BACTERIA. 155 



The list is a very long one. They could be called, 

 in general, poisons. They are sometimes termed 

 germicides. A chemical that kills one species may 

 not have any effect upon another. They are often 

 killed by the excreta due to their own growth. 



Bacteria require food just .as other living things 

 do. Their food must be in a moist, or liquid, con- 

 dition. Wherever there is dead animal or vege- 

 table matter in a moist state, with the temperature 

 sufficiently warm, bacteria will begin to grow and 

 multiply and cause decomposition. When the tem- 

 perature is low, such as in ordinary freezing, winter 

 weather, their action ceases; but it begins again 

 when the temperature rises much above freezing. 



Living, healthy plants and animals have the 

 power to resist the attacks of bacteria. When life 

 ceases, the resisting power ceases; so that a plant 

 or animal begins to decay as soon as it is dead, 

 especially in warm, moist weather. This decay is 

 always due to the growth of bacteria. 



RELATION OF BACTERIA TO THE FERTILITY OF 

 THE SOIL. 



It is a well-known fact among farmers that the 

 fertility of the soil is soon exhausted by the succes- 

 sive raising and clearing off of crops, if nothing is 

 given back in return. For long ages the earth has 

 produced countless numbers of plants and animals 

 which have derived their sustenance from the soil. 

 All this time the soil has not only retained its 



