156 THE ESSEX NATURALIST.
In company with C. Okenii were numbers of a large Spirillum
(probably Thiospirillum jenense or T. sanguineum), another
of the sulphur bacteria, but, although of a yellowish red colour,
these could have had very little, if any, effect upon the general
coloration of the water.
In a short time after collection, the organisms, which in
the pond had been generally distributed through the water,
sank to the bottom of the bottle, forming an intensely red-purple
mass. When the pond was visited six days later there was no
coloration of the water, but C. Okenii was fairly common in the
bottom sediment.
One or two visits were made to the pond during the spring
and summer of 1925 without any trace of red coloration being
noticed, but on the 13th September, exactly one year after
the red coloration was first seen, the southern end of the pond,
where not covered with duckweed, was again found to be red.
On this occasion, however, the colour was only a dull red and
not very striking, although a number of partially submerged
floating leaves were seen to be covered with a layer of the red
material of considerable intensity. The red colour was found
to be caused, as before, by great numbers of Chromatium Okenii
with which were associated a certain number of Thiospirillum.
The occurrence of water turned into "blood" has frequently
been recorded, from the time of the first Egyptian plague to the
present day. There is no doubt that in most cases the colour
has been due to living microscopic organisms, but these may
have been of several different types. The commonest cause
of the red colour is probably one or other of several species of
Euglena, e.g. E. sanguinea, E. haematodes, &c., which have the
power of developing the colouring matter carotin under certain
conditions. Red-coloured water can also be brought about
by forms of sulphur bacteria other than Chromatium, especially
by Lamprocystis roseopersicina and Thiospirillum sanguineum.
An alga, Botryococcus Braunii, has also been recorded as produc-
ing a red-coloured water in the late summer, and the same effect
is sometimes due to the presence in great numbers of reddish
coloured water-fleas, usually of the genus Daphnia.
The two principal conditions necessary for the development
of red sulphur bacteria appear to be a stagnant water rich in
sulphuretted hydrogen, and good illumination. In this latter