34
THE ESSEX NATURALIST.
content necessary for fungal decay of wood is given by Koehler
[22] as 20% of the oven-dry weight. In this country well
seasoned timber would have a moisture content of about 15%,
probably nearer 20% in winter, so that it might be regarded as
just about immune from fungal attack, but in damp places and
out of doors the moisture content would rise well above the
critical figure. This critical figure varies for different fungi;
thus of two dry rot fungi Merulius lacrymans requires less water
for its growth than does Coniophora cerebella: in general the sap
stain fungi require more moisture than the rots.
Growth of any fungus proceeds most rapidly at a particular
temperature. Temperatures of between 21° C. and 26° C. may be
regarded as the most favourable in this respect. High tempera-
tures are more dangerous than low ones, for, although the latter
may inhibit growth, they do not necessarily cause death. The
effects of temperature on the rate of growth of certain wood
rotting fungi have been studied by Cartwright and Findlay [7];
they worked with cultures, so that the fungi were not studied
under natural conditions, but there seems no reason to assume
that their results do not indicate a different reaction to tempera-
tures in the fungi which they studied. In Polyporus sulphureus
the optimum temperature was about 30° C, and there was no
growth at 35° C, whereas in Polyporus dryadeus a white butt
rot of oak, the optimum was about 25° C, and at this temperature
growth was about four times as rapid as at 9° C. or 30° C. In
Merulius lacrymans, which does not occur in the tropics, growth
was best at 23° C, and there was no growth at 30° C, while in
Schizophyllum commune, a fungus common in tropical timbers,
the optimum temperature was between 30° and 35° C, and the
maximum between 42° and 44° C. A warm, moist climate is
specially favourable to fungus growth.
A sufficient supply of oxygen is normally available, so that
oxygen is not likely to be a limiting factor in the growth of wood
inhabiting fungi. By reason of the deficiency of oxygen, how-
ever, submerged timber is immune from fungus attack ; when the
lack of oxygen is borne in mind the remarkable durability of
wooden piles is less surprising. Koehler [22] states that in the
living tree the heartwood seems more prone to fungal attack than
the sapwood, and explains this as due to the presence of air in the
heartwood, while the cells of the sapwood are full of water, and, as