THE PATH OF EVOLUTION 



and the evolution of carbon dioxide gradual, so that 

 the yeast is not buoyed up by it to the surface, but 

 remains mostly at the bottom of the vat. This con- 

 stitutes the lower yeast (Unter Hefe) of the lager 

 beer breweries. It is anaerobic ; that is, its life and 

 growth is independent of the presence of Oxygen. 

 The fermentation thus produced is less liable to be 

 contaminated with the growth of other mycodermic 

 growths that might cause acidity or viscidity, and is 

 best adapted to the fermentation of weak worts that, 

 under more rapid action, would pass beyond control. 

 At a higher temperature, say about 70 F., the upper 

 yeast (Ober Hefe) is formed. The growth is much 

 more rapid and the evolution of Carbon Dioxide 

 more violent. Towards the latter part of the process 

 a large portion of the cellules contain spores, which 

 serve as new centres of growth, and which are car- 

 ried off by the escaping gas and disseminated in the 

 atmosphere. The fermentation of wine or other fruit 

 juices is of a similar nature, though it is not necessary 

 to add the yeast from previous operation as required 

 in beer brewing, the grape juice obtaining from con- 

 tact with the outer skin of the fruit when crushed 

 the germ of the yeast plant, whose growth starts the 

 fermentation. 



Other widespread members of the family are the 

 Mucor, or Mould plants, that attack most organic 



230 



