Walk into a damp forest on a dark night, switch off your light and wait. At first, you may see nothing. Then a faint green glow appears along a fallen tree trunk, or tiny mushroom caps emerge among the leaves like a handful of little lamps.
Light without heat
This phenomenon is called bioluminescence. Several species of fungi produce light through a chemical reaction in their cells, belonging to the same broad family of reactions that makes fireflies glow. Unlike fireflies, however, fungi do not flash: their light is steady and faint, often coming both from the fine threads running through decaying wood and from the mushrooms themselves.
People have observed this phenomenon for centuries. In English it has long been known as “foxfire”, an apt name for a glow often glimpsed only out of the corner of your eye.
Scientists are still investigating why some fungi glow. One theory is that the light attracts insects that help spread spores; another is that it is simply a by-product of the fungi's chemistry. For now, the forest still holds some secrets.
How to see it for yourself
Rain helps: the glow is usually easier to see when the wood and ground are wet. Choose a night with little or no moonlight, stay away from streetlights and give your eyes plenty of time to adjust to the darkness – longer than you might expect. If you need light to find your way, a dim red light helps preserve your night vision.
Look, but do not pick. Glowing fungi are part of the slow process that turns dead wood into soil, and some fungi are poisonous. Stay on the path, go with someone who knows the area and leave everything as you found it.
Then switch off your torch and wait. The forest is brighter than it looks.
