and the causes affecting its recent diminution. 73
existed there, or be found only in an ill-developed or abnormal
condition. Now, owing partly to the trees in most parts of
the Forest being at present too crowded and grouped together,
partly to the large quantity of scrub and undergrowth
surrounding their stems, and partly to the frequent and
general pollarding of the hornbeams, neither sunlight nor
rain can find such immediate and direct access to their
trunks and branches as lichens essentially require for their
due evolution.6 But for these adverse influences we might
have expected that the spores or soredia of such species and
varieties as grow upon the old and felled trees would have
germinated and gradually developed into perfect plants on
the other trees growing in their neighbourhood. This,
however, generally speaking, is by no means the case, and
it is only on the more isolated trees, occurring in open
situations, exposed to sunshine, wind, and rain, that any
well-developed lichens are to be found.7 For though, as
already observed, the longest-lived of all vegetables, lichens
are at the same time so delicately constituted that, if the
atmospherical conditions under which they grow are in any
way interfered with, those already developed immediately
begin to decay, and those in the process of development
speedily perish. Hence, instead of finding the trunks of the
trees covered with these "creatures of light and air," we
usually find only protohyplne of species of Pyrenomycetes
and various algals, such as Protococcus—the "creatures of
damp and shade."8
6 Thus also is it that the birch, and more especially the holly, which
elsewhere yield a considerable number of species are here so destitute of
almost any trace of lichen-growth.
7 It is to be observed that, though lichens greedily imbibe water for
their nourishment, yet when this is not naturally and in due course dried
up by the sun and wind having free access to their substrata, it becomes
injurious to the life of such species as are not aquatic or semi-aquatic.
8 According to Schwendenerism, it is just in these circumstances when
we have the fungal mycelia ready to lay hold upon the algals, that
lichens would be found in various stages of evolution. Facts, however,
are in direct antagonism to this hypothesis; and so far from lichens
being thus manufactured, I have repeatedly observed in the Forest that
when the pro-mycelium of Pyrenomycetes comes into contact with and
invests in its meshes the Protococcus, the latter is speedily destroyed.