238 THE FUNGI 



practical one of combating germs. So important are the bacte- 

 ria in disease and hygiene that a science has developed, called 

 bacteriology, with elaborate methods of its own to which special- 

 ists give their entire attention. 



CLASS VI. THE YEASTS, OK SACCHAROMYCETES 



259. The yeasts. The yeasts are much larger than the bac- 

 teria, and have a more complex cell structure, for there is 

 present a clearly defined nucleus. The cells reproduce in a 

 peculiar manner called 'budding, and the yeasts are frequently 

 termed budding fungi. Small extensions are put forth from the 

 cells (Fig. 209, A), which, after increas- 

 ing in size, become cut off from the par- 

 ent structure. The parent and daughter 

 cells frequently remain attached in short 

 chains or clusters. The relationships of 

 the yeasts are very obscure, but there 

 are reasons for believing them to be de- 

 generate conditions derived from some 

 types of higher fungi whose spores are 



FIG. 209. Yeast (Saccha- kn Wn at times to P aSS ***** yeast-like 



romyces cereoisice) forms when cultivated in sugary solutions. 



A, vegetative cells, show- Yeasts are chiefly interesting as the 



ing method of budding ; agents of alcoholic fermentation by which 



J3, spore formation 



sugar dissolved m water is changed into 



alcohol and the gas carbon dioxide. The alcoholic nature of 

 wines, beers, ales, and hard cider is due to the fermentation of 

 grape juice, wort, or sweet cider, all of which contain sugar, and 

 the froth and bubbles of gas which escape from the fermenting 

 fluid is carbon dioxide. The yeasts are distributed very widely, 

 and they are sure to be introduced by dust into any sugar solu- 

 tion that is not sealed up. Therefore weak sugar solutions fer- 

 ment spontaneously if left exposed, although it is the practice 

 in the manufacture of beers and some wines to use special kinds 



