MICRO-ORGANISMS. 151 
We have seen that nitrate, by the action of the denitrify- 
ing bacteria, is reduced to free nitrogen gas. In the course 
of ages there must necessarily have “been a depletion of the 
world’s combined nitrogen were there not a compensating 
action at work in the fixation of nitrogen by certain algae 
and higher plants. There is thus a cycle in the alteration 
of albumen, viz., albumen-ammonia-nitrate-nitrogen-albu- 
men. 
The Fixation of Nitrogen. 
The interest in the fixation of the nitrogen question is 
centred around the Legwminose, whose roots are studded 
with nodules, within which are bacteria of a particular kind. 
By some means, at present unknown, these cause the plant 
to absorb and elaborate gaseous nitrogen. Two of the 
most recent suggestions are that the bacteria secrete an 
enzyme, which causes fixation in the leaves, and that the 
plant absorbs nitrogen to form a compound which 1s 
elaborated by the bacteria and returned to the plant. The 
fixative power is not restricted to the Leguminose, but is 
found in other plants. Likewise, bacteria have been isolated 
that fix nitrogen directly—a thing that has not been proved 
for the nodule bacteria. It has, however, been placed 
beyond doubt that in poor soils the genera of the Legu- 
minose will not grow unless the bacteria peculiar to them- 
selves are present. There are races of the nodule-former for 
each plant.genus ; for example, the bacterium in the nodule 
of the vetch is different from that in the bean, and the lupin 
bacterium differs from both. 
Nitragin and Alinit. 
Pure cultures of individual nodule bacteria are sold under 
the name of “ nitragin,”’ for the purpose of scattering over 
the ground before sowing, say, a crop of peas, or for m 
fecting the seeds previous to sowing. Nitragin is a culture 
of the bacteria in gelatine which has been liquefied and then 
solidified, so that the bacteria are diffused .throughout the 
solid gelatine. Experiments with these pure cultures have 
been contradictory, and it is apparent that they are only 
useful when a plant is introduced into a field for the first 
time. In such a case, a bag of soil from an old field, where 
the plants have been growing, would probably serve equally 
well. 
Under the name of “ Alinit,” a bacterium has been placed 
upon the market for the purpose of increasing the cereal 
crop. Just as in the case of nitragin, field-experiments with 
this bacterium have been inconclusive and contradictory. 
