DIFFERENCES BETWEEN DIFFERENT SPECIES OF BACTERIA.

While bacteria are thus very simple in form, there are a few other
slight variations in detail which assist in distinguishing them.
The rods are sometimes very blunt at the ends, almost as if cut
square across, while in other species they are more rounded and
occasionally slightly tapering. Sometimes they are
surrounded by a thin layer of some gelatinous substance, which
forms what is called a capsule (Fig. 10). This capsule may connect
them and serve as a cement, to prevent the separate elements of a
chain from falling apart.

Sometimes such a gelatinous secretion will unite great masses of
bacteria into clusters, which may float on the surface of the
liquid in which they grow or may sink to the bottom. Such masses
are called zoogloea, and their general appearance serves as one of
the characters for distinguishing different species of bacteria
(Fig. 10, a and b). When growing in solid media, such as a
nutritious liquid made stiff with gelatine, the different species
have different methods of spreading from their central point of
origin. A single bacterium in the midst of such a stiffened mass
will feed upon it and produce descendants rapidly; but these
descendants, not being able to move through the gelatine, will
remain clustered together in a mass, which the bacteriologist
calls a colony. But their method of clustering, due to different
methods of growth, is by no means always alike, and these colonies
show great differences in general appearance. The differences
appear to be constant, however, for the same species of bacteria,
and hence the shape and appearance of the colony enable