256 



YEAST (SACCHAROMYCES) 



outgrowth (Fig. 140, b) which slowly enlarges and assumes the 

 form of the parent, from which it becomes separated by gradual 

 constriction ; if this process of budding takes place rapidly, the 

 cells do not immediately separate, and thus the chains (Fig. 140, c) 

 above mentioned are formed. A stage, capable of a prolonged 

 resting period, can also be obtained, for instance by growing 

 Yeast on the surface of a raw Potato ; under these circumstances 

 the cell-contents undergo division into four, and each part 

 becomes surrounded by a thick wall (Fig. 140, d). 



There are a number of different species of Yeast which 



tt. 



9- 



FIG. 140. Yeast (Saccharomyces). a-c, various individuals, showing 

 general form and multiplication by budding ; d, individual containing 

 resting spores ; e., cell-structure, ch., chromatin threads ; g., glycogen 

 vacuole ; n., nucleolus with surrounding chromatin ; v., volutin 

 granules; va., nuclear vacuole. - (a-d after Wettstein ; e after Wager.) 



ferment various sugars and split them up into alcohols (mainly 

 ethyl alcohol) and carbon dioxide. The chemical change is 

 brought about by an enzyme zymase (cf. pp. 53, 55) which can, 

 with some difficulty, be extracted from the cells ; in addition 

 the latter contain invertase and other ferments. The mode of 

 action of zymase is complicated and not yet fully understood, 

 but it is known that fermentation depends on the contemporary 

 presence of phosphates (cf. p. 57). The alcohol present in beer, 

 wine, etc., is formed by the action of Yeasts, while the carbon 

 dioxide simultaneously evolved is compressed into cylinders and 

 sold as a by-product. In the manufacture of beer, malt (i.e. germi- 



