FIXATION OF FREE NITROGEN 799 



been grown and which contains the nodule-bacteria, the plants 

 do not die, but grow luxuriantly and increase in nitrogen-content 

 to a very great degree. At the same time an extensive develop- 

 ment of nodules is observable on their roots. 



By growing . nodule-bearing plants in closed vessels and 

 analysing the composition of the enclosed air both before and 

 after growth Schloesing and Laurent showed that the amount of 

 free nitrogen which disappeared from the air corresponded with 

 the amount gained by the plants. 



Although there is no doubt about the fact that without the 

 nodule-bacteria leguminous plants cannot utilise free nitrogen, 

 the manner in which the combination of plant plus bacteria 

 effects the nitrogen-fixation is still a matter of some uncertainty. 



Mazd and Golding have shown that the nodule-bacteria in pure 

 cultures apart from leguminous plants, are able to fix and utilise 

 free nitrogen of the atmosphere to a certain extent, and it is 

 most probable that leguminous plants growing in association 

 with the nodule-bacteria obtain their abundance of nitrogen 

 not from the air direct, but as it were second-hand from the 

 bacteria. 



No increase of nitrogen is observable actually in the soil 

 on which the plants are growing, and there appears to be no 

 evidence for the supposition that the nitrogen is first fixed in the 

 soil and subsequently taken up by the roots of the leguminous 

 plant. 



The explanation which appears to agree most with known facts 

 is that the nitrogen is fixed by the nodule-bacteria in the tissues 

 of the nodule, and that the nitrogenous compounds produced 

 are subsequently absorbed and utilised by the leguminous 

 plant. 



The amount of combined nitrogen present in the nodules is 

 found to increase gradually from the time they first appear up to 

 the time when they are plump and their hypertrophied tissue 

 completely filled with bacteroids. 



