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Q: During a nocturnal walk in the woods of the Belgian Ardennes, I found a
piece of rotten wood. To my surprise, I saw a soft glow inside. As I write
this, after 24 hours in darkness, the wood is still glowing. What causes
this?

A: This type of glow was noted by Aristotle more than 2000 years ago. It is
a bioluminescence caused by a woodrotting fungus and is kin to the light of
the glow-worm, firefly and lantern fish. The mechanism is the oxidation of
luciferin, a light-emitting compound, mediated by the enzyme luciferase.

Many microorganisms luminesce, including the vegetative filaments of fungi.
One of the most common and spectacular is the honey fungus toadstool
(Armillaria mellea). It is a serious and destructive parasite of many trees
and is probably what the questioner saw. The infested wood, and the straplike
strands which spread the fungus beneath the bark, glow with a greenish light.
A closely related North American toadstool (Clitocybe illudens) is named jack-
o’-lantern for the same reason.

A: The honey fungus is a very common parasitic mushroom on trees and woody
shrubs. It spreads by root-like growths called rhizomorphs. The tips of the
rhizomorphs and the mycelium of the fungus are luminescent. Soldiers in the
trenches during the First World War used to attach pieces of rotting wood to
their helmets to avoid colliding in the dark, and in the Second World War the
fire watchers in timber yards covered brightly glowing wood with tarpaulins
for fear that enemy aircraft would spot it.

The light emitted by bioluminescence will probably vary in strength over
time; keeping the wood moist (but not saturated) and at a temperature between
10° and 25° C should maximise the out-put. There is no known
biological benefit for luminescence in fungi; perhaps the light attracts
invertebrates that then eat the smaller invertebrates grazing on the fungal
mycelium.