FERMENTATION 99 



reproduces itself. When the buds reach a certain size, 

 they separate from the parent plant, continue to take in 

 nourishment from the surrounding liquid by osmosis, 

 increase in size to maturity, and then give rise to other 

 plants, by repeating the process of budding.^ Under 

 favorable conditions new plants are formed very rapidly 

 by budding, so that in the course of a few hours the total 

 number of yeast plants will have enormously increased, 

 notwithstanding the fact that some of them in the mean- 

 time may have died. This increase in number may be 

 noted with the naked eye, by observing the increase of 

 turbidity or opalescence of the yeast-mixture after it has 

 stood for an hour or more. The yeast cakes of commerce 

 consist largely of starch and millions of tiny yeast plants, 

 skimmed from the surface of a fermenting liquid, and then 

 pressed together. 



100. The Active Agent in Fermentation. — After it be- 

 came recognized that the presence of the yeast plant is 

 necessary, in order to have alcoholic fermentation, it re- 

 quired careful study before it was discovered that if the 

 yeast cells, after being disintegrated by grinding, are all 

 filtered out of the mixture, the filtered liquid still re- 

 tains the power to cause fermentation. From this it was 

 learned that the active agent, or immediate cause of the 

 process, is not the yeast itself, but some substance or 

 substances produced by the yeast. These substances are 

 the real ferments or enzymes, and are secreted by the 

 living protoplasm of the yeast. At least three different 

 enzymes are known to be produced by yeast. The one 



^ Another process of reproduction of yeast, by the production of 

 endospores, need not here be described. 



