[Sidenote: Function and life of bacteria.]

The discoveries that some species of nitrifying bacteria and perhaps
pigmented forms are capable of carbon-assimilation, that others can fix
free nitrogen and that a number of decompositions hitherto unsuspected are
accomplished by Schizomycetes, have put the questions of nutrition and
fermentation in quite new lights. Apart from numerous fermentation
processes such as rotting, the soaking of skins for tanning, the
preparation of indigo and of tobacco, hay, ensilage, &c., in all of which
bacterial fermentations are concerned, attention may be especially directed
to the following evidence of the supreme importance of Schizomycetes in
agriculture and daily life. Indeed, nothing marks the attitude of modern
bacteriology more clearly than the increasing attention which is being paid
to useful fermentations. The vast majority of these organisms are not
pathogenic, most are harmless and many are indispensable aids in natural
operations important to man.

[Illustration: FIG. 13.--A series of phases of germination of the spore of
_B. ramosus_ sown at 8.30 (to the extreme left), showing how the growth can
be measured. If we place the base of the filament in each case on a base
line in the order of the successive times of observation recorded, and at
distances apart proportional to the intervals of time (8.30, 10.0, 10.30,
11.40, and so on) and erect the straightened-out filaments, the
proportional length of each of which is here given for each period, a line
joining the tips of the filaments gives the curve of growth. (H. M. W.)]

Fischer has proposed that the old division into saprophytes and parasites
should be replaced by one which takes into account other peculiarities in
the mode of nutrition of bacteria. The nitrifying, nitrogen-fixing,
sulphur- and iron-bacteria he regards as monotrophic, _i.e._ as able to
carry on one particular series of fermentations or decompositions only, and
since they require no organic food materials, or at least are able to work
up nitrogen or carbon from inorganic sources, he regards them as primitive
forms in this respect and terms them _Prototrophic_. They may be looked
upon as the nearest existing representatives of the primary forms of life
which first obtained the power of working up non-living into living
materials, and as playing a correspondingly important _role_ in the
evolution of life on our globe. The vast majority of bacteria, on the othe