THE BIO-CHEMISTRY OF ANIMALS AND PLANTS 679 



cause of it reduce nitrates to nitrites and ammonia, and finally 

 to free nitrogen. So active are some of them in pure cultures 

 that the process of nitrogen evolution resembles an alcoholic 

 fermentation and has been aptly termed "nitrate fermentation." 

 There cannot be the slightest doubt that by this process there 

 is continually set free on the surface of the earth an enormous 

 quantity of nitrogen which is apparently lost to organic life. 



N-Fixation. — A solution of this apparent contradiction, 

 although already foreseen by Berthelot, has finally been 

 furnished and proved by the exact experimental work of 

 Winogradsky. He succeeded in 1893 in isolating a bacillus, 

 called by him Clostridium Paste urianum, from the soil which 

 is able to utilise directly the nitrogen of the air for building 

 up its protein substance. A large number of similar microbes, 

 mainly anaerobes, have subsequently been isolated by other 

 workers. That the absorption of nitrogen by these organisms 

 takes place in large quantities is proved by experiments on trial 

 fields, and it has been calculated that about 30 kg. nitrogen per 

 acre are made available yearly. 



This is, however, not the only way by which the atmo- 

 spheric nitrogen is utilised by organic life. 1 It has long been 

 accepted amongst farmers as a well-established fact that wheat 

 grows better on fields on which previously leguminous plants 

 such as clover had been grown. Biologists took no notice 

 of this result of practical experience until quite recently. The 

 whole process has now been completely cleared up, the work 

 beginning with the careful investigations of Hellriegel in 1886. 

 He was the first to establish accurately the fact that leguminous, 

 plants possess the property of absorbing atmospheric nitrogen, 

 and he showed the connection of this property with the 

 formation of root nodules and the presence of bacteria. To 

 Beijerinck (1886) belongs the merit of having succeeded in 

 isolating these bacteria in pure cultures and showing that 

 a bacillus, called by him B. radieico/a, enables the plant by a 

 symbiotic process to absorb elementary nitrogen from the 

 atmosphere and to make use of it for its growth. These 



1 An allusion might here be made to the successful modern methods of 

 nitrogen fixation from the air by electro-chemical means (calcium-cyan-amide, 

 calcium nitrate). As it has been stated that the saltpetre fields of Chili will be 

 exhausted within the next fifty years, such considerations are of practical 

 importance. 



