102 



ELEMENTARY SCIENCE 



from such soil are greatly affected by the hidden life which 

 swarms among their roots, and finds its nourishment in 

 rotting branches and decaying leaves. 



Chief among these micro-organisms of the soil we should 

 note the bacteria, which, as you may already know, are 



very abundant else- 

 where as well as in 

 soil (see Fig. 44). 

 Doubtless you have 

 heard that bacteria 

 cause disease, and 

 perhaps you natu- 

 rally think of them 

 as the enemies of 

 mankind. It is 

 true that many dis- 

 eases are caused by 

 bacteria; it is also 

 true that most of 

 these diseases 

 might be prevented 

 by careful living. 

 And yet the bene- 

 fits which bacteria 

 confer on mankind 

 are quite as impor- 

 tant as the harm 

 they do. It is the 



green plants which make our food; but it is the bacteria, 

 chiefly, which reduce the dead parts and bodies of organ- 

 isms to a condition in which they can be built up into food 

 again. Just as we depend on the work of green plants for 



FIG. 44. Various kinds of bacteria. 



