DEVELOPMENT OF PLANTS 151 



bacteria, and were it not for these organisms 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 cornpounds 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 Decomposition. — The great majority of bacteria 

 live as saprophytes upon organic matter. There are several 

 classes of these plants. One kind lives upon nitrogenous matter, 

 causing a decomposition known as putrefaction. These bacteria 

 only effect a partial decomposition of the organic matter, forming 

 a great variety of substances which are still quite complex and 

 often characterized by their unpleasant odors or poisonous prop- 

 erties. Another kind completes the work of the bacteria of putre- 

 faction, bringing about a decomposition known as decay. Here 

 the end products are much simpler, consisting of carbon dioxide, 

 nitrogen and water, and practically harmless and without odor. 

 The immense importance of these little plants in the economy of 

 nature is evident. The accumulation of material not only is 

 prevented but changes of decomposition by a long series of chem- 

 ical reactions, break down the complex substances, simple organic 

 and inorganic compounds that serve as food for plants. This in 

 brief is the story of the organic world. Plants construct complex 

 compounds from simple ones and die. Bacteria now reduce them 

 to simple substances which become again the food of succeeding 

 generations. Thus bacteria assist in keeping up the fertility 



