CLASSIFICATION OF THE YEASTS 47 



to form the number of nuclei characteristic of the species, 

 each nucleus becoming a spore. In some forms the cells 

 which fuse are of the same size; in others they differ. In 

 one the spores fuse in pairs during the process of germi- 

 nation and the young yeast cells are produced by budding. 

 Occasionally yeast cells surround themselves with an 

 unusually heavy membrane and pass into a resting state. 

 Such cells are termed chlamydospores. 



The Classification of Yeasts 



Relationships of True Yeasts. — The development of asci 

 and ascospores by yeasts show that their relationship is 

 primarily to the group of fungi termed ascus-fungi or 

 Ascomycetes. Yeasts are sometimes conveniently sepa- 

 rated into true yeasts and false yeasts. The former are 

 those which have already been described. To the latter 

 ■ belong certain forms which have every appearance of being 

 the vegetative stages of true yeasts, but which do not pro- 

 duce spores, and are frequently not active in causing alco- 

 holic fermentation. An organism of the latter type is 

 sometimes termed a torula. 



Classification of the Yeasts. — Botanists recognize a dozen 

 or more different genera of yeasts. Only two are of suffi- 

 cient importance to be considered here. The first of these 

 is the genus Saccharomyces, which includes those common 

 yeasts which multiply by process of budding, and Schiso- 

 saccharomyces, in which the cells multiply by a process oi 

 fission. 



Saccharomyces. — To this genus belong most of the yeasts 

 of economic importance, including those which are used in 

 wine making, in manufacture of beer and commercial 

 alcohol, and in bread making. Many different species have 

 been described and several different methods of classifica- 

 tion have been used. 



One common method of differentiating yeasts is based 



