WORK OF BACTERIA 



49 



tables tissues are changed into gases which dissipate in- 

 to the air or reunite into 

 compounds that form a 

 part of the soil. These 

 then become once more 

 food for plant life, and 

 this, in turn, for the sus- 

 tenance of animals. 



Bacteria are the agents 

 of decay by which all or- 

 ganic materials are re- 

 turned to the soil or the 

 air. Thereby life is not 

 only made possible, but 

 also is sustained. What 

 the conditions would be 

 were these invisible agents 

 to cease their beneficent 

 work of scavenging can be 

 scarcely imagined. Life 

 as we know it on this earth 

 could not exist were these 

 dust-plants not present. 



All animal life is de- 

 pendent directly or indirectly upon the vegetable 

 kingdom for sustenance. Man takes both animal and 

 vegetable food, but he is not able to manufacture this 

 food out of the inorganic elements. 



Plants use for their food gases, water, and various 

 salts usually dissolved in the water. In sunlight the 



FIG. 30. A VARIETY OF PEA. 



soil with the 

 nitrifying bac- 



Grown 

 proper 

 terta. 



Grown under the same 

 conditions without the 

 bacteria. 



