68 BACTERIOLOGY FOR NURSES 



not accumulate unless bacteria are present. A rather strange 

 fact is that this fixation of nitrogen is not accomplished by any 

 one species alone, but only takes place when two. or three are 

 acting together. The name azobacter is generally applied to the 

 group. 



A second method by which bacteria aid in reclaiming this dis- 

 persed nitrogeri is in combination with some of the higher plants, 

 chiefly beans, peas, and clover. When growing in soil that con- 

 tains little or no nitrogen, these plants will, during their growth, 

 be found to have accumulated a considerable store of combined 

 nitrogen in their tissues. It is evident that the only possible source 

 of supply is the nitrogen of the air that permeates the soil. When 

 a plant gains its nitrogen in this manner it develops upon its roots 

 little protuberances known as root nodules or tubercles, which when 

 examined microscopically are found to be nests of bacteria. By 

 what process the plant and the bacteria growing together succeed 

 in extracting the nitrogen from the air is not known. The plant 

 continues to increase the store of nitrogen in its roots, stem, and 

 leaves probably during the whole of its normal growth. Finally 

 it dies, and, falling upon the ground becomes buried. It is imme- 

 diately seized by the decomposition bacteria and the destructive 

 changes already described begin. The eventual result is, that 

 which seemed lost nitrogen is once more converted into nitrates 

 and again forms part of the cycle. 



Bacteria, then, play a threefold role in the cycle : (1) they reduce 

 complex nitrogenous substances to simpler and more stable ones 

 such as may be used by plants for food ; (2) they build up those 

 that are too simple into suitable compounds ; (3) they fix the nitro- 

 gen of the air by an unknown process, but by one that makes it 

 again available for the nourishment of plants. 



It is important to note that almost the entire cycle takes place 

 upon the surface or in the upper layers of the soil. A few feet 

 below the surface there are very few bacteria ; consequently a car- 

 cass buried deep or sewage placed too low are not acted upon so 

 completely. At a depth of six feet very few organisms are found. 



It is extremely difficult to determine the exact number of bac- 



