Switch off your headlamp on the right kind of night, in the right kind of wet, decaying woodland, and wait for your eyes to adjust. On a fallen branch, or salting the leaf litter, you may see it: a faint, steady, greenish light, coming from nothing that is burning. No heat. No flame. Just a cold glow leaking out of dead wood, as if the forest floor had its own dim constellation. People have a name for it older than the word "biology": foxfire.
This isn't a trick of tired eyes, and it isn't stored-up daylight bleeding back out. It's a living chemical reaction — an actual fungus, manufacturing light. Established And it has been quietly astonishing people for a very long time.
A light older than the science
The oldest surviving note about foxfire is Aristotle's, around 382 BC: he described a light coming from decaying matter that, unlike fire, was cold to the touch. Established Pliny the Elder later wrote of wood that glowed in olive groves. For centuries it was a beautiful mystery — folk-named "fairy fire," lumped in with will-o'-the-wisps and other things that glimmer at the edge of the dark. Folklore It was even pressed into service: the crew of the Turtle, a hand-cranked submarine from the American Revolution, used a smear of foxfire to light the compass and depth gauge in the pitch-black hull. (A nice legend credits Benjamin Franklin with the idea; the surviving letters suggest he was only consulted later, when the cold made the foxfire stop glowing.) Established
The actual culprit wasn't pinned down until 1823, when someone examined the glowing timbers propping up a mine and traced the light not to the wood but to the fungus growing through it. Established The mushrooms, it turned out, were the lamp.
How many, and what colour
Glowing is not some universal fungal trick — it's a rare and scattered one. Only around 130 species of fungi are known to be bioluminescent, a count that has roughly doubled in the last fifteen years as people have gone looking with better cameras. Established They aren't a single family; the talent crops up in several separate lineages of gilled mushrooms, including the honey fungus (Armillaria), the jack-o'-lantern and ghost fungi (Omphalotus), and the bitter little Panellus stipticus that lights up logs across eastern North America. Established
And here's a small, telling detail: they nearly all glow the same colour. Across these unrelated fungi, the light lands in a narrow band of green, peaking around 520 to 530 nanometres. Established When biology converges on one answer from many directions, it's usually a hint that the answer matters — a thread we'll pick up at the end.
The lamp, taken apart
The light works the same way a firefly's does, with different parts: a small molecule called a luciferin, and an enzyme called a luciferase that burns it. Established In fungi, the luciferin is a compound named 3-hydroxyhispidin, and — this is the lovely part — it's built from caffeic acid, one of the most ordinary molecules in the plant world, a building block of the very wood these fungi are busy eating. The luciferase grabs the luciferin, oxygen joins in, and the molecule offloads its spare energy as a single photon of green light. No heat wasted. Cold fire, exactly as Aristotle said.
For most of those 2,000 years this was a black box. Then in 2018 a team worked out the entire chemical loop — every enzyme that builds the luciferin from caffeic acid, burns it for light, and recycles it to start again. Established It was the first complete bioluminescence pathway ever fully described in any eukaryote (the big-celled branch of life that includes fungi, plants, and us). And they proved they had it right in the most charming way possible: they moved the fungal genes into ordinary yeast, and then into plants — and the yeast glowed, and the plants glowed, lit from within by a borrowed mushroom. Established
So why do they do it?
Here is where wonder and honesty have to share a sentence. We can explain, molecule by molecule, how a mushroom makes light. Why it bothers — what the glow is for — is a younger and shakier question.
The best answer we have comes from a 2015 study of Neonothopanus gardneri, a fungus that lights up the coconut groves of northern Brazil so vividly the locals call it flor-de-coco, the coconut flower. Established The researchers found two things. First, the glow isn't constant: it's governed by a circadian clock, an internal timer that ramps the light up at night and damps it by day — the fungus is choosing when to shine. Second, they set out fake mushrooms made of acrylic resin and lit them from inside with green LEDs. The lit decoys pulled in rove beetles, flies, wasps and ants in far greater numbers than identical dark ones. Established The implication is elegant: the light is an advertisement, drawing in insects that then blunder off carrying the fungus's spores — a glowing version of what a flower does with colour and scent.
Where we get honest
That's a satisfying story, and it's tempting to stamp it onto every glowing fungus in the world. We shouldn't — because that clean result came from one species, and the next species pushed back.
When another team ran much the same experiment on the Australian ghost fungus, Omphalotus nidiformis, its glow attracted no more insects than the dark controls did. Contested The spore-advertising idea simply didn't hold there. And it's an awkward fit for a lot of luminous fungi anyway: many do their brightest glowing in the mycelium — the threads woven deep through rotting wood and soil, where there's no night sky to signal against and few flying insects to court. A lantern buried inside a log is hard to explain as a billboard.
So the honest state of play is this: "fungi glow to attract spore-carriers" is well supported in at least one species and genuinely unsettled as a universal rule. Contested The light may do different jobs in different fungi — an insect lure here, a warning to hungry grazers there, and in some cases perhaps no job at all, just an unavoidable flicker thrown off by the chemistry of breaking down wood, light for its own sake. We don't yet know which. And there's something fitting about that: a whole kingdom kept a private source of light burning for reasons we still can't fully read — which is a better story than a solved one.
Next in the Bestiary: a fungus that doesn't flee radiation but grows toward it — the black molds thriving inside the Chernobyl reactor, and the honest limits of what we know about how. See you there.