6
THE ESSEX NATURALIST.
in my mind the various cases in which more or less striking phe-
nomena noticeable in the field were entirely due to microscopic
plants and animals. I found that the number of such cases was
more considerable than might have been anticipated, and that
very many of them were to be observed in our own immediate
neighbourhood. Here then was a subject which seemed to
combine on the one hand an obvious appeal to members of a
Field Club, and on the other the possibility for me to stick to my
own particular last. And thus it comes about that I want to try
to interest you for a little while in some of what I have called in
the title of this address "Field phenomena," which can be shown
to be brought about by organisms only to be determined by the
aid of the microscope.
If we look at the matter from an ecological point of view there
are obviously two principal classes of such phenomena, namely,
those which are sub-aerial, like the "Red Snow" already referred
to, and those which are aquatic, occurring in the waters of lakes,
ponds, streams, etc.
The commonest, most constant and most striking of the
sub-aerial phenomena is no doubt the green powdery incrustation
which occurs on the trunks and branches of trees, wooden palings,
etc., and particularly on trees in woods. I have used the word
"striking" as applicable to this green growth on trees, and I think
with justification, for if it were not so familiar an object it would
at once arrest our attention. Even as it is one cannot but be
startled sometimes by the intensely vivid glowing radiance of
certain patches of this green incrustation when illuminated by a
shaft of sunlight. In any case it is certain that our woodland
trees, especially when leafless, would lose much of their charm
and picturesqueness if it were not for the phenomenon in question.
But its very commonness seems to have the effect of dulling our
curiosity and even those who have had some botanical training
and who can tell readily enough that the phenomenon is produced
by a microscopic alga, do not, as a rule, push their enquiries
further. Nevertheless, there are many most interesting problems
connected with this green coating of trees, as I shall now endeavour
to show.
In the first place it must be observed that there are two
distinct types of green incrustation, often occurring side by side
or even intermixed on the same trunk, one of a pure green and