Derbyshire feather-moss  (Thamnobryum angustifolium)

Classified as Critically Endangered in the UK, and Endangered by the IUCN Red List. Protected under Schedule 8 of the Wildlife and Countryside Act (as amended).

Although clearly related to the widespread moss Thamnobryum alopecurum, this species has more distinctive narrower leaves with very coarsely-toothed tips. Like its relative, the species has a tree-like structure, but its stems are more slender and their branches are further apart.

This moss is probably endemic to Britain, with the only known specimens being found in Derbyshire. Its location is kept secret.

Derbyshire feather moss is found on vertical limestone rock faces, which are damp, and shaded. It also grows under water.

Mosses are an ancient group of plants, and often the first to colonise a bare surface. They do not produce flowers or seeds, but usually reproduce vegetatively or by developing capsules, the fruiting bodies which contain spores. Neither do they have roots. They maintain their footholds with rhizoids, with which they anchor themselves to rock or the ground. Derbyshire feather-moss has never been observed to produce fruiting bodies. When growing underwater it forms dark green clumps, but as the water recedes in the summer months the moss desiccates, and the stems can turn a reddish-brown. Limestone encrustations, which form on the underside of the plant, are particularly apparent at this time of year.

The chief threat to this moss is deterioration in the quality of the water within its only native area.

Derbyshire feather moss is included in English Nature's Species Recovery Programme (SRP). Because of its rarity and the risk from unscrupulous collectors, its main locations are kept secret. Anyone wishing to see it must contact the site manager first. This may seem like an extreme measure, but with a species this rare, and comprising the only known populations in the world, it is dangerous to take risks.

In order to safeguard the future of this moss, not only is it vital to protect the sites where it occurs in the wild, but attempts are also being made to propagate the moss ex-situ. This will enable possible re-introductions back into the wild, and it also offers an insurance against extinction. To loose any species, even something as easily overlooked as a moss, would further impoverish our natural world.

• Capsules: in mosses and liverworts, the spore-bearing structure, held aloft on a stalk called a seta. Capsules have a variety of shapes. Most moss capsules have a mouth, which is covered by a lid until the spores become ripe and the lid falls off, revealing a single or double ring of teeth, known as the ‘peristome’. The spores are released and dispersed in the wind. In liverworts the capsules do not have lids; when the spores are ripe the capsule splits into four, releasing the spores.
• Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
• Ex-situ: measures to conserve a species or habitat that occur outside of the natural range of the species. E.g. in zoos or botanical gardens.
• Rhizoids: thread-like structures that help to anchor the plant to the substrate, and absorb minerals and water. In liverworts they consist of a single cell, in mosses they are multi-cellular.
• Spores: microscopic particles involved in both dispersal and reproduction. They comprise a single or group of unspecialised cells and do not contain an embryo, as do seeds.
• Vegetative reproduction: type of asexual reproduction (reproduction without recombination of genetic material) that results in the propagation of plants using only the vegetative tissues such as leaves or stems. The resulting plant is genetically identical to the original plant. A well-known example of this is the reproduction of strawberry plants from 'runners'.