenance of life. Let us briefly consider the case of an ae'ro- 

 biotic plant. We have seen that, under normal conditions, 

 the principal features of its metabolism are probably these : 

 it absorbs oxygen ; this oxygen is fixed by the protoplasm 

 as intramolecular oxygen, and the protoplasm undergoes 

 decomposition with evolution of carbon dioxide. It is not 

 too much to assume that, when the supply of oxygen is cut 

 off there is a reserve of intramolecular oxygen, and that the 

 life of the plant is maintained for a time by the decomposition 

 of protoplasm-molecules in which intramolecular oxygen 

 is still present. But now fermentative decomposition be- 

 comes energetic. It might be suggested that it is excited 

 and maintained by the decomposition of the protoplasm 

 which is still going on in virtue of the presence of the reserve 

 of intramolecular oxygen, and, as a matter of fact, Pasteur 

 has observed that alcoholic fermentation is most active when 

 the Yeast which excites it has been previously in contact 

 with free oxygen. But this suggestion is untenable for 

 various reasons. In the first place fermentation continues 

 so long that it is impossible to imagine that intramolecular 

 oxygen is present during the whole period. Secondly, if 

 this suggestion be valid, then fermentation ought to go on 

 most actively in the presence of free oxygen, and it has 

 been shewn that this is not the case. Finally, it fails to 

 account for the existence of anaerobiotic plants. Pasteur's 

 view of the significance of fermentative decomposition is this, 

 that it is the expression of the effort of the organism to 

 obtain oxygen from substances which contain it in combi- 

 nation. Another possible view is this, that the organism 

 obtains by the fermentative decomposition of other substances 

 that necessary supply of energy which, in the presence of 

 free oxygen, it obtains by the decomposition of its own 

 protoplasm-molecules. In the case of most aerobiotic plants 

 the end, whatever it is, is only imperfectly attained, for the 

 vitality of the organism becomes gradually diminished, fer- 

 mentation becomes less and less . active, and the organism 

 either dies at once, or passes for a longer or a shorter period 

 into a state of suspended animation from which it can only 



