REPORT ON THE LICHENS OF EPPING FOREST. 91
to science only from examples gathered in Surrey two years
ago, we have since found in three distinct stations in Epping
Forest, though never in masses larger than a half-crown.
If a particular form of Lichen should happen to become
totally extinct within the Forest, there is but little chance of
its recolonizing the district, considering the distance of other
large tracts of woodland from the locality ; should, however,
certain forms be struggling on in the leprous condition, there
may, under the conditions now obtaining, be a recovery similar
to that which we have noted with regard to the ground forms.
Throughout the Forest there are, upon tree-trunks, in places
where conditions are unfavourable for vigorous lichen-growth,
powdery patches of a grey-green or yellowish colour, which
consist of algal cells, among which are scattered numerous
fungus hyphal threads. Some of these patches are probably
imperfect lichen-plants which have failed to develop normally
owing to the adverse conditions ; by the older lichenologists
they were classified with Lichens under distinct generic names,
as Lepraria and others. It is possible that lichens may persist
in this imperfect leprous form for long periods, and, should
more favourable conditions arise, then develop normally
and produce a perfect thallus with ripe apothecia ; but much
careful investigation is still needed in this connection before a
definite statement can be put forward. It is indeed by no means
certain that a lichen finds this leprous, rudimentary condition
a disadvantage to it in its peculiar mode of life. Its nutrition
would seem to be as well secured in this state as in its more
perfect one, after building up an elaborate thallus ; while,
as to reproduction, many lichens fail, except at rare intervals,
to produce fertile fruits, and trust to simpler, adventitious
methods of self-distribution. The most usual way in which
this is effected is by what are termed soredia, which are simply
small pulverulent masses of intermixed algal cells and fungal
hyphae produced, often in enormous abundance, upon the
upper surface, or at the extremities of the lobes of the thallus,
at spots where the cortex is ruptured, and which have the
potentiality of reproducing the lichen-thallus when blown by
the wind to suitable stations. Now, these soredia are but
localised patches of leprous thallus, and are the normal means
of self-reproduction in many lichens ; so that it is not