e
life history of these organisms in general, how they live, move and
react toward their environment.

~Nature of Bacteria.~ Toadstools, smuts, rusts and mildews are known to
even the casual observer, because they are of evident size. Their
plant-like nature can be more readily understood from their general
structure and habits of life. The bacteria, however, are so small, that
under ordinary conditions, they only become evident to our unaided
senses by the by-products of their activity.

When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
these tiny, rapidly-moving organisms he thought they were animals.
Indeed, under a microscope, many of them bear a close resemblance to
those minute worms found in vinegar that are known as "vinegar-eels."
The idea that they belonged to the animal kingdom continued to hold
ground until after the middle of the nineteenth century; but with the
improvement in microscopes, a more thorough study of these tiny
structures was made possible, and their vegetable nature demonstrated.
The bacteria as a class are separated from the fungi mainly by their
method of growth; from the lower algae by the absence of chlorophyll,
the green coloring matter of vegetable organisms.

~Structure of bacteria.~ So far as structure is concerned the bacteria
stand on the lowest plane of vegetable life. The single individual is
composed of but a single cell, the structure of which does not differ
essentially from that of many of the higher types of plant life. It is
composed of a protoplasmic body which is surrounded by a thin membrane
that separates it from neighboring cells that are alike in form and
size.

~Form and size.~ When a plant is composed of a single cell but little
difference in form is to be expected. While there are intermediate
stages that grade insensibly into each other, the bacteria may be
grouped into three main types, so far as form is concerned. These are
spherical, elongated, and spiral, and to these different types are given
the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural,
_cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a
corkscrew serve as convenient models to illustrate these different
forms.

[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_,
represent different types as to form: _a_, coccus, _b_, bacillus, _c_,
spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or
cluster coccus; _f_ and _g_, di