298 



GENERAL MORPHOLOGY OF PLANTS 



sugar by the yeast, alcohol is also formed. These two products 

 of fermentation by the yeast plant are of great importance. The 

 evolution of the gas carbon dioxide in bread making forms 

 large numbers of gas bubbles in the dough which causes it 

 to "rise." When the "risen" dough is baked, the heat causes 

 these bubbles to expand and the bread "rises" more and becomes 

 "light." The same yeast plant is used by brewers for ferment- 

 ing the malt in the manufacture of beer. It is now a domestic 

 or industrial form. There are, however, many wild yeasts, 



Fig. 246. 

 Fermentation tube with culture of yeast. 



Fig. 247. 



Fermentation tube filled CO> from action 

 of yeast in a sugar solution. 



which are abundant on all sorts of vegetables and multiply in 

 decaying parts. Some of these wild yeasts are associated with 

 certain bacteria, forming little nodules which have been used by 

 natives of certain countries in making fermented drinks, as 

 "ginger beer" from the "ginger-beer" plant, or "kumiss" from 

 mare's milk fermented by throwing in some of these grains, which 

 is used by some of the tribes of the Caucasus, 



