FUNGI AND BEECH
239
the Hartig net. All the evidence suggests that it is the
larger fungi that are concerned. Tracing mycelial connexion
between a mycorrhizal root and a fungus fruit-body is almost
impossible, since the hyphae are so fine and since the
sporophore may be some distance from the root. It is only
by pure culture experiments that the ability of a fungus
to enter into mycorrhizal relations with a tree can be
demonstrated. The eminent Swedish botanist Melin has
been responsible for most of the critical work in this con-
nexion. He raised "sterile" plants from surface-sterilised
seeds planted in sterilised soil. He took precautions to
exclude all extraneous micro-organisms. He then inoculated
the sterile soil with a fungal species suspected of forming
mycorrhiza. If typical fungus-roots were produced, then
the suspicion was proved to be correct. In this way he has
demonstrated that species of Boletus, Russula, Lactarius
and many more form mycorrhizae with conifers. However,
such critical methods have, apparently, not yet been applied
to beech, and, although we may suspect that species of
toadstools regularly found under beech are concerned in
mycorrhiza formation, the proof is lacking.
The physiological relationships of mycorrhiza still remain
obscure. It is not known how necessary is the association
either for the tree or for the fungus, nor is it known what
each derives from the other—it seems clear that forest trees
are healthy only if mycorrhiza is present, but whether its
presence is the cause or the effect of the health of trees is
as yet unproven.
Much work will be needed before it is established which
fungal species of the beechwood are truly mycorrhizal.
Such species are probably deriving something essential from
the tree, but in addition they are, no doubt, utilising food
from decaying beech leaves. It seems certain, however,
that some of the fungal species of the ground flora are
entirely litter decomposers with no ability to form
mycorrhiza with the trees.
It must be emphasised that the fungi as we see them in
a beechwood are only the fruit-bodies—the spore-producing
structures. The vegetative part—the feeding mycelium—
is concealed below the dead, fallen leaves. This is the more
permanent part of the fungus, and may live for many years,