162 SIGNIFICANCE OF BACTERIA 



independent of organic matter for their support, deriving the 

 necessary energy for their growth from the decomposition of 

 inorganic matter. This also is absolutely the case with the 

 nitrifying bacteria which are unable to live upon organic matter 

 and it should be added that these latter forms effect the same 

 decomposition of carbon dioxide as noted in photosynthesis 

 (see page 12). This is an important fact, for we see that there 

 are two methods for effecting tr^ storing up of energy. Green 

 plants are the principal agents in this work. They build up, 

 synthesize, such substances as sugars by utilizing the energy of 

 the sunlight for this reason the process is termed photosynthesis. 

 Certain bacteria also build up compounds by utilizing the energy 

 of chemical reactions. This process is therefore termed chemo- 

 synthesis. 



We should now stop for a moment and consider what the real 

 significance of these bacterial forms is. Green plants build up 

 complex substances from certain elements and simple com- 

 pounds. In the first steps of this work they may be assisted by 

 certain bacteria and fungi. These complex compounds are now 

 reduced by other bacteria to their original state so that they may 

 be used again by the green plant. This in a word is the story 

 of the organic world. Certain substances in the air and in the 

 soil are going through an endless rotation or cycle of chemical 

 changes ever changing from simple to complex, from a com- 

 plex state back to their simple form. So then the green plant is a 

 temporary carrier of certain elements that, owing to their combi- 

 nations, represent a certain amount of stored up energy. We 

 and the plant and the bacteria utilize these substances in the 

 same way. We eat them, i. e., decompose them. This is 

 practically the only way that life can be maintained and the end 

 result of this eating, this decomposition, is that the elements 

 making up the compounds are returned unchanged to the earth, 

 to go again through the cycle of changes. 



4. Disease. In contrast to the kinds of bacteria mentioned 

 above there is another group that live as parasites on plants 

 and animals producing disease either by destroying the tissue 

 and sapping the vitality or by the production of poisonous 



