VITAL PHEXOMEXA OF BACTERIA 99 



albuminous substances, which are frequently first peptonized and then 

 further decomposed. Typical putrefaction occurs only when oxygen is 

 absent or scanty; the free passage of air through a culture of putrefac- 

 tive bacteria an event which does not take place in natural putrefaction 

 very much modifies the process : first, biologically, as the anaerobic 

 bacteria are inhibited, and then by the action of the oxygen on the 

 products or by-products of the aerobic and facultative anaerobic 

 bacteria. 



As putrefactive products we have peptone, ammonia, and amines, 

 leucin, tyrosin, and other amido substances; oxyfatty acids, indol, 

 skatol, phenol, ptomains, toxins, and, finally, sulphuretted hydrogen, 

 mercaptans, carbonic acid, hydrogen, and, possibly, marsh-gas. 



Nitrifying Bacteria. According to recent observations, nitrification is 

 produced by a small, special group of bacteria, cultivated with diffi- 

 culty, which do not grow on our usual culture media. From the inves- 

 tigations of Winogradsky it would appear that there are two common 

 micro-organisms present in the soil, one of which converts ammonia 

 into nitrites and the other converts nitrites into nitrates. - 



Conversion of Nitrous and Nitric Acids into Free Nitrogen. This process 

 is performed by a number of bacteria. The special nitrate-fermenting 

 bacteria, however, were first accurately described by Barri and Stutzer. 

 In their exhaustive investigation they first isolated from horse-manure 

 two bacteria, neither of which was alone capable of producing nitrogen 

 from nitrates, but which together in the presence of oxygen, but never 

 without it entirely, decomposed nitrates energetically. Later a second 

 denitrificating bacillus was found, B. denitrificans II, which by itself 

 was able to produce nitrogen from nitrates. 



The practical importance of these organisms is that by their action 

 large quantities of nitrates in the soil, and especially in manure, may 

 become lost as plant food by being converted into nitrogen. 



Nitrogen Combination. The bacillu* radiocola of Beyerinck, which 

 was isolated by him, has the power of assimilating nitrogen from the 

 air. This bacillus is found in the small root nodules of various legu- 

 minous plants (pease, clover, etc.), and can be obtained from these in 

 cultures. Different varieties exist in different kinds of legumes, each 

 kind of legume apparently having a special variety of bacteria adapted 

 to it, and not every variety is capable of producing nodules in all legumes. 

 There are certain "neutral" varieties, however, exjtfkg free in the soil 

 and not adapted to any special legume, and thd o he able to 



form nodules in different legumes. 



By the aid of these root bacteria, which e|pRance to the roots 

 and there produce this nodular formation, We leguminous plants are 

 enabled to assimilate nitrogen from the atmosphere. ' This explains the 

 reason why poor, sandy soils become gradually fruitful when pease, 

 lupine, and other varieties of legumes are grown upon them and then 

 turned under with the plough. It is not known exactly how this assimi- 

 lation of nitrogen occurs, but it is assumed that the zoogloea-like bac- 

 teria, called bacteroids, constantly observed in the nodules, either alone 



