42 theorip:s on the absorption of nitrogen 



Beijerinck, in 1890, showed that Bacillus radicicola 

 assimilated nitrates, and this property has since been 

 assigned to Azotobacters as well. This conversion of nitrates 

 and other soluble nitrogenous compounds into insoluble 

 compounds provides a stimulus and favours the fixation of 

 atmospheric nitrogen. 



In these circumstances the Leguminos^ serve a double 

 object in agriculture; they do not merely fix atmospheric 

 nitrogen, but also favour the same function in bacteria. 



Such then are the theories derived from the various 

 phenomena. Should they be confirmed, our ideas and know- 

 ledge of soil fertility will develop. In fact, the American 

 theories on toxins accumulated in the soil by plant growth 

 have already altered the old line of research, and there seems 

 to be no prima facie reason why this doctrine should not 

 be confirmed when one considers that the Leguminosce are 

 incapable of yielding the same crop when cultivated con- 

 tinuously in the same soil. Is it impoverishment of the soil ; 

 is it a degeneration of the particular bacteria associated with 

 the various Leguminosce; or is it the accumulation of the 

 toxins they produce? All these three may be contributory 

 causes, but it is impossible at present to say which pre- 

 dominates. What is likely is the fact that in the soil, 

 which Berthelot has designated as a "living world," there 

 is doubtless a perpetual struggle between the various bac- 

 teria, and, should it be a harmful species which gains the 

 upper hand, the good species disappear. 



As regards the evolution of special races of bacteria, there 

 are thus grounds for believing in the existence of numerous 

 varieties more or less adapted to certain plants and to certain 

 environments. 



We shall content ourselves with a mention of the theory 

 of Bon^ma and Thiel on the assimilation of nitrogen by 

 plants through oxidation of free nitrogen by ferric hydrate. 



Researches have also been made on the influence of 

 sugar. This carbohydrate appears to have the property of 



