3 AGRICULTURAL BACTERIOLOGY. 



are apparently produced entirely outside of the body of the 

 bacteria, and are not in any sense excreted products. .It is, 

 for example, quite certain that the alcohol which arises from 

 the fermentation of sugar is not an excretion of the yeast cell, 

 but that, by some agency, the sugar is broken up outside of 

 the yeast, and that alcohol and carbon dioxide arise as prod- 

 ucts of its decomposition. Bacteria in some cases act in a 

 similar manner (e. g. t nitrification). Just how they can ac- 

 complish such transformation outside of their own bodies has 

 not been clear. Various suggestions have been made to 

 explain the process and in some instances we have a partial 

 knowledge of the method by which such changes are pro- 

 duced. 



