= The vegetable kingdom to which the bacteria
belong consists of plants of the most varying size and nature. Those
of most common acquaintance are the green plants varying in size
from those not visible to the naked eye to the largest trees.
Another class of plants known as fungi or fungous plants do not
contain chlorophyll, the green coloring matter, but are usually
colorless and, as a rule, of small size; among them are included
such forms as the mushrooms, smuts, rusts and mildews, as well as
the molds and yeasts. The bacteria are closely allied to this latter
class. When first discovered they were thought to be animals because
of the ability of some forms to move about in liquids.

The bacteria, like other kinds of living organisms, possess a
definite form and shape. They are the simplest in structure of all
the plants, the individual organism consisting of a single cell. The
larger and more highly organized forms of life are made up of many
microscopic cells, and the life of the individual consists of the
work of all the cells. The bacteria are very comparable to the
single cells of the higher plants and animals, but in the case of
the bacteria the single cell is able to exist apart from all other
cells and to carry out all of its life processes including
reproduction.

=Forms of bacteria.= With the multicellular organisms much variation
in form is possible, but with these single-celled organisms the
possible variation in form is greatly limited. Three well marked
types occur among the bacteria: the round or coccus form (plural
cocci); the rod-shaped or bacillus (plural bacilli); and the
twisted or spirillum type (plural spirilla). Most organisms of
special significance in dairying belong to the coccus or bacillus
group.

=Size of bacteria.= The bacteria, as a class, are among the smallest
of living objects. None of them are individually visible to the
naked eye, and they can be so seen only when clumps or masses are
formed in the process of growth.

[Illustration: Fig. 1.--Forms of Bacteria. A, coccus; B, bacillus;
C, spirillum.]

While there is considerable relative variation in size, yet in
actual dimensions, this difference is so small as to make careful
microscopic determinations necessary. An average diameter may be
taken as about one thirty-thousandth of an inch, while the length
varies naturally several fold, depending upon whether the type under
observation is a coccus or a bacillus.

It is very