304 BIOLOGY AND ITS MAKERS 
of energy left in the nitrogenous substance. They cause the 
nitrogen and the hydrogen to unite with oxygen in such a way 
that there are produced nitrous and nitric acids, and from 
these two acids, through chemical action, result the nitrites and 
the nitrates. These substances are then utilized by the plant 
in the manufacture of protoplasm, and the plant is fed upon 
by animal organisms, so that a direct relationship is estab- 
lished between these lower forms of life and the higher plant 
and animal series; a relationship that is not only interesting, 
but that helps to throw an important side-light upon the 
general nature of vital activities, their kind and their reach. 
In addition to the soil bacteria mentioned above, there 
are others that form association with the rootlets of certain 
plants and possess the power of fixing free nitrogen from 
the air. 
The nitrifying bacteria, are, of course, of great importance 
to the farmer and the agriculturist. 
It is not our purpose, however, to trace the different 
phases of the subject of bacteriology to their conclusions, but 
rather to give a picture of the historical development of this 
subject as related to the broader one of general biology. 
