FIELD PHENOMENA DUE TO MICROSCOPIC ORGANISMS. 11
fungus, so that their pure green colour is blended with the white
of the hyphal threads, thus producing the characteristic appear-
ance referred to, On the whole it appears that Trcbouxia, when
forming part of the lichen, can survive even drier conditions than
Pleurococcus, for, so far as I have been able to see any difference
in position, the grey-green incrustation occurs more frequently
on the upper surfaces of inclined trunks and branches, whereas
Pleurococcus is more frequent on vertical or slightly overhanging
surfaces. This greater tolerance of desiccation, however, pretty
certainly does not reside in the Trebouxia plant itself, but is due
to the protection afforded by the envelope of hyphal threads.
This, no doubt, is one of the benefits it receives in compensation
for its imprisonment by the fungus.
Next to the green incrustations on trees, etc., the most
noticeable sub-aerial field phenomena, due to microscopic or-
ganisms, are probably the coloured patches which are sometimes
found on the ground or on stones and rocks. These may be of
various colours, the commonest being different shades of green,
but shades of orange, red, purple and black also occur.
As regards the green patches found on bare ground it must be
remarked that many, perhaps most, of these are caused by the
thread-like protonema of mosses. I do not propose, however,
to say anything about these. Of the strictly microscopic or-
ganisms producing green coloration of the ground the most
frequent is probably the filamentous alga Hormidium flaccidum
in one or other of its various forms. Plants belonging to the
genus Hormidium are very closely allied to the aquatic genus
Ulothrix, but most of them have taken to a sub-aerial mode of
life as we have already seen to be the case in Stichococcus, another
near relative of Ulothrix. They are, however, decidedly more
terrestrial in habit than species of Stichococcus, and have not
entirely given up reproduction by means of aplanospores, and
even by zoospores, as both Pleurococcus and Stichococcus appear
to have done. On Leyton Flats and other parts of the Epping
Forest area which remain somewhat water-logged during the
autumn and winter, considerable patches of Hormidium flaccidum
are to be found in the early part of the year. Later these become
dried up, and no doubt all the cells in many of the filaments, and
perhaps a good many in the others are killed, but the aplanospores
and some cells survive and start the plant again as soon as the