Brief descriptions of the rare and scarce ferns

THE RARE NATIVE FERNS
(occur in few sites)

Anogramma leptophylla, Jersey Fern.
In Britain, this small Mediterranean species is found only in a few sites in Jersey and Guernsey, on lane banks. It forms tufts of finely dissected fronds more likely to be confused with a seedling Umbellifer than with any other fern. It is the only British fern that has short-lived sporophytes formed each year from perennial gametophytes. The sporophytes usually die during the dry summer,

Asplenium onopteris, Irish, or Acute-leaved, Spleenwort, Western Black Spleenwort
This Mediterranean species occurs in the extreme south of Ireland but has not been identified in the wild in Britain. It is one of the parental species involved in  the origin of the much more widespread A. adiantum-nigrum, which it most closely resembles and from which it is sometimes not easily distinguished. It grows in dry, warm, sites in open woodland. It has narrower and more pointed pinnae and pinnules than most forms of  A. adiantum-nigrum.

Cystopteris diaphana, Wintergreen Bladder Fern.
This Mediterranean and Macaronesian species was only recently identified as almost certainly a British native and it has no established English name. Since 2000 it has been described from a few shady river-bank and lane-bank sites in Cornwall and Devon, as well as in a ruined garden fernery in Argyll, Scotland where it is perhaps a surviving introduction. Now its existence in Britain has been recognised, it may well be found elsewhere in the milder parts of Britain and Ireland. Not easy to distinguish morphologically from C. fragilis, a very variable species, it differs in occurring on more acidic substrates and, more strikingly, in being wintergreen. Although C. fragilis is one of the first British ferns to expand fronds in the spring, in some forms as early as March, it is also one of the first to die back in late summer, especially in more exposed sites. Even though a second flush of a few fronds may occur in the autumn, all fronds later die back and the plant is dormant throughout the winter. By contrast, plants identified as C. diaphana in Britain have green fronds throughout the winter. It is not known whether C. diaphana would maintain its distinctive wintergreen characteristic if it grew in areas with less mild winters.

Cystopteris dickieana, Dickie’s Bladder Fern
This small fern is similar to Cystopteris fragilis, and is known only from localised sea-caves in Kincardineshire and an inland Scottish site. It tends to have more crowded pinnae than C. fragilis, and is less dissected. Examination of the spores helps to distinguish the two taxa, with the spores of C. dickieana being rugose, as opposed to the spiny spores of C. fragilis.

Cystopteris montana, Mountain Bladder Fern.
Found in moist, shady, rocky sites above 700m in the Central Scottish Highlands, and rare even there, this small fern has finely-dissected, (broadly) triangular, tripinnate fronds arising singly from a creeping rhizome.  It is absent from the rest of Britain.

Ophioglossum azoricum, Small Adder’s Tongue
Apart from some sites in the New Forest, this fern is restricted to scattered island and peninsular sites up the west coast of Ireland and Britain, from Isles of Scilly to Shetland, where it grows in short turf close to the sea. It is similar to the Common Adder’s Tongue, but with fewer sporangia (rarely more than 12 pairs cf. rarely fewer than 12) and smaller (less than 8cm), and relatively broader, fronds (up to 5x4cm) often occuring in groups of two or three. Fronds emerge in May and die back in September.

Ophioglossum lusitanicum, Least Adder’s Tongue.
In Britain, this species is known only from a few sites in Guernsey and the Isles of Scilly where it grows in exposed coastal turf. Similar to O. azoricum but smaller (less than 1.5cm). Differs from both the other British Adder’s Tongues in growing during the winter; fronds emerge in October and die back in May.

Vandenboschia speciosa, Killarney Fern
This is a large Filmy Fern with a (broadly) triangular or obovate frond up to 30cm long. The fronds are finely dissected and the translucent, membranous, dark green, blade is reminiscent of seaweed or thick polyvinyl cling-film in texture It is known as a sporophyte from only 20-30 constantly damp sites, almost all in west Britain, all closely guarded secrets. It is less rare in Ireland, particularly in the south-west. In recent years, this species has been found as permanent populations of independent gametophytes in dark, humid conditions in caves and under rock ledges at more than a 100 localities over a much wider area of the wetter parts of Britain.  More localities are being found every year. Unlike most ferns, the gametophyte forms a long-lived branching filament, one cell wide.  It spreads as a green, moss-like, mat by filament growth but it also forms detachable propagules that can be dispersed, probably by water, with the potential to start a new colony.  In Madeira, where the species is most common, the life cycle is completed normally but in most British localities, the gametophyte colonies seem to be unable to produce mature sporophytes and even antheridia and archegonia are rarely observed.  Sporelings have been seen in a few English populations but most are very small and apparently incapable of further development.

Woodsia alpina, Alpine Woodsia
Although found throughout the North-Temperate zone including Norway, it has been estimated that only about 1000 plants of this species grow in Britain. It occurs at only a few sites in the mountains of Scotland and North Wales where it is restricted to wet, lime-rich, rocks above about 750m.  It is a small fern (fronds rarely more than 8cm), with (narrowly) oblanceolate or parallel-sided blades, just bipinnate at the base, and (broadly) triangular lobed pinnae that are about as long as they are wide and have up to 3 distinct pinnule pairs.  The stipe is jointed near the base.  There are few if any long jointed hairs on the blade surfaces and no scales on the pinna midribs.  It is not always easy to distinguish from poorly-grown specimens of Woodsia ilvensis (see below)

