ZYMOGENIC BACTERIA 203 



are characterized by the hydrating or oxydizing action of 

 enzymes on carbohydrate compounds. 



All animals and plants are composed, in addition to nitrog- 

 enous compounds, of a large percentage of carbon com- 

 pounds. During the life of all animals, carbon in the form of 

 carbon dioxide (CO 2 ) is expired continually into the air. 

 Part of the carbon dioxide thus expired by animals is taken up 

 by the chlorophyll-bearing plants and higher pigmented algae, 

 and, by the aid of the kinetic energy of the sunlight, is ab- 

 sorbed, fixed, and subsequently made into complex organic 

 compounds. In the process of plant respiration, a small 

 amount of carbon in the form of carbon dioxide is again re- 

 turned to the air. The remaining portion of the carbon is 

 not liberated until the plant dies and is disintegrated. There 

 is also a large amount of carbon and nitrogen liberated when 

 the animal dies and is decomposed. 



The carbon is liberated from the animal and plant body by 

 the activity of bacteria. It escapes here also in the form of 

 carbon dioxide. The zymogenic bacteria and the saprogenic 

 bacteria, which will be considered later, are responsible for 

 the liberation of these compounds. The nitrogenous com- 

 pounds are acted upon chiefly by the saprogenic or putre- 

 factive bacteria, while the carbohydrate compounds and 

 organic acids, which are in a measure non-putrefactive, are 

 acted upon by the zymogenic or fermentive bacteria. 



The term " fermentation " is used by various writers in a 

 variety of different ways. The term will be used in this dis- 



