SOME FUNGI IN WOOD. 33
attack wood are concerned, it is impossible to draw a sharp distinc-
tion between parasites and saprophytes : it is a nice point, whether
a fungus living in the heartwood of a living tree is a parasite
or saprophyte, for the heartwood, even in a live tree, is dead.
The situation of suitable food supplies in a tree will obviously
determine which part of the wood becomes infected : sap stain
fungi, by reason of the food they require, are naturally limited
more or less to the sapwood, and mainly to the rays, since it is
these cells chiefly which store food materials. Further, although
the food supply may be adequate, it may not be available because
of the presence of toxic substances, or, at least, of substances
which inhibit fungus growth; among such substances may be
included tannins, resin and some oils, and possibly other sub-
stances, like alkaloids. There are more of these substances in
the heartwood than in the sapwood, hence it will be more
protected in this respect from the attacks of fungi. In Oak,
Cartwright and Findlay [8] demonstrated that the sapwood is
more readily attacked than the heartwood, and here there can
be little doubt that tannin is responsible for the increased
immunity of the heartwood: Lutz [24, 25] has shown that some
fungi, like Stereum hirsutum and Polyporus betulinus, will grow-
on oak heartwood after most of the tannin has been removed.
Nevertheless, in some fungi which normally grow on heartwood
of oak, growth in culture is better if the substratum contains a
certain amount of tannin. Cartwright [5] found that the
growth of Fistulina hepatica, a cubical brown rot of oak heart-
wood, was slightly stimulated by the addition of a little tannin
to the culture medium, while in Polyporus sulphureus, which
produces a similar type of rot in oak heartwood, the stimulation
was more marked. In contrast the growth of Polystictus versi-
color, which mostly attacks sapwood, was reduced to about a
third, and that of Daedalea quercina, which attacks oak sapwood
much more rapidly than the heartwood, was reduced nearly as
much; in Merulius lacrymans, a dry rot fungus, addition of
tannin brought about a cessation of growth. In Fistulina
hepatica especially, Cartwright suggests the possibility that the
tannin may be broken down to provide the fungus with sugars.
Fungi are unable to develop in dry wood, and their growth
is stopped, except in very rare instances, if the wood which they
have invaded becomes dry. The minimum (critical) moisture