850 
The Micro-wganisms of the Soil. 
is normal. The experiments weie carried on in sand containing a 
suitable nutritive solution. Some of the pots were sterilised ; to 
others infusions from soil were added. In all those, and in only those, 
to which fresh infusions of lupin soil had been added the lupins 
developed normally, bearing the well-known nodules on their roots, 
and contained, when harvested, conspicuously larger amounts of 
idtrogen than the soil and infusion could have given them. Wher- 
ever the infusion had not been added, or where it had been sterilised 
at 212 deg. or even 160 deg. F., the development remained abnormal, 
the production scanty : nodules continued absent and the harvested 
plants contained less nitrogen than had been oflered them. 
According to Ward, Breal, and Prazmowski, nodules will grow 
on plants free from them when infected with an infusion from 
nodules of other plants. 
Beyerinck has named the infecting organisms, of which there 
may be many varieties. Bacterium radicola. With the growth of 
the nodules the behaviour of the plant towards nitrogen is changed, 
and the just mentioned independence begins ; this has been proved 
by an almost superabundance of experiments. Still the explanation 
of the manner in which the nitrogen is acquired is not definitely 
settled. The first inference would be that the root-inhabiting 
bacteria possess the power of assimilating atmospheric nitrogen, and 
that tlie higher plants as hosts, harbouring these bacteria in their 
roots, use the nitrogen compounds so produced. Thus there would 
exist a case of symbiosis (literally, a living together') between Split 
Fungi (Schizomycetes) and the higher plants. W e cannot be too slow in 
accepting this seemingly simple explanation ; still the difficulty of a 
correct interpretation does not alter the fact that the leguminous 
plants acquire free nitrogen from the atmosphere, and that the refuse 
of their roots thus enriches the soil. They may be called nitrogen- 
collectors, in contradistinction to the gramineous nitrogen -consumers. 
Berthelot has long contended that the free soil can “ fixate ” 
nitrogen. He considers a sandy and clayey nature of the soil 
essential ; it must admit of free access of air, must not be too moist, 
and must be rich in potash and poor in nitrogen. Gautier and 
Drouin claim that the presence of humous substances causes in- 
crease of nitrogen. 
Soils free from organic substances do not fixate nitrogen, or the 
gain is slight. The presence of ferric oxide, so long considered 
capable of fixing nitrogen, has no efiect. Berthelot, as well as most 
investigators in this line, attributes the fixation to the activity of 
nitrogen-fixing chlorophyll-free bacteria. In most cases the amount 
is much less than that obtained in soils with leguminous plants. 
No inorganic soil constituents are known to possess the power of 
fixing nitrogen, and it is questionable whether humous substances 
can directly do this. 
In 1881 Atwater claimed that peas during their growth obtained 
large quantities of nitrogen from the air. Atwater and Woods 
made another series of eighty-nine experiments ; the result is re- 
corded in their admirable paper in the American Chemical Journal, 
