270 



Hereby hydrogen and much carbonic acid are produced, so that it is then necessary 

 also to determine the hydrogen. But this fermentation may be prevented by intro- 

 ducing the material very loosely into the burette, so that there are but few points of 

 contact between the nodules, and the air can freely pass between. Under such con- 

 ditions there is no danger that free nitrogen will be formed; this only occuring through 

 the action of the denitrifying bacteria on nitrates, which substance is in the nodules 

 completely absent. 



Of the tubercles of yellow lupine we used in our experiments quantities of 

 100 grs., 500 grs., and later even of i kil. In some experiments we had the root 

 tubercles cut off, in others the roots with the tubercles were left united with large 

 pieces of the stem, so that eventually formed nitrogen compounds might be able to 

 flow into the stem. All our estimations, however, showed that not in a single case 

 the slightest fixation of nitrogen by the tubercles was observable. As at first we 

 doubted of the accuracy of our results obtained with relatively little material, we after- 

 wards used the just mentioned larger quantities, but this did not make any difference 

 either. Besides the two said species we still examined several times 10 to 20 grs. of 

 the nodules of Vicia faba, and once about 15 grs. nodules of Robin ia pseudo-acacia, 

 but other results were not obtained. 



As our researches did not last longer than 12 to 20 days it might be objected 

 that we have not sufficiently imitated the conditions of the plants in the field. Further, 

 that in these experiments the growth of the tubercles, together with that of the whole 

 plant, was excluded. Although these objections have not been refuted in the pre- 

 ceding, it is still highly improbable that nitrogen fixation would be associated with 

 the growth of the tubercles and not with the augmentation of the bacteria out of 

 the plant. Principal, however, is the fact that if within the nodules nitrogen fixation 

 were to take place, which might have escaped our attention, the concerned quantity 

 must certainly be extremely small. When we now consider how difficult it is to 

 collect a few grams of tubercles for example of Robinia, it is clear that if this material 

 is to be of any significance for such a great tree, its nitrogen-fixing power must be 

 enormous. The experiments, however, show that the tubercles are wholly inactive or 

 nearly so, hence there can be no question of attaching to them any importance con- 

 cerning the nitrogen nutrition, whilst yet nitrogen fixation by this tree is as certain 

 as for lupine and serradella and even on a much larger scale. So the nitrogen 

 nutrition of the Papilionaceae can only be indirectly connected with the bacteria of 

 the nodules. In my opinion this relation can only exist in the herbaceous species and 

 in the germ plants of the shrubs and trees of that plant order, but in full-grown 

 specimens of the woody species such al Robin ia pseudo-acacia the presence or the 

 absence of the nodules is wholly indifferent. Likewise on the roots of shrubs, such as 

 Sarothamnus vulgar is, Spartium scoparium, Genista anglica, and Genista pilosa in 

 full-grown condition, the number of tubercles is so small, their volume so insignificant 

 to that of the whole plant, that even if they were able to assimilate some free nitrogen 

 their slight activity could not possibly explain the rich nitrogen store of the whole 

 plant. 



Hence, the at present generally accepted explanation of the peculiar behaviour of 

 the Papilionaceae cannot be correct. New researches, especially with Phaseolus, are 

 desirable. 



