44
THE ESSEX NATURALIST.
woodlands may be sufficient to account for its scarcity;
its intolerance to higher temperatures may in part account
for the fact that it is less troublesome in the United
States where summer temperatures are higher, and where,
thanks to central heating, the buildings are warmer in winter
than are those of northern Europe. Merulius can only start
an infection in wood provided there is enough moisture for
its spores to germinate; other conditions, such as oxygen
and temperature, play their part as well. Once established in
the timber the hyphae begin to break down the cell walls,
producing in the incipent stages a slight discoloration, but
later a cubical brown rot: during the breakdown of the cell
walls a good deal of water is formed as a bye product and this,
of course, ensures that the fungus has an adequate supply of
moisture; in fact, once established, the fungus is able to attack
quite dry timber. Apart from the hyphae which break down the
wood, there are others which run over the surface of the wood
as greyish white strings; these are composed of three types of
hyphae, hyphae similar to those which occur in the wood, thick-
walled hyphae whose function is mechanical, and long hyphae
whose function is that of conduction. By means of these
strings the. fungus can spread, for they can travel over concrete
and steel girders, or even through mortar and the like, and
bring about fresh infections at a considerable distance from the
original one. Poria vaporaria attacks timber in the same way
as Merulius, but it possesses less specialised rhizomorphs and
can spread over dry timber only to a very limited extent.
Its conducting hyphae are often thinner than those of Merulius,
and white in colour, and owing to the fibre-like hyphae which they
contain are tough and flexible when dry, not brittle as are those
of Merulius; this contrast serves as a valuable aid to identifi-
cation in the absence of sporophores, as also do the white masses
of mycelium, which differ from those of Merulius (which, although
at first whitish may become yellowish) in retaining their white
colour. Coniophora terebella Pers., is another frequent cause of
dry rot in buildings, but requires more moisture than those
already mentioned; it breaks down the wood in a similar
manner. The presence of Merulius in timber may render it
liable to infection by Coniophora, thanks to the moisture which
Merulius secretes. Moreover, it has been shown, although