122 MAINE AGRICULTURAL EXPERIMENT STATION. 



HOW IS THE NITROGEN FIXED? 



There has been a question whether, under the influence of the 

 symbiosis, the higher plant was enabled to fix the free nitrogen 

 of the air by its leaves. It seems probable, however, that the 

 nodule-bacteria fix the nitrogen within the plant, and that the 

 higher plant then absorbs the nitrogenous compounds pro- 

 duced. 



Among the most important recent contributions to the sub- 

 ject are those of Nobbe and Hiltner, who claim* that the assim- 

 ilation bears a direct relation to the formation of bacteroids. In 

 many cases plants growing in rich soil and well supplied with 

 nodules, when inoculated with pure cultures of Bacillus radicicola 

 behaved very differently; some growing considerably in the 

 amount of nitrogen, and others apparently suffering from nitro- 

 gen hunger. Examination proved that the nodule producing 

 organisms were unchanged in the weak plants, while in the 

 thrifty ones the bacteria were changed to bacteroids. The con- 

 clusions drawn were that "(i) tubercles in which bacteroid for- 

 mation does not occur are injurious instead of beneficial to the 

 host plant (the unchanged bacteria are then merely parasites;) 

 (2) the unchanged bacteria present in tubercles seem to have no 

 relation to the nitrogen fixation by legumes ; (3) the more vig- 

 orous the bacteria the less tendency there is toward bacteroid 

 formation; (4) the assimilation of nitrogen begins with the for- 

 mation of bacteroids."f 



Nobbe and Hiltner claim further that the bacteroids are 

 formed by repeated division of the tubercle germ without the 

 separation into isolated individuals. This continued division 

 usually takes place transversely, producing an elongated 

 growth, although lateral protuberances often arise making a 

 branched and irregular appearance. They liken the swollen 

 branched bacteroids to a gill respiration, the nitrogen being 

 absorbed by the water and thus coming to the absorbing sur- 

 faces in a dissolved condition.! 



*Lanclw. Vers. Stat. 42: 459 et seq. 



tCitecl by Russell, Bot. Gaz. 19, 291, (1884). 



jrbid. 



