TlhG Sources of the Nhiroijen of our Leguminous Crops. 691 
conditions, be great gain of nitrogen if only the soil be suitably 
infected. Nor would there be any such actual gain of nitrogen in 
nitrogen-free soils, as there undoubtedly is, if the source of the 
nitrogen, either of the parasite or of the host, were essentially 
the supplies of combined nitrogen within the soil. 
Further, one assumption is, that the organisms become dis- 
tributed in the soil both during the life of the host and after- 
wards, and that the fixation takes place under their agency 
within the soil itself, rather than in the course of the develop- 
ment of the bacteria in symbiosis with the higher plant. 
Another assumption is, that the fixation takes place in the soil 
itself under the influence of microbes existing within it, and 
that the higher plant assimilates the resulting combined nitrogen. 
As bearing upon these points, it may be obsei'ved that in the 
experiments with peas in 1888 there was practically no gain of 
nitrogen within the soil itself, which it may be sujiposed there 
would have been if the fixation had taken place within it, and 
the host had acquired its gain from the compounds there pro- 
duced. Indeed, the evidence at present at command certainly 
does not point to the conclusion that the gain of nitrogen by 
Leguminosie under the influence of microbe infection of the 
soil, and nodule-formation, is due to fixation by organisms within 
the soil itself, indepenrTently of the symbiosis. It is obvious, 
too, that, so far as free nitrogen may be fixed by microbes within 
the soil, independently of connection with a higher plant, the 
resulting nitrogenous compounds should, directly or indirectly, 
be available to plants generally, whether leguminous or non- 
leguminous. 
On this point it may be remai'ked that about thirty years 
ago Boussingault concluded, from the results of vegetation ex- 
periments made in 1858 and 1859 in mixtures of rich soil and 
sand, that free nitrogen had been fixed within the soil by the 
agency of mycodermic vegetation, and that the nitrogenous 
products which remained within it were largely in the form of 
organic detritus. Subsequently, however, he considered that 
there was not satisfactorv evidence that free nitrooren is fixed 
* within the soil under the influence of the development of lower 
organisms. It is, nevertheless, of interest to observe that those of 
his results in 1858 and 1859 which showed any material gain of 
nitrogen, either in the vegetable matter grown, or in the soil, 
were obtained with Leguminosa', and that in the case in which 
there was the greatest gain in the plants themselves, he records 
that there were numerous tubercles on their roots. In one other 
case, in which, however, only sand was used as soil, and the 
gain in the plant was but small, he also observed tubercles on 
z z 2 
