BACTERIA IN NATURAL PROCESSES. 103 



nitrites (Fig. 25 at D). Now these compounds are 

 not ordinarily within the reach of plant life. The 

 luxuriant vegetation of the globe extracts its ni- 

 trogen from the soil in a form more complex than 

 either of the compounds here mentioned ; for, as 

 we have seen, it is nitrates chiefly that furnish 

 plants with their nitrogen food factor. But ni- 

 trates contain considerable oxygen. Ammonia, 

 which is one of the products of putrefactive de- 

 composition, contains no oxygen, and nitrites, an- 

 other factor, contains less oxygen than nitrates. 

 These bodies are thus too simple for plants to 

 make use of as a source of nitrogen. The chem- 

 ical destruction of the food material which results 

 from the action of the putrefactive bacteria is too 

 thorough, and the nitrogen foods are not yet in 

 condition to be used by plants. 



Now comes in the agency of still another class 

 of micro-organisms, the existence of which has 

 been demonstrated to us during the last few years. 

 In the soil everywhere, espe- 

 cially in fertile soil, is a class 

 of bacteria which has received 

 the name of nitrifying bacteria 

 (Fig. 26). These organisms 

 grow in the soil and feed upon 

 the soil ingredients. In the FlG - 26. -Soil bacteria 

 course of their life they have ^^^ *" 

 somewhat the same action upon 

 the simple nitrogen cleavage products just men- 

 tioned as we have already noticed the vinegar- 

 producing species have upon alcohol, viz., the 

 bringing about a union with oxygen. There are 

 apparently several different kinds of nitrifying 

 bacteria with different powers. Some of them 

 cause an oxidation of the nitrogen products by 



