Tiny fern-moss (Fissidens exiguus)

Kingdom	Plantae
Phylum	Bryophyta
Class	Bryopsida
Order	Fissidentales
Family	Fissidentaceae
Genus	Fissidens (1)
Size	Height: 1.5-2.5 mm (2)

Classified as Near Threatened in Great Britain (3), and receives general protection under Schedule 8 of the Wildlife and Countryside Act 1981 (2). It is classified as Rare in Europe (4).

As the common name suggests, the tiny fern-moss is very small, measuring between 1.5 and 2.5 mm in size (2). Its identification requires detailed microscopic study, and there are doubts as to its species status (4).

This species was originally identified in North America, and was not accepted to occur in Europe until 1953 (4). It is rare in Britain, occurring in the Weald in East Sussex and Kent, with very isolated populations in Somerset, Powys, south Devon, Warwickshire and Cumbria (2). In Europe, this moss has become extinct in Denmark and Germany (3). If tiny fern-moss is found to be a species, its conservation in Great Britain will be of international importance; the 16 known sites in England represent most of the European population (4).

Inhabits sandstone rock close to the water level, and shallow water in streams and rivers (3).

Mosses, hornworts and liverworts form a group of simple plants called bryophytes (5). Bryophytes lack many of the more complex structures of the higher plants, such as a vascular system, and flowers. They do not have roots, instead they have structures called ‘rhizoids’ which absorb water and anchor the plant to the substrate. All bryophytes have an interesting life cycle consisting of two main parts, called the gametophyte and sporophyte generations (6). Plants in the gametophyte stage can reproduce sexually. Male organs (antheridia) produce male sex cells, which actually move to the female sex organs (archegonia) (6) through water droplets (7). Fertilisation occurs and a plant develops called a ‘sporophyte’, which remains attached to the plant. The sporophyte releases spores from within a capsule; the spores disperse and develop into a new gametophyte stage plant (6).

Although the precise threats facing this species are not fully understood, the following are thought to be factors: eutrophication and other types of water pollution, and decreased shading caused by tree removal (3).

As a UK Biodiversity Action Plan (UK BAP) priority species, the tiny fern-moss has a Species Action Plan, which guides its conservation. This plan aims to maintain all current populations and set up ex-situ populations before 2005 (3). This moss is included in English Nature’s Species Recovery Programme, which has part-funded research and survey work in conjunction with Plantlife, the wild plant conservation charity (4). This research has discovered that this moss may not merit species status, and has suggested that taxonomic studies to determine the status of this moss should precede any conservation work (4).

Information authenticated by Plantlife, the wild plant conservation charity:
http://www.plantlife.org.uk

• Eutrophication: nutrient enrichment of aquatic or terrestrial ecosystems.
• 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.
• Gametophyte: a life cycle stage in plants, which has one set of chromosomes (threads of DNA protein) in the cell nucleus (a condition known as ‘haploid’), which arises from a spore (which is also haploid). Sex cells (gametes) are produced during the gametophyte stage. This is the dominant life-cycle stage in liverworts and mosses.
• 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.
• Sporophyte: the stage of a plant life cycle that produces spores (microscopic particles used in dispersal and reproduction). This stage is diploid (in the cell nucleus there are two sets of chromosomes - threads of DNA protein) and is dominant in ‘higher’ plants such as flowering plants.
• Vascular system: in plants, the system that allows water and nutrients to move around.

1. National Biodiversity Network Species Dictionary (August, 2002)
   http://www.nhm.ac.uk/nbn/
2. The Environment Agency. (2000) Focus on Biodiversity. The Environment Agency, Bristol.
3. UK BAP Species Action Plan (July, 2002)
   http://www.ukbap.org.uk
4. Holyoak, D.T. (2001) Tiny fern-moss Fissidens exiguuus. Plantlife report 181. ISBN 1 872613 52 7. Plantlife, London.
5. Church, J.M., Hodgetts, N.G., Preston, C.D. and Stewart, N.F. (2001) British Red Data Books: mosses and liverworts. Joint Nature Conservation Committee, Peterborough.
6. Mosses and Liverworts in Wales (August, 2002)
   http://home.clara.net/adhale/bryos/
7. Egerton, H. and Jones, F. (1998) Nature Encyclopedia. Dorling Kindersley, London.