Crouch by the mouth of a cave, or peer into the hollow left under the roots of a fallen tree, and now and then the darkness glows quietly back at you: a cool green-gold light, like a cat's eye caught in a torch beam. Yet there is no torch. The glimmer comes from a moss, Schistostega pennata, the luminous moss, and the soft light it throws from the dimmest of corners has seeded folk tales right across the northern world.
A light from almost no light
First the wonder and the let-down together: the moss makes no light of its own. It neither burns nor glows in the chemical way a firefly does, but instead gathers the faint light reaching its shaded den and bounces it back at whoever is looking. The secret lies in its youngest stage, the protonema, the green thread every moss sends out from a germinating spore. In most mosses that thread is a plain tangle of filaments; in Schistostega some of its cells swell into rounded, almost spherical beads, each behaving like a minute lens. Dim light entering a bead is focused onto a cluster of chloroplasts at the back wall, which soak up the red and blue for photosynthesis and bounce the leftover green straight back out the way it came, which is just why the returning light reads green. An eye sitting in its path catches the glow; shift your head a hand's width and it blinks out, since you have stepped clear of the narrow cone the cells throw back. It is the very principle that fires up a cat's eyes or a roadside reflector, here pressed into service by a plant scratching a living from the last scraps of daylight.
Where it hides
Schistostega seeks out exactly the places other plants shun: deep shade, unbroken damp and air that scarcely stirs. Look for it in the cool backs of caves and rock overhangs, in the gaps between sandstone boulders, in old mine adits and the mouths of badger setts and rabbit burrows, and on the raw soil clinging to the upended root-plate of a wind-thrown tree. It holds to acid, lime-free ground, sandstone and gritty soil rather than chalk, and to the cooler temperate and boreal reaches of the Northern Hemisphere. The light it asks for is barely more than a rumour of day, far less than any flowering plant could turn to account, which is just how it keeps these black recesses to itself.
Goblin gold and dragon's gold
A wink of gold from a dark hole, gone the moment you reach for it, was never going to slip past the folk imagination. Across Europe the moss picked up names like goblin gold, dragon's gold and elf-gold, and more than one hopeful soul is said to have dug into a bank after buried treasure only to come away with a fistful of damp earth. In Japan it is hikarigoke, the shining moss, and at several caves and hollows where it gathers in quantity it is guarded as a Natural Monument, visited for the pale fire it shows in the gloom. People have always loaded moss with meanings out of all proportion to its size, a habit picked over in moss myths and old beliefs.
The fern that comes after
The glow belongs to the protonema, the seedling stage; the moss proper, the leafy plant that grows on from it, is a far quieter affair, easily walked past once you stop hunting the light. Its shoots are small and fragile, a translucent pale green, the leaves set in two neat rows along flattened stems so that each reads like a miniature fern frond or a single soft feather. The species name, pennata, means feathered, for just that look. These little fronds stand barely a centimetre high and carry no glow at all, so the plant that gives the species its name turns out to be the least striking thing about it. To get the two-stage life behind all this straight, the green thread first and the leafy plant second, the groundwork waits in what a moss actually is.
Look, and leave it where it lies
Seeing luminous moss for yourself takes patience more than effort. Find promising ground, a sunken lane cut through sandstone, the back of a cave, the underside of a tipped-up root-ball in damp woodland, then get your eye down low, let it adjust to the dark, and move slowly until the green spark answers. A torch held close beside your temple, angled to throw light back toward your own face, will often coax it out. The one thing to resist is digging: prise the moss from its niche and you wreck the exact stillness and gloom it leans on, and it will not take hold again on an open bank. This is a plant to be visited rather than collected, in the spirit of the field manners gathered in collecting moss responsibly.