THE LIVING TREE 117
The cork forms a watertight and gastight layer and while it
effectively prevents the ingress of materials it also effectively stops
their egress and thus prevents gaseous exchange! This difficulty
is obviated by the provision of areas filled with loose cork,
through which gases may readily pass : these lenticels are usually
quite small, although they form conspicuous horizontal lines
sometimes several inches long, in such trees as Cherry and Birch.
The tree must also solve another problem, that of dealing with
its old dead wood, for only the peripheral wood appears to be of
physiological use to the tree and the older dead wood—heart-
wood—is probably non-functional, although it has been suggested
that it may serve as a water reservoir. Composed, as it is, to a
large extent of cellulose, the wood furnishes an attraction to
some types of fungi and is sometimes attacked, so that the tree
suffers from heartrot. But the heartwood forms a convenient
repository for waste materials of the tree's metabolism and sub-
stances like gum, tannins and various colouring matters are often
deposited there so that the heartwood is frequently darker than
the sapwood : some of these substances, like tannins, appear
to be inimical to most fungi and consequently afford a measure
of protection to the tree.
Some of the parasites which prey upon the wood enter the
tree through wounds and wounds are common enough in trees,
even if we exclude those produced by external causes. Thus the
dead, shaded branches eventually rot and fall off and leave
wounds which are soon covered by fungal spores. Wounds,
however, are covered up by the tree, again by cork, for a cork
layer—callus—gradually encroaches on the exposed surface
and finally, unless it is very large, seals it off. Similarly dead
branch stubs are gradually engulfed by the increasing growth in
girth and as we have seen, scars left by dead and decayed roots are
also effectively covered by cork. Very frequently copious produc-
tion of gum or resin occurs in the region of a wound and these
substances may act in the manner of antiseptics and retard, if they
do not stop, the development of fungal parasites. In some trees
some of the unwanted wood is shed, and it is recorded that branches
over two inches in diameter are shed in the Kauri (Agathis).
Although the living tree is beset by a host of predatory fungi,
its roots may form an association with certain fungi to the mutual
advantage of both tree and fungus. This fungus-root association
—mycorrhiza—has been the subject of one presidential address
to our Club, and need not detain us long here. It is an excellent
example of the phenomenon of symbiosis, but the harmonious
relationship between tree and fungus is scarcely a suitable example
for the moralist. There is no abnegation of the individual, tree
or fungus, to the common good, but rather a state of armed