To a casual observer, it looks like a patch of old moss, which is what it is. But show one of the tiny fronds to a bryologist, and he will quiver with excitement. Bryologists are students of mosses - and thamnobryum angustifolium is so rare that the site near Bakewell in the Derbyshire Peaks is now closely protected.
How the plant came to exist here is a mystery. Bryologists believe it may have been there since the ice age, and possibly became separated from identical mosses that became extinct.
Its rarity has been known to botanists for a century, but it was thought to grow also in Ireland and on sea cliffs on Madeira. However, scientists have found that these other growths belong to similar but different species.
Once naturalists realised the Derbyshire specimen might be unique, they hurriedly added it to the list of plants protected by the 1981 Wildlife and Countryside Act. This means that anyone disturbing it can be fined heavily.
The moss covers an area of about three square metresat the mouth of an underground stream. Its colour varies from dark green to shades of brown and the length of its fronds also varies, the longest being about two inches.
Douglas Gilbert, a biologist and guardian of the moss, warns off rock-climbers from the site: 'There is an overhang where they used to practise directly above the moss. It wouldn't take many boots to destroy it for all time.'
There is also the danger that unscrupulous collectors might try to grab handfuls.
'We are aware of this risk,' says Mr Gilbert. 'But they would be wasting their time. I don't think it would grow away from this habitat. Also, it could prove expensive. The Act allows fines of pounds 2,000 a plant. Each frond is a separate plant and there would be hundreds in a handful.'