Sources of Nitrogen in Plants. 345 
in the air can supply the plants with what they obtain, 
‘and probably the only assumption which remains is that 
the Papilionaceae have the power of making the free nitrogen 
of the air available for their life-purposes.’ 
Now it has been certainly shown by Boussingault that 
even the Papilionaceae are unable directly to assimilate the 
elementary nitrogen ; but this does not exclude the possibility 
that something of the kind may occur indirectly, and we 
have now to examine a few observations which may point 
to something of the kind. 
Berthelot has shown that free nitrogen may be absorbed 
by the soil and converted into compounds, probably by 
means of schizomycetes or micro-organisms of some kind. 
The roots of Papilionaceae are provided with tubercular 
swellings full of ‘ bacteria ’.* 
It has been stated above that in the researches some plants 
did well and others worse : now, Hellriegel finds that those 
plants which are still in the starvation-phase have either 
no tubercles or very few and insignificant ones, whereas the 
plants which are flourishing have many well-developed speci- 
mens on the roots. ‘ The more plants we investigated, the 
more we were convinced that the development of the root- 
tubercles stands in the closest, strictest relation to the growth 
and assimilation of the whole plant.’ 
Now, notice the following experiments. On May 25 were 
taken forty vessels filled with soil devoid of nitrogen, and 
two pea-seeds placed in each. Then ten of these vessels 
were watered with soil-washings— the authors say, ‘ Resting 
on the fact that in every normal culture-soil micro-organisms 
exist in abundance, we took some of the fertile soil of our 
culture-field, stirred it up with five times the quantity of 
distilled water, and after a short settling gave 25 cc. of this 
quasi-solution to each vessel.’ 
Bearing in mind that the experiment began on May 25, 
1 Here the author is following older views as to the nature of the contents of 
the tubercles : they are not bacteria, but yeast-like gemmules budded off from the 
mycelium of a true fungus. (See paper in Phil. Trans. 1887, pp. 539-562.) 
