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. ^*^' wimpler SubstanceS. 



