doubtedly
contribute in bringing the surface into contact with larger supplies of
oxygen and other fluids in unity of time, are not so rapid or so extensive
when compared with other standards than the apparent dimensions of the
microscopic field. The microscope magnifies the distance traversed as well
as the organism, and although a bacterium which covers 9-10 cm. or more in
15 minutes--say 0.1 mm. or 100 [micron] per second--appears to be darting
across the field with great velocity, because its own small size--say 5 x 1
[micron]--comes into comparison, it should be borne in mind that if a mouse
2 in. long only, travelled twenty times its own length, _i.e._ 40 in., in a
second, the distance traversed in 15 minutes at that rate, viz. 1000 yards,
would not appear excessive. In a similar way we must be careful, in our
wonder at the marvellous rapidity of cell-division and growth of bacteria,
that we do not exaggerate the significance of the phenomenon. It takes any
ordinary rodlet 30-40 minutes to double its length and divide into two
equal daughter cells when growth is at its best; nearer the minimum it may
require 3-4 hours or even much longer. It is by no means certain that even
the higher rate is greater than that exhibited by a tropical bamboo which
will grow over a foot a day, or even common grasses, or asparagus, during
the active period of cell-division, though the phenomenon is here
complicated by the phase of extension due to intercalation of water. The
enormous extension of surface also facilitates the absorption of energy
from the environment, and, to take one case only, it is impossible to doubt
that some source of radiant energy must be at the disposal of those
prototrophic forms which decompose carbonates and assimilate carbonic acid
in the dark and oxidize nitrogen in dry rocky regions where no organic
materials are at their disposal, even could they utilize them. It is
usually stated that the carbon dioxide molecule is here split by means of
energy derived from the oxidation of nitrogen, but apart from the fact that
none of these processes can proceed until the temperature rises to the
minimum cardinal point, Engelmann's experiment shows that in the purple
bacteria rays are used other than those employed by green plants, and
especially ultra-red rays not seen in the spectrum, and we may probably
conclude that "dark rays"--_i.e._ rays not appearing in the visible
spectrum--are absorbed and employed by these a