What are mosses?

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What are mosses?

Mosses may be small, unassuming and not as well known as flowering plants but they are just as complex and extremely interesting and they perform an important role in the health and function of our environment.

They belong to a diverse group of organisms called Bryophytes that also includes Liverworts and Hornworts. Bryophytes are among the simplest of the terrestrial plants. The word “Bryophyte” comes from the Greek words “bryon” meaning “tree-moss” and “phyton” meaning “plant”.

Mosses are among the earliest plants to become adapted to living on dry land, having evolved from algae.

Small in size they may be but mosses are one of the largest groups of land plants and can be found throughout the world in a variety of habitats.

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Moss characteristics

Moss gametophyte © Ita McCobb

Mosses are small flowerless plants that do not have a true vascular system and therefore are unable to draw water and nutrients up from the ground over a distance, which is why they are unable to grow very large. This feature distinguishes them from Ferns and flowering plants.

Instead of roots, mosses have rhizoids – fine brown laments, resembling roots that anchor them to the ground but do not draw up water.

Mosses absorb water and nutrients mainly through their leaves, which are usually only a single cell in thickness.

Mosses do not have flowers or fruit and instead of seeds they have spores.

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Lifecycle of moss

Moss mat © Ita McCobb

Mosses have a two-stage lifecycle, known as “alternation of generations”. These two stages are called gametophyte (this is the green part of the Moss we see) and sporophyte (this is formed of a capsule containing the spores and lament). The sporophyte generation produces spores that are capable of germinating and develop into the gametophyte generation that produces male and female sex organs and ensure sexual reproduction.

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Moss statistics

Acrocarpous moss © Ita McCobb

There are an estimated 12,000 species of Moss that are classified as Bryophytes, which are among the group of plants that reproduce by spores.

Bryophyte species are generally among the first to colonize open ground and may well be the ancestors of the first plants on Earth. Other Bryophytes are Hornworts (Anthocerotophyta) and Liverworts (Marchantiophyta).

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Mosses & ecosystems

Moss & grey Lichen © Ita McCobb

As with all Bryophytes, mosses play an important role in regulating ecosystems because they provide a vital buffer system for plants that live close by and benefit from the water and nutrients that mosses collect.

Bryophytes are very good indicators of habitat quality as many plant species in this group are sensitive to levels of moisture in the atmosphere, which are lower in disturbed habitats because there is less shade there.

Mosses play an important role in our ecosystems, as they are crucial for soil stabilization and water retention, helping in the prevention of floods and landslides.

With other Bryophytes mosses are important in the carbon cycle, with peat mosses storing a huge amount of carbon in both Arctic and temperate zones.

Mosses are also important habitats for a wide variety of plants, insects and fungi and are used by other wildlife such as squirrels for lining their dreys, birds for lining their nests and dormice that hibernate in nests beneath moss.

Mosses are also reliable indicators of air pollution risks to ecosystems as they acquire most of their nutrients directly from the atmosphere.

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Threats to Moss

The threats to mosses are many and include both habitat loss and habitat degradation.

Certain aspects of modern agriculture such as overgrazing, under-grazing and the widespread use of herbicides and fertilizers, pose a threat to moss survival as does the felling and clearing of trees in forests.

As mosses do not have roots they get most of their nutrients directly from the air and rain rather than the soil so they are particularly sensitive to atmospheric pollutants and many have become extinct from some urban and industrial environments.

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Misuse of moss-plant names

It is interesting to note that there are several plant names in which the word “moss” is misused!

Although certain seaweeds are called Sea mosses, no mosses are found in salt water. Spanish moss (Tillandsia usneoides) is not a moss but a flowering plant in the same family as the pineapple! Reindeer moss (Cladonia rangiferina) is not a moss but Lichen found in the arctic tundra, Irish moss (Chondrus crispus) is a red algae and Club moss (Lycopodium) is a relative of Ferns!

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Common mosses

Moss in wetland habitat © Ita McCobb

Examples of common mosses are:

  • Acrocarpous mosses – Dicranum,Tortella, Barbula and Polytrichum.
  • Pleurocarpous mosses – Hypnum, Thuidium, Amblystegium, Brachythecium and Plagiothecium mosses.

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Resources: United Nations Environment Programme (UNEP), Division of Technology, Industry and Economics; Botany for gardeners, Brian Capon, Third edition, Timber Press; Colour Identification Guide to the Grasses, Sedges, Rushes and Ferns of the British Isles and north-western Europe, Francis Rose, Viking, The Penguin Group; Royal Botanical Gardens, Kew, London; Missouri Botanical Garden; Latin for gardeners, Lorraine Harrison, Royal Horticultural Society; International Botanical Congress (IBC); RHS Gardeners’ encyclopaedia, Christopher Brickell ed., Dorling Kindersley; The Royal Horticultural Society; The Wildflower Key, Frances Rose, Warne; Collins tree guide, Owen Johnson & David More,William Collins; Oxford English Dictionary, Oxford University Press.