FIXATION OF FREE NITROGEN. 159 



It is seen that, stated very briefly, the general result was mtwgen 

 that at the third period of growth of the peas in sand, the in yot- 

 amount of dry matter of the nodules was very much dimin- 

 ished, the percentage of nitrogen in the dry matter was very 

 much reduced, and the actual quantity of nitrogen remaining 

 in the total nodules was also very much reduced ; in fact, 

 the nitrogen of the nodules was almost exhausted. The peas 

 grown in rich soil, however, maintained much more vegeta- 

 tive activity at the conclusion, and showed a very great 

 increase in the number of nodules from the first to the third 

 period ; and with this there was also much more dry sub- 

 stance, and even a greater actual quantity of nitrogen in the 

 total nodules at the conclusion. Still, as in the peas grown 

 in sand, the percentage of nitrogen in the dry substance of 

 the nodules was very much reduced at the conclusion. 



In the case of the plant of longer life — the sainfoin — there 

 was, both in sand and in soil, very great increase in the 

 number of nodules, and in the actual amount of dry substance 

 and of nitrogen in them, as the growth progressed. The per- 

 centage of nitrogen in the dry substance of the nodules also 

 showed, even in the sand, comparatively little reduction, and 

 in the soil even an increase. In fact, separate analyses of 

 nodules of different character, or in different conditions, 

 showed that whilst some were more or less exhausted and 

 contained a less percentage of nitrogen, others contained a 

 high percentage, and were doubtless new and active. 



Thus the results pointed to the interesting conclusion that An inter- 

 in the case of the annual, when the seed is formed, and the es , tm P cm ' 



CvUSIOTl 



plant more or less exhausted, both the actual amount of 

 nitrogen in the nodules, and its percentage in their dry 

 substance, are greatly reduced; but that with the plant of 

 longer life, although the earlier-formed nodules become ex- 

 hausted, others are constantly produced, thus providing for 

 future growth. The results of this new series of experiments, Root-nod- 

 taken together with those of the quantitative series, also serve ^ es ar * d . 

 further to show that there is intimate connection between the nitrogen. 

 gain of nitrogen by Leguminosas, and the development of 

 nodules on their roots. 



The alternative explanations of the fixation of free nitrogen Alternative 

 in the growth of Leguminosae seem to be — tions^f'the 



1. That under the conditions of the symbiosis the plant fixation of 

 is enabled to fix the free nitrogen of the atmosphere by its f ree nitro ~ 



1 gen. 



leaves. 



2. That the nodule- organisms become distributed within 

 the soil, and there fix free nitrogen ; the resulting nitrogenous 

 compounds becoming available as a source of nitrogen to the 

 roots of the higher plant. 



