WORK OF BACTERIA 
40 
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. 
(a) Grown in soil with the 
proper nitrifying bac¬ 
teria. 
(b) Grown under the same 
conditions without the 
bacteria. 
