118 THE ESSEX NATURALIST
neutrality in which neither organism is sufficiently strong to take
advantage of its partner.
Very roughly, we may divide the mycorrhiza into those in
which the fungus lives within the tissues of the root—the endo-
tropic type—and those which form a sort of felt about the
outside, the ectotrophic type. The latter are found on the
short roots and are common in the Fagaceae and Betulaceae,
in Willows and Poplars and in members of the Abietineae :
the fungi are mostly Hymenomycetes—some of the common
woodland fungi. Except on the most fertile soils this fungal
association appears to be essential for some trees and exotic
trees which fail to establish themselves may be suffering from
lack of the appropriate mycorrhizal fungus. We have much more
to learn of this relationship. Although it is not yet known if
mycorrhiza are common to all trees, we know that under very
favourable conditions trees can thrive without these fungi, but
there is no doubt that the mycorrhizal habit is very widespread
among forest trees: for the successful growth of many such trees
the mycorrhizal association is often essential and it is interesting
to speculate whether, in many instances, trees are not dual
organisms—comparable to lichens—depending for their satis-
factory growth on their association with fungi.
Time will not allow of a more extended exploration or we
might examine other ways in which the tree is adapted to its
environment. There is the problem of the ascent of water and
mineral salts from the soil to the leaves. At one time it was
thought that the limits of height had been reached and that taller
trees were mechanically impossible because water would be unable
to rise to the leaves : it is now suggested that this problem might
not become serious until trees had attained a height approximating
to that of Ben Nevis (4,406 feet); the estimates of height made
by the most enthusiastic dendrologists are still somewhat below
this figure. There is the problem of the transport of food
material and its storage and to mention but one of the many
others, the variation of individuals in a species, permitting the
species to adapt itself to an ever-widening range. But enough
has been said at least to indicate that the tree is a very wonderful
organism, beset with numerous and difficult problems which
it has solved to permit of the satisfactory continuance of the race.
LITERATURE
No attempt has heen made to give a complete bibliography, but the books
and papers in the following list may be consulted for fuller details of the
matters dealt with here.
Boulton. E. H. B. and Jay, B. A. 1946. British Timbers. London.
Boyce, J. S. 1938. Forest Pathology. New York.