r
hand, which are ordinarily termed saprophytes, are _saprogenic_, _i.e._
bring organic material to the putrefactive state--or _saprophilous_, _i.e._
live best in such putrefying materials--or become _zymogenic_, _i.e._ their
metabolic products may induce blood-poisoning or other toxic effects
(facultative parasites) though they are not true parasites. These forms are
termed by Fischer _Metatrophic_, because they require various kinds of
organic materials obtained from the dead remains of other organisms or from
the surfaces of their bodies, and can utilize and decompose them in various
ways (_Polytrophic_) or, if monotrophic, are at least unable to work them
up. The true parasites--obligate parasites of de Bary--are placed by
Fischer in a third biological group, _Paratrophic_ bacteria, to mark the
importance of their mode of life in the interior of living organisms where
they live and multiply in the blood, juices or tissues.

[Sidenote: Nitrogen bacteria.]

When we reflect that some hundreds of thousands of tons of urea are daily
deposited, which ordinary plants are unable to assimilate until
considerable changes have been undergone, the question is of importance,
What happens in the meantime? In effect the urea first becomes carbonate of
ammonia by a simple hydrolysis brought about by bacteria, more and more
definitely known since Pasteur, van Tieghem and Cohn first described them.
Lea and Miquel further proved that the hydrolysis is due to an
enzyme--urase--separable with difficulty from the bacteria concerned. Many
forms in rivers, soil, manure heaps, &c., are capable of bringing about
this change to ammonium carbonate, and much of the loss of volatile ammonia
on farms is preventible if the facts are apprehended. The excreta of urea
alone thus afford to the soil enormous stores of nitrogen combined in a
form which can be rendered available by bacteria, and there are in addition
the supplies brought down in rain from the atmosphere, and those due to
other living debris. The researches of later years have demonstrated that a
still more inexhaustible supply of nitrogen is made available by the
nitrogen-fixing bacteria of the soil. There are in all cultivated soils
forms of bacteria which are capable of forcing the inert free nitrogen to
combine with other elements into compounds assimilable by plants. This was
long asserted as probable before Winogradsky showed that the conclusions of
M. P. E. Berthelot, A. Laurent an