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. 1 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 
1 Another process of reproduction of yeast, by the production of 
endospores, need not here be described. 
