What are bryophytes?
Bryophytes are the oldest land plants on earth, and have been around for 400 million years or more. Although small, they can be very conspicuous growing as extensive mats in woodland, as cushions on walls, rocks and tree trunks, and as pioneer colonists of disturbed habitats.
They comprise three main taxonomic groups: mosses (Bryophyta), liverworts (Marchantiophyta) and hornworts (Anthocerotophyta) which have evolved quite separately.Worldwide there are possibly 10,000 species of mosses, 7000 liverworts and 200 hornworts.
Most bryophytes have erect or creeping stems and tiny leaves, but hornworts and some liverworts have only a flat thallus and no leaves.
How do bryophytes live their lives?
Bryophytes have a two-stage life cycle (alternation of generations). The ‘gametophyte' generation is the green photosynthetic part (the familiar moss or liverwort plant) attached to the substrate by threads (rhizoids), and the ‘sporophyte' generation which consists of a stalk and capsule which are dependent on the gametophyte for support and nutrients. The capsule when ripe releases thousands of tiny spores. Sexual reproduction occurs on the gametophyte generation and requires water for fertilisation. Many bryophytes also produce ‘gemmae': tiny buds, discs or leaf fragments which spread the plants vegetatively.
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Mosses by their very nature are soft and absorbent and in many cultures have been traditionally used as bedding, padding and packing materials. Linnaeus gave Fontinalis antipyretica (a common north European aquatic moss) its Latin name because he observed Laplanders using it to caulk chimneys.
Bog moss (Sphagnum) in particular has many uses: it is the major component of peat used as fuel and compost, as a horticultural substrate, and in the past as nappies and surgical dressings because of its absorbent and antiseptic properties. In World War I, the public around Edinburgh were encouraged to collect Sphagnum for shipment to the battle fronts for medical use. Commercial harvesting of mosses (e.g. for making wreaths and decorations) is now a serious conservation issue.
More important perhaps are the vital ecological roles of bryophytes. On bare and disturbed ground they are primary pioneers helping other plants to gain a foothold. In bogs and forest (especially in the montane tropics) they absorb huge quantities of water, thereby acting as a sponge and maintaining humidity over dry periods and preventing rapid run-off and flooding (excessive flooding in India is thought to be partly due to loss of bryophyte cover in Himalayan forests). They act as a home to many plants and animals, particularly invertebrates, and provide a moist foothold for many other plants such as ferns and orchids in mossy or ‘Elfin' forest, as in the oceanic woods of western Scotland or the mossy forests of the Andes and Himalayas.
Bryophytes show a wealth of adaptive features to all kinds of climates, substrates and habitats. Many are precise indicators: of rock type such as Tortella tortuosa on limestone, Andreaea and Racomitrium on acidic or granitic rocks, of acid bogs, eg. Sphagnum species, of rich fens, eg. Tomentypnum nitens, of metalliferous rocks and soil, eg. Ditrichum plumbicola and Grimmia atrata, of pollution levels such as Dicranoweisia cirrata which thrives on tree bark in polluted areas and many pollution-sensitive species on trees in unpolluted areas, eg. Antitrichia curtipendula. In dry climates many are drought-resistant ‘xerophytes' in contrast to delicate ‘mesophytes' growing in wet habitats.
Scotland has a globally important bryophyte flora of about 920 species, because of its geographical position, its range of climatic types, and diverse geology. The western seaboard is rich in oceanic mosses and liverworts, such as the tropical Cyclodictyon laetevirens and Myurium hochstetteri. In the south-east the sunny sea-banks have a Mediterranean flora. Perhaps the richest habitats are in the mountains: the Ben Lawers range, well-known for its alpine flora, is even richer in rare bryophytes. The high Cairngorm plateau has several rare arctic and alpine species, particularly in snow-beds, such as Marsupella arctica and Andreaea blyttii, whilst the mountains of the North-west Highlands have a unique bryophyte-dominated community the ‘Northern Hepatic Mat' dominated by a range of rare liverworts, some of which (e.g. Pleurozia purpurea, Anastrophyllum alpinum) show astonishing disjunctions to the Himalayas and the mountains of British Columbia, thousands of miles apart. This juxtaposition of arctic habitats and an oceanic climate combine to make Scotland's bryoflora unique in the world. Tropical and arctic species can literally be found within ten or twenty miles of each other.
Scotland is important historically in bryology in that many European species were first discovered here: eg. Anastrepta orcadensis, Marsupella nevicensis, Pohlia scotica, Bryoerythrophyllum caledonicum, Plagiochila atlantica. Pioneers such as Sir William Hooker, Archibald Menzies and James Dickson first explored Scotland for bryophytes at the end of the eighteenth century. Tetrodontium brownianum, named after the celebrated Scottish botanist Robert Brown, was first discovered at Roslin near Edinburgh by Brown in his student days, and still thrives there.
Mosses and liverworts are common throughout the Royal Botanic Garden Edinburgh at Inverleith but especially on the Peat Walls, in the Rock Garden and in the Cryptogamic Garden. These are all native species. In the glasshouses a number of exotic species thrive, especially in the Fern House and the Peat and Rock Houses, such as the spectacular Hypopterygium tamarisci (formerly called H. atrotheca). These have probably been introduced accidentally with other plants. Many native species grow vigorously, especially the thalloid liverwort Conocephalum conicum which covers large areas of the Fern House in shiny mats. Few are cultivated, though many plant pots are colonised by the weedy greenhouse liverwort Marchantia polymorpha.
At Dawyck Botanic Garden there is a wealth of native bryophytes typical of southern Scotland. The clean air ensures luxuriant growth of epiphytic bryophytes including the locally rare Leucodon sciuroides and Neckera pumila. The Dawyck Cryptogamic Sanctuary now provides a secure haven for a good range of common species typical of upland woods.
In the Benmore Botanic Garden are many of the more oceanic species typical of 'western' native Scottish woodlands. In places bryophytes form the dominant ground flora and along with the ferns form an attractive natural ground-cover under the Rhododendrons and conifers. At Benmore three southern hemisphere liverworts, Lophocolea semiteres, L. bispinosa and Telaranea tetradactyla have become naturalised in the garden. Close to Benmore are some remnants of ancient Scottish oakwood, with very rich communities of oceanic bryophytes including Plagiochila atlantica. A project is now under way to experimentally re-create an area of native woodland with its component bryophyte flora within the garden itself.