34 Bacteria in Relation to Country Life 



fixing bacteria can readily utilize the nitrogen gas of 

 the air, which is inaccessible to the other species. 



Enzymes. Valuable knowledge 

 of the mechanism of decomposition 

 by bacteria is supplied by the in- 

 vestigations on the enzymes, other- 

 wise known as unorganized fer- 

 ments. Enzymes are chemical 

 substances of complex constitution, 

 produced by living organisms, both 

 plants and animals. 



In the human body certain 

 enzymes are found, capable of 

 transforming starch into sugar; 

 others capable of breaking up pro- 

 tein; still others capable of disin- 

 tegrating fats, and pre- 

 paring them for assimi- 

 lation. The enzymes 

 in the saliva, stomach, 

 liver, and pancreas, are 

 secreted by the animal 

 body for a specific pur- 

 pose in aiding the 

 digestion and assimila- 

 tion of food. A familar 

 instance of enzyme 

 action is that of pepsin, 

 the function of which 

 is to change protein in- 



Fig. 13. Apparatus for filtering cultures. to simpler Substances. 



