^6 BACTERIA, YEASTS, AND MOLDS 



the process of bread making, too, fermentation has been 

 known almost as long; for we read in literature of 

 leavened and unleavened bread at least three thousand 

 years ago. 



Although fermentation was thus long known, its cause 

 remained a mystery until the nineteenth century. The 

 type of fermentation which we are considering is in all 

 cases produced by essentially the same agency, a group 

 of plants called yeasts. It is not always the same species 

 of yeast, for the group includes quite a large number of 

 different species. The commercial product is simply one 

 kind that has been cultivated for commercial purposes ; 

 but there are many others in nature not under cultivation 

 which may conveniently be called wild yeasts. All of the 

 kinds are, however, very similar in appearance, have the 

 same general characters, and are closely related to each 

 other. 



The yeast plant was first seen two centuries ago by a Dutch 

 microscopist, who found fermented liquors filled with minute 

 bodies, the significance of which he did not understand ; 

 nor was it known that they cause fermentation until about 

 the third decade of the nineteenth century. At that time 

 it was quite conclusively demonstrated that these minute 

 bodies were living organisms, capable of feeding, growing, 

 and multiplying, and having a very close relation to the 

 phenomena of fermentation. It was soon shown also 

 that it was their growth that produced the fermentation, 

 since this phenomenon would not occur unless these 

 organisms were not only present but also growing and 

 multiplying. In our study we must first learn the nature 

 of the yeast plant. 



