142 IMPORTANCE OF BACTERIA 



dnce a form of decay called fermentation. It is also a familiar 

 fact that many diseases are due to these plants. 



Attention has already been called to certain bacteria of the first 

 group mentioned above as very important plants in keeping up 

 the fertility of the soil. These bacteria form tubercles in the 

 roots of plants belonging to the bean family where they combine 

 the free nitrogen of the air into compounds that can be absorbed 

 by the plants, page 57. Another very important kind that may 

 be considered in connection with the tubercle-forming bacteria, 

 live in the soil and change the simple nitrogen compounds, as 

 ammonia, that result from the decay of plant and animal life 

 into nitrites. For this reason they are commonly called the 

 nitrifying bacteria. Were it not for these bacteria the ammonia 

 compounds would quickly escape from the soil as a gas and thus 

 the principal source of nitrogen in nature for the plant would be 

 lost. 



Numerous forms of bacteria produce deposits of sulphur and 

 iron by oxidizing the lower compounds of these minerals and 

 are thus the cause of the white and yellow slimy masses of sul- 

 phur and iron in springs and streams. Some of these bacteria 

 appear to be independent of organic matter for their support, 

 deriving the necessary energy for their growth from the de- 

 composition of inorganic material. It should be added that the 

 nitrifying bacteria effect the same interchange of gases as noted 

 in photosynthesis and that some of the sulphur bacteria possess 

 a reddish pigment that decomposes carbon-dioxide in the same 

 way. This substance may be related to chlorophyll and it has 

 been suggested that it may represent an earlier state in the evo- 

 lution of chlorophyll. 



(d) Bacteria of Decay. — The great majority of bacteria live 

 as saprophytes upon organic matter. There are two classes of 

 these plants. The first class live upon nitrogenous matter, caus- 

 ing a decomposition or decay known as putrefaction and thus 

 removing the remains of the animals and plants. The immense 

 importance of this in the economy of nature is evident. The 

 accumulation of material not only is prevented but changes of 

 decav, by a long series of chemical reactions, break down the 



