mind. That some of them do cause disease is indisputable, since
bacteriologists have, by their watchful and careful methods, separated
almost a single plant from its surroundings and congeners, planted it
free from all contamination, and observed it produce an infinitesimal
brood of its own kind. Animals and patients inoculated with the plants
thus cultivated have rapidly become subjects of the special disease
which the particular plant was supposed to produce.

The difficulty of such investigation becomes apparent when it is
remembered that under the microscope many of these forms of vegetable
life are identical in appearance, and it is only by observing their
growth when in a proper soil that they can be distinguished from each
other. In certain cases it is quite difficult to distinguish them by
the physical appearances produced during their growth. Then it is only
after an animal has been inoculated with them that the individual
parasite can be accurately recognized and called by name. It is known
then by the results which it is capable of producing.

The various forms of bacteria are recognized, as I have said, by their
method of growth and by their shape. Another means of recognition is
their individual peculiarity of taking certain dyes, so that special
plants can be recognized, under the microscope, by the color which a
dye gives to them, and which they refuse to give up when treated with
chemical substances which remove the stain from, or bleach, all the
other tissues which at first have been similarly stained.

The similarity between bacteria and the ordinary plants with which
florists are familiar is, indeed, remarkable. Bacteria grow in animal
and other albuminous fluids; but it is just as essential for them to
have a suitable soil as it is for the corn or wheat that the farmer