80 THE EEACTIONS OCCURRING IN SOILS. IV. 



formation of nitric acid. The nitrogen must have been 

 obtained from the air. Subsequent experiments of Wino- 

 gradski showed that the bacillus was anaerobic and if air were 

 present could only assimilate free nitrogen by the aid of other 

 micro-organisms which may have acted by removing the dis- 

 solved oxygen from the solution.. The amount of nitrogen 

 assimilated seems to bear some relation to the sugar consumed, 

 but the action is greatly affected by the presence of combined 

 nitrogen. 



Cultures of an organism (known as Bacillus ellenbachensis), 

 said to have similar properties, were made in Germany and 

 sold under the name of " aliiiit." They have not been very 

 successful in practice. 



Another class of micro-organisms is of great importance in 

 agriculture, viz., those which flourish in the nodular swellings 

 on the roots of certain leguminous plants. 



The great question as to the possibility or otherwise of 

 utilising the free nitrogen of the air has excited much attention 

 and an enormous amount of research has been devoted to its 

 solution. That leguminous crops apparently increased rather 

 than diminished the amount of nitrogen in the upper part of 

 the soil, although they contained large quantities themselves, 

 had been observed and use had been made of the fact in 

 agriculture. No satisfactory explanation as to how this was 

 effected was forthcoming until, in 1886, Hellriegel published an 

 account of the bacteria which he found in the root nodules 

 possessed by clover and other leguminous plants. In later 

 papers, in conjunction with Wilfarth," he clearly showed that, 

 living in these nodules were bacteria (Bacillus mclicocola), 

 which have the power of bringing about the assimilation by 

 the parent plant of the free nitrogen of the air. From other 

 investigations by the same chemists, and also by Nobbe and 

 Hiltner, Schloesing and Laurent, and others, it has been 

 proved that the various leguminosae have different bacteria, 

 and that assimilation of free nitrogen by a plant depends upon 

 the presence in the soil of the particular micro-organism 

 capable of growing in symbiosis] with it. The importance of 



* See abstracts in J.C.S. 1888, 742 ; 1889, 640. 



t By this term is meant the living together of two organisms for their mutual 

 welfare, as distinguished from parasitism, in which one oiganism preys upon another to 

 its own advantage, but to the injury of the host. 



