Polypodium cambricum L., Southern Polypody

Account Summary

Growth form and preferred habitats

The fronds of P. cambricum are broadly triangular and can be up to 50 cm in length, with the edges of the pinnae often quite serrate. New fronds are produced later than in the other two Polypodium species, in the autumn and winter, and they are reputedly the least frost hardy of the three species. It has been suggested that in Britain and Ireland Southern Polypody is mainly confined to lowland sites within a few hundred metres of sea-level, often coastal and lying within the 4°C winter minimum isotherm (Page 1997). Spore-producing sori are confined to the upper third of the frond; young sori are more oval than in other forms of polypody, and when the plant is vigorously vegetative, no sporangia are formed, rendering the plants sterile.

Species identification

Variation in both vegetative and reproductive characters is so great in the genus Polypodium that P. cambricum (the diploid) and P. interjectum (the hexaploid allopolyploid fertile hybrid derived from a cross between P. cambricum and P. vulgare s.s., followed by spontaneous chromosome doubling), tend to overlap in many respects. The presence of relatively large, branched colourless threads (ie paraphyses), within the sorus among the sporangia, are unique to P. cambricum, and taken in conjunction with 'good spores' (ie fully formed, fertile, regular-shaped, non-aborted ones), they provide the best distinguishing character for the species (Roberts 1970; Page 1997).

Having said this, these are very much laboratory microscopic characters, and furthermore care must be taken not to confuse the much larger, more branched paraphyses with the minute glandular hairs which occur scattered on the lower surface of the frond in all European species of Polypodium. These hairs are much smaller than true paraphyses, being usually only 3 or 4 cells long – with their glandular, inflated terminal cells mostly coloured a dark reddish-brown (Roberts 1970 – see p. 128, Fig. 6 to clarify these differences).

Fermanagh occurrence

While this perennial is the most distinctive and readily identified of the Polypodium species or hybrids in Britain & Ireland, it is also very definitely the least frequent of the three species in Fermanagh and elsewhere in these islands (Page 1997). So far, P. cambricum has been recorded in just 37 of Fermanagh tetrads, 7.0% of those in the VC.

Southern Polypody is a strongly calcicole species and, as the Fermanagh tetrad map indicates, it typically grows in the W & S of the county on limestone natural rock outcrops. Often it occupies the more sheltered parts of cliffs, crevices in old lime-mortared walls, or ground on the steep-sloping rocky floors of hazel woods, where it is capable of forming large stands. Locally, it has never been found in artificial habitats such as quarries, nor growing as an epiphytic on the bark of trees, behaviour that may be confined to more maritime districts than occurs in Fermanagh (Jermy & Camus 1991).

In drier, more exposed upland sites, which are usually limestone gorges and cave mouths, P. cambricum associates with Asplenium ruta-muraria (Wall-rue) and Ceterach officinarum (Rustyback). On the other hand, when occurring in shade in Ash and Hazel woodland in constantly damp and humid conditions, it consorts instead with other shade-tolerant species such as Hedera helix (Ivy), Polystichum setiferum (Soft Shield-fern) and Phyllitis scolopendrium (Hart's-tongue).

British and Irish occurrence

The current knowledge of the occurrence of P. cambricum in Britain & Ireland, as displayed at the hectad level in the New Atlas and the 2005 New Atlas of Ferns, shows it is both southern and western in Britain, although stretching northwards as far as mainland Argyll in W Scotland (VC 98) (Preston et al. 2002; Wardlaw & Leonard 2005). The same two maps show it very much more widely scattered across the whole of Ireland. The distribution looks patchy, however, suggesting that the recording effort is distinctly uneven. More work is required to arrive at an accurate picture of the real distribution of this fern.

World occurrence

P. cambricum is entirely confined to W and S Europe, being sparse and rather disjunctly spread from the British Isles to Portugal, N Africa and eastwards through the Mediterranean islands to Greece and Turkey (Jalas & Suominen 1972, Map 140; Page 1997).

Name

The species Latin epithet 'cambricum' means 'of Wales' (Cambria) (Gilbert-Carter 1964).

Hybrids

While diploid P. cambricum is by far the least common of the three Polypodium species in Britain & Ireland, its hybrids with the other two species are even rarer. The hybrid between P. cambricum and P. interjectum (P. × shivasiae) should be actively looked for as both species are calcicole and P. interjectum (being very much the more common parent) probably occurs at or near every P. cambricum site. So far only four records of this hybrid have been discovered in Fermanagh.

Threats

Clearance of woodland.

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