238 Nature-Study Agriculture 



multiply very rapidly at first, producing great masses 

 of what we call " mother of vinegar " ; but when the 



acid has become strong it 

 checks their increase. Their 

 numbers are also reduced by 

 FIG. 191. Diagram showing how a microscopic animal germs 



bacterium multiplies by pinching in calle d " animalcules." 



These are much larger than 



bacteria, and they use many of the bacteria for food. 

 Furthermore, the white corpuscles in the blood of the 

 larger animals destroy many harmful bacteria, and 

 different varieties of bacteria destroy each other. 



The work of Just as there are very many kinds of the ordinary 

 plants upon the earth, so there are very many kinds of 

 bacteria, drawing their food from different animal or 

 vegetable sources and producing various results. Some 

 kinds of bacteria aid in causing the decay of materials 

 that they feed upon, and decay is a most necessary 

 process in nature. Without it, dead vegetable and 

 animal matter would not disappear and become a part 



Decay a of the soil, adding to the fertility of the land. The 

 carbon removed from the air and built into plant sub- 

 stance would never be returned to the air in the form 

 of carbon dioxid gas (Chapter Two), and soon plants 

 would be unable to grow for want of it. We usually 

 think of decay as an unpleasant process, but it is so 

 only while it is going on. When decay is complete, 

 there is nothing left of the substance decayed but some 

 perfectly pure gases which mingle with the air, and a 

 little mineral matter as odorless and as pure as any 

 other part of the soil (Fig. 192). 



