68 NITRIFICATION AND DENITRIFICATION 
tions a very small amount of organic matter acts like an antiseptic. 
In the soil, however, the nitrifiers behave differently, and are not 
checked in their growth by such small quantities of organic matter 
as serve to check them in laboratory solutions. This injurious 
action of organic matter is a curious phenomenon. The more 
highly organized the compound, the more decided its checking 
action, and thus, the more valuable the material for ordinary kinds 
of bacteria, the greater its injury upon the nitric bacteria. These 
bacteria thus grow under conditions detrimental to nearly all 
other bacteria, but will not grow under the conditions which other 
species find most favorable. A more sharp contrast can hardly be 
conceived. Not only bacteria, but all other colorless plants are 
obliged to depend upon organic food as a source of energy, in 
this respect resembling animals; but here is a group of organisms 
that does not need organic matter. They do not, therefore, need 
any other living o1ganisms to interpose between them and the 
mineral world, but may develop under conditions in which they 
are supplied with mineral substances alone. It is more surprising 
perhaps to find that they do not need light, but can utilize the 
mineral substances while growing in perfect darkness This fact 
was at first conceived as quite contrary to our general ideas of the 
relation of life to physical energy. We have supposed that the 
only source of energy for living things is sunlight, and that this 
energy is stored up by green plants in the form of chemical com- 
pounds of high complexity. The animals and colorless plants 
use these stores as food, breaking them up and using the energy 
liberated for their own use. But here we have organisms which do 
not require organic material as a source of energy and are not able 
to utilize sunlight itself directly. Evidently they must obtain 
their energy from some other source than that which is commonly 
utilized by animals and plants. That they have a source of energy 
at command is evident from the fact that they can assimilate CO. 
and build it into their own tissues, a process that requires energy. 
The present belief is that they obtain their energy from the oxida- 
