NITROGEN-GATHERING OR NITROGEN-FIXING BACTERIA IOL 
nitrates. This proves that some living organisms are concerned, 
and the fact that no viszble plants are growing in the soil shows that 
the higher plants do not produce the result The only conclusion 
that can be drawn, therefore, is that microorganisms are the agents 
for reclaiming free nitrogen from the atmosphere and fixing it in 
the earth in some form of nitrogen compounds, which eventually 
become nitrates and, thus, plant foods. 
Such facts plainly pointed toward bacteria or allied organisms 
as the real agents for fixing nitrogen from the air, and this sugges- 
tion once made was quickly followed by the isolation of the bac- 
teria concerned. It is now a demonstrated fact that the power of 
gathering atmospheric nitrogen and fixing it in the soil belongs to 
bacteria, and during the last fifteen years much study has been 
devoted to the microorganisms concerned. It appears that there 
are two general types of such nitrogen fixations associated with 
different classes of bacteria: (1) Nitrogen fixation by bacteria alone 
(non-symbiotic). (2) Nitrogen fixation by bacteria in connection 
with legumes (symbiotic). 
NON-SYMBIOTIC FIXATION 
These are soil bacteria that are able to produce an increase of 
nitrogen in ordinary soil without the aid of other organisms. Qf 
these, two types have been thoroughly studied, the first acting 
anaerobically, and the second aerobically. (2) The first one that 
was found with this power was isolated from soil and named 
Clostridium™ pasteurianum (Fig.20). Itis an anaerobic bacterium, 
and in culture media will not grow in the presence of oxygen. In 
the soil, however, it is thought to be associated with a second 
bacterium that is aerobic, the latter absorbing the oxygen so that 
the anaerobic form can grow. The Clostridium alone is able to 
assimilate nitrogen if kept in an oxygen-free atmosphere. In its 
* Clostridium is a generic name not commonly used and therefore not mentioned. 
onp 13. It is applied to spore-bearing rods, that swell on sporulation and become 
spindle-shaped. 
