20
THE ESSEX NATURALIST.
the bottom of a pond is sometimes caused by certain filamentous
bacteria.
I have now completed my survey of those kinds of phenomena
due to microscopic organisms which can be spoken of as phe-
nomena in the literal sense, for they have all been effects which
have appealed to our sense of vision. But before taking leave
of this subject it ought to be noted that microscopic organisms
can also produce effects in the field which appeal to our sense of
smell. Apart from the very unpleasant sulphuretted hydrogen
produced by the micro-organisms of putrefaction, quite noticeable
fishy odours may be generated by such forms as Volvox, Eudorina,
Uroglena, Peridinium, etc., when present in the water in great
quantity. The blue-green algae generally are characterised by a
peculiar rather unpleasant grassy or earthy smell, but some other
forms, such as many Diatoms, give off more sweet-smelling or
even aromatic odours.
In conclusion, I feel I ought to apologise for having had to
refer to such a number of organisms whose names can be familiar
to only a few present, but I hope I have at least been able to
make it evident to all that many phenomena which we meet with
on our field excursions are only to be understood by a study of
their microscopic producers.
THE BIRDS OF THE COLNE VALLEY IN
1925 AND 1926.
By W. E. GLEGG, F.Z.S., M.B.O.U.
(With One Plate.)
THE present contribution is the fourth of what, it is hoped,
will be a complete series of papers on the birds of the
Essex estuarial rivers, articles dealing with the Crouch, Blackwater
and Stour having already appeared in this journal. The Colne
Valley possesses those characteristics which one generally
associates with the coast of Essex, that is, wide stretches of
marshy land, deeply indented by creeks, broken up by a net-work
of fleets, and enclosed by many miles of twisting and doubling
sea-wall. The course of the estuary is much shorter than the
others of Essex, as it may be said to start at that curious jumble