THE YEASTS CHAPMAN 303 



The microscope having definitely shown yeast to consist of minute 

 living cells — that is to say, of a living organism — it became of high 

 interest and importance to study its life history, and to ascertain 

 what connection, if any, there was between the vital functions of the 

 organism and the phenomena of fermentation. It was very quickly 

 recognized that yeast usually reproduced by budding, but in 1839 

 Schwann appears to have observed for the first time that it was also 

 capable of reproducing by the formation of ascospores. 



About the year 1856 Pasteur commenced his epoch-making re- 

 searches on fermentation, and brought to bear on the subject that 

 experimental skill and wonderful energy which were characteristic 

 of all his work. To deal with this in any detail would be impossible 

 within the limits assigned to a presidential address, and would, more- 

 over, take me outside the scope of my subject. It will, however, 

 suffice for my purpose if I point out that to Pasteur belongs the 

 honor of having definitely enunciated a physiological theory of fer- 

 mentation — that is to ssij, a theory which associated the formation of 

 the various fermentation products directly with the life activity of 

 the yeast organism. It is true that in some important respects his 

 views have had to undergo modification as the result of more recent 

 work, but the essential fact still remains that the production of 

 alcohol and carbon dioxide during the process of fermentation is 

 indirectly the result of the life activity of the yeast cell. 



In 1897 Buchner made the very important and interesting obser- 

 vation that the liquid contents of the yeast cell, when added to a 

 fermentable liquid, are able to excite fermentation without the 

 presence of any cells at all. He showed that the production of 

 alcohol and carbon dioxide were the result of the activity of an 

 enzyme secreted by the cell, to which he gave the name zymase. 

 As in the case of other enzymes, zymase is very sensitive to external 

 conditions, and is also highly selective in respect of its chemical 

 activities. Thus, so far as is known, the hexoses alone, and of these, 

 only four (d-glucose, d-mannose, d-galactose, and d-fructose) are 

 directly fermentable; and before the fermentation of other sugars, 

 such as maltose and cane sugar, can take place, it is necessary that 

 they should be converted into one or other of these hexoses. This 

 is, in all cases, effected by enzymes which are secreted by the yeast, 

 and it is very interesting to note that certain yeasts, while secreting 

 invertase, and therefore capable of fermenting cane sugar, do not 

 secrete maltase, and are therefore incapable of fermenting maltose. 

 Then again, there are a few yeasts which, in addition to secreting 

 invertase and maltase, secrete lactase, and are therefore capable of 

 fermenting milk sugar. Every yeast cell is, in fact, a minute 

 laboratory in which chemical changes of the utmost complexity are 



