THE FUNGI 



245 



the yeast-cell. It is spherical or ellipsoidal in shape, thin- 

 walled, and contains nearly colorless protoplasm, which in- 

 cludes oily droplets known as vacuoles and a well-defined 

 nucleus. 



Multiplication ordinarily takes place by a process of hud- 

 ding by which the nucleus divides and a new cell is pushed 

 out from the old 

 one and finally cut 

 off from it by a new 

 cell-wall (Fig. 171). 



Spore formation 

 occurs when yeast is 

 grown with scanty 

 nourishment. The 

 protoplasm breaks 

 up into small, nearly 

 spherical masses, 

 often two or four in 

 number, and these 

 form spores which re- 

 sist unfavorable con- 

 ditions better than 

 the ordinary yeast- 

 cell. 



316. Alcoholic Fermentation. — The growth of yeast in 

 a liquid which contains sugar destroys part or all of the 

 sugar, changing it mostly into alcohol and carbon dioxide. 

 The alcohol remains mixed with the liquid, while most of 

 the carbon dioxide escapes in little bubbles. 



In bread making the yeast grows at the expense of sugar 

 in the dough (derived from the flour). The bread rises as 

 the bubbles of gas escape, at the ordinary temperature, and 



Common Yeast very Highly 

 Magnified. 



a and 6 show vacuoles ; c shows a nucleus n inside 

 of the yeast-eell; d shows a budding cell with 

 the nucleus dividing; e shows the cell divided, 

 the new cell containing a bit of the old nucleus. 



