FIELD PHENOMENA DUE TO MICROSCOPIC ORGANISMS. 19
form balls up to 3" or more in diameter. When the water is
particularly clear it is probable that the effect of sunlight upon
the activities of the plants is so strong that oxygen is produced
in sufficient quantity to form temporary bubbles, which remain
entangled among the algal threads and so render the balls of
"sponge-weed" buoyant.
In the foregoing cases of microscopic organisms accumulating
at the surface, the total amount of material involved has been
considerable, but there are also cases in which only a mere film
or pellicle is formed, although still producing a noticeable effect.
Naturally such films are usually only found in small and sheltered
pieces of water. They may be green, when they are usually due
to some species of Euglena or Chlamydomonas in what is known
as the "palmella" state, or red, when they may be due to
Euglena haematodes, or iridescent, when they may be due to
bacteria in the "zoogloea" condition, or golden, when they are
due to a very small Chrysomonad, Chromulina Rosanoffii.
Yet another category of aquatic phenomena is that in which
striking colour effects are sometimes brought about by the
abundant growth of microscopic organisms on the bottom and
sides of ponds and streams. Such appearances can, of course,
only become effective when the water is clear and not very deep.
A good example of this type of effect is the rich velvety brown
coating which is often to be seen on the stones, etc., at the
bottom and sides of gently running streams, margins of ponds,
etc. This is caused by a dense growth of Diatoms, usually of many
different kinds, living together, including species of Navicula,
Nitzschia, Surirella, Synedra, etc. But more frequently the
colour will be green and then it is most frequently caused by the
luxuriant development of one or a few species of green or blue-
green algae. Mr. Harris tells me that in shallow clear streams, in
some parts of Devonshire, Schizochlamys gelatinosa paints the beds
a vivid green, and I have seen a somewhat similar effect in a little
stream in Wanstead Park produced by an undetermined species
of Chlamydomonas. Very dark, almost blackish green, effects
of this character are usually due to species of the blue-green alga
Oscillatoria or an allied type. A white coating of the bottom of
small stagnant ponds may be due to the development of the
ciliated Infusorian Spirostomum ambiguum in great numbers,
and a white edging or coating of decaying leaves, sticks, etc., at