8o SACCHAROMYCES less. 



By experimenting with several flasks of this kind it can 

 be proved that fermentation goes on as well in darkness as 

 in light, and that it is quite independent of free oxygen. 

 Indeed the process does not go on if free oxygen — i.e., 

 oxygen in the form of dissolved gas — is present in the fluid ; 

 from which it would seem that Saccharomyces must be able 

 to obtain the oxygen, which like all other organisms it 

 requires for its metabolic processes, from the food supplied 

 to it. 



The process of fermentation goes on most actively 

 between 28° and 34° C : at low temperatures it is com- 

 paratively slow, and at 38° C. multiplication ceases. 



If a small portion of yeast is boiled so as to kill the 

 cells, and then added to a flask of Pasteur's solution, no 

 fermentation takes plac^ from which it is proved that the 

 decomposition of sugar is effected by the living yeast-cells 

 only. There seems to be no doubt that the property of 

 exciting alcoholic fermentation is a function of the living 

 protoplasm of Saccharomyces. The yeast-plant is therefore 

 known as an organised ferment : when growing in a sac- 

 charine solution it not only performs the ordinary metabolic 

 processes necessary for its own existence, but induces 

 decomposition of the sugar present, this decomposition 

 being unaccompanied by any corresponding change in the 

 yeast-plant itself. 



It is necessary to mention, in this connection, that there 

 is an important group of not-living bodies which produce 

 striking chemical changes in various substances without 

 themselves undergoing any change : these are distinguished 

 as unorganised ferments. A well-known example is pepsin, 

 which is found in the gastric juice of the higher animals, 

 and has the function of converting proteids into peptones 

 (see p. 12) : its presence has been proved in the Mycetozoa 



