72 On the Lichen-Flora of Epping Forest,
quently it is only upon trees that have attained a very
considerable age that we can expect to find them fully
developed. Hence, when such trees are extensively destroyed,
the lichens growing upon them necessarily perish, and, for
reasons subsequently to be noticed, their spores, which may
have germinated elsewhere upon younger trees, may never
become developed into perfect plants. It is no doubt to
the wholesale destruction of these older trees, especially oak,
ash, and elm, that many of the species in Forster's herbarium,
and some of these the most interesting, had become extinct
previous to my earlier excursions to the Forest. That this
cause has been in active operation since then is entirely con-
firmed by my recent observations in all parts of the Forest.
More especially is this marked in the neighbourhood of High
Beach, which, in point of elevation, as also in other respects,
was (as it still is in a lesser degree) the best adapted for
lichen-growth. As the result of the felling of so many old trees
in the portion extending from the "King's Oak" to Copthall
and the "Wake Arms," and particularly of those which then
lined the roadsides, a large proportion of the lichens in the
above list has become either quite extinct or extremely rare.
It is true that there are still extensive clumps of old beeches
extant in many parts, but here, as generally elsewhere, the
beech is remarkably destitute of lichens—the most charac-
teristic, and indeed the only well-developed species here
growing upon it being Lecanora commoides.
Another cause of the diminution of the lichen-flora of the
Forest is the want of due access of light and moisture to the
existing trees. Lichens have very appropriately been termed
by Mr. Berkeley "the creatures of light," inasmuch as none
of them grow, or can grow, in dark places, while it is only a
few that are rightly developed even in shady places, under
special conditions. Their nourishment also is derived not,
as has been sometimes alleged, from the different substrata
upon which they occur, but exclusively from water—water
from the clouds, streams, lakes, or the sea, with the different
ingredients contained in it, and suited for their growth.
Hence where light and moisture cannot rightly penetrate,
lichens must either entirely die out, if they have previously