394 THE FOOD OF PLANTS 



Negative results do not necessarily prove that free nitrogen cannot 

 be assimilated in a given plant under any circumstances, for potential 

 powers may lie dormant which are exercised only under special conditions. 

 Isolated root-tubercle bacteria assimilate free nitrogen feebly or not at all, 

 although under other conditions or in their normal habitat this power 

 may be strongly developed. The root-tubercle bacteria are aerobic both 

 when isolated and when present in the root-tubercles (Sect. 70), and are 

 probably supplied with oxygen by the cells of the latter. It is possible 

 moreover that aerobic forms capable of assimilating free nitrogen may exist 

 in the soil. 



It is also extremely probable that this power is possessed by different 

 organisms in different degrees, and that we may pass by stages of transi- 

 tion to plants which are entirely or almost entirely devoid of this property. 

 Winogradsky has actually isolated from soil two other bacteria, which 

 feebly assimilate free nitrogen, but only when the presence of a small 

 trace of a suitable nitrogen-compound renders their growth and development 

 possible. Puriewitsch 1 finds the same to be the case with Pcnicilliu-in 

 glaucinn and Aspergillus niger, but no such power has as yet been detected 

 in any of the algae, or in any of the higher plants. 



None of these plants seem to require free nitrogen, nor apparently do 

 the root-tubercle bacteria, whereas C. Pastewianuin, according to Wino- 

 gradsky, is only able to exist when supplied with free nitrogen and thus 

 enabled to exercise its normal nitrogen-assimilating activity. This bac- 

 terium is unable to grow in bouillon even in the presence of free nitrogen ; 

 the presence of a little ammonium sulphate does not retard its growth, 

 indeed a portion of the nitrogen it requires may be obtained from this 

 source. Again, an abundant supply of nitrates causes the assimilation 

 of free nitrogen by leguminous plants to decrease to a very marked 

 extent. 



Winogradsky was unable to isolate any other bacterium than C. Pas- 

 teurianum with an active power of assimilating nitrogen, and he has 

 shown that the bacteria isolated by Berthelot upon nutrient gelatine were 

 incapable of any such fixation. It is, however, possible, that other 

 nitrogen-assimilating bacteria may exist, but that they are unable to 

 develop under the cultural conditions employed, so that C. Pastcnrianum 

 is not necessarily the only micro-organism which enriches the soil \vith 



1 Her. cl. Bot. Ges., 1895, p. 343. Frank (Hot. Zeitung, 1893, p. 146; Landw. Jahrb., 1892, 

 Bd. xxi, p. 6) and Berthelot (Ann. d. chim. et d. phys., 1893, vi. ser., T. xxx, p. 427) state that 

 Pcnicillium and a few other fungi assimilate free nitrogen. Sestini and del Torre (Versuchsst, 1876, 

 Bd. xix, p. 8) and Jodin (Compt. rend., 1862, T. LV, p. 612) obtained similar results at an earlier 

 date, but these latter researches are inconclusive, and objections may be made even to the ones first 

 mentioned. \Vinogradsky (III, 1895, 1. c., p. 350) could detect no power of fixing nitrogen in an 

 Aspergillus he examined. 