Woodsia ilvensis, Oblong Woodsia
Though not uncommon in Scandinavia, including the nearest parts of Norway, this is Britain’s rarest fern, partly because of over-collecting in the 19th Century.  Now it is known at only three widely separated localities in Scottish mountains, a similar number of localities in the mountains of north-west Wales, and one site in the English Lake District. At most sites, individual populations consist of only 1-4 ‘tufts’ (some ‘tufts’ may consist of more than one individual), with a total for Britain of fewer than 100 known ‘tufts’.  It is a small (fronds rarely more than 10cm) bipinnate fern with a (sometimes narrowly) obovate blade, and triangular or obovate lobed pinnae that are typically at least twice as long as wide and have up to 6 distinct pinnule pairs.  The stipe is jointed near the base.  The blade surfaces are covered in numerous long jointed hairs and there are narrow scales on the undersurface of the pinna midribs.  The margin of the cup-shaped indusium is extended into long hair-like projections.  When young, this fern has been confused with Cystopteris fragilis (qv) which it can superficially resemble.

THE SCARCE BRITISH NATIVE FERNS
(absent from most of Britain but sometimes locally common)
The 6 scarce native species are absent from most of Britain and Ireland, and confined to a limited habitat range, but may be locally common within their restricted distribution.

Adiantum capillus-veneris, Maidenhair Fern
This distinctive small to medium-sized fern with drooping fronds is restricted to sea cliffs of west England, Wales and Ireland. The stipe and rachis are purple-brown and wiry. The sori are marginal on fan-shaped light-green pinnules.

Asplenium obovatum ssp. lanceolatum, Lanceolate Spleenwort (=A.billotii)
This Mediterranean species is restricted to the western fringes of Britain and Ireland. In Britain, it is rare outside Cornwall and north-west Wales and in Scotland is known only from isolated sites close to the sea in Kintyre and west Sutherland. Although generally considered to be a species of rocky sites on the coast, in Cornwall it occurs in lane-banks inland at higher altitudes, as for example on the fringes of Bodmin Moor at over 260m (850ft) altitude and about 20km (12miles) inland.  This small fern most closely resembles the Black Spleenwort, Asplenium adiantum-nigrum (qv) but is distinguished from it by the shape of the blade outline as well as by less obvious differences in the shape of the pinnae.  The longest pinnae of the obovate blade occur about one-third of the way from base to tip, and the lowest two pinnae are downward pointing.

Athyrium distentifolium, Alpine Lady Fern.
This medium-sized fern is restricted to hollows above about 600m in the Scottish Highlands, often where snow lingers.  It is a circumpolar species, absent from the rest of Britain.  When fertile, it can be distinguished from A. filix-femina, the Common Lady Fern, which it usually closely resembles, by the sori, which are small and circular, with little or no indusium, and mainly in the upper half of the frond.  There is a distinct form with more finely divided pinnae and reflexed fronds which lie almost flat and have sporangia concentrated in the lower half.  Breeding experiments have revealed that all the differences are inherited together in a simple Mendelian fashion. Consequently this form is recognise as a variety (var. flexile) of A. distentifolium, even though in the past it was considered a distinct species (A. flexile).  A. distentifolium var. flexile is endemic to Scotland, where it is even rarer than the typical species, with which it usually occurs.

Dryopteris cristata, Crested or Fen Buckler Fern
Now apparently extinct in Scotland, and absent from Wales and Ireland, this lowland fern is almost entirely confined to wet fens in East Anglia. Among native species, it most closely resembles D. carthusiana, but the narrow, upright, fronds are distinctive.

Dryopteris submontana, Rigid Buckler Fern.
This upland fern is found only in crevices in limestone pavement and screes, and, apart from a few outlying sites in mountains of North Wales and north-west English Midlands, is restricted to a limited area of the northern Pennines. Superficially, it most closely resembles a small Male Fern but is easily distinguished. Unlike the Male Ferns, the fronds are dull grey-green and narrowly triangular with a ‘matt’ surface due to the many small yellow glandular hairs, visible under a hand lens, which emit a balsam-like fragrance when newly unfurled fronds are lightly rubbed.

Pteridium pinetorum, Northern or Pinewood Bracken.
This fern is found only in remnants of the ancient Caledonian forest (either in or close to surviving native Scots Pine forest, or where such forest has previously existed) but there it can occur in large stands. It is included by Page (1997) in his Flora as a species distinct from Common Bracken, Pteridium aquilinum, but in a taxonomic revision (Telopea 10(4), p.798, 2004; Fern Gazette 18(3), pp.101-109, 2008) Thomson awards it only subspecific status as P. aquilinum subspecies pinetorum (C. N. Page & R. R. Mill) J. A. Thomson. While easily recognised as a bracken, several features make P. pinetorum easy to distinguish from P.aquilinum, especially when the two species grow side-by-side as they sometimes do in the Pine forest. The fronds are shorter than those of Common Bracken and of different habit and shape. The rachis is angled at about 60o to the vertical at the junction of first pair of pinnae (in Common Bracken, the stipe and rachis are near-vertical). The lowest pinnae are the longest, so the blade is broadly triangular (in Common Bracken, the second or third pair of pinnae are almost always the longest). All pinnae are more or less in the plane of the rachis, thus facing upwards (in Common Bracken they also face upwards, but as a result of being rotated on the vertical rachis).These characteristics give the dense stands a distinctive appearance. Viewed from a distance, dense populations of P. pinetorum typically form a continuous and uniformly knee-high flat green surface. Among other, more subtle, differences, fronds of P. pinetorum expand rapidly in early summer, all pinnae unfurling simultaneously, whereas those of Common Bracken unfurl slowly and sequentially so that when the lowest pair of pinnae is almost fully expanded, the upper ones are still at the crozier stage.

Print Friendly, PDF & Email

The website for people who like ferns