Polypodium interjectum Shivas, Intermediate Polypody

Account Summary

Growth form and preferred habitats

Typical specimens of this species (a fertile hexaploid hybrid formed by allopolyploidy between the other two British Isles Polypodium species) have ovate to narrowly oval fronds, generally longer than those of P. vulgare s.s. and with at least the lowest pair of pinnae bent forwards (ie inflexed), to form a 'V' shape. The fronds are leathery, evergreen and frost and drought resistant, fresh ones being produced in late summer and autumn, ie later in the year than P. vulgare s.s. and before those of P. cambricum (Southern Polypody) (Jermy & Camus 1991).

The plant is weakly calcicole or may prefer near-neutral conditions. A study of fern species with respect to root cation-exchange properties carried out in W Europe discovered the pH of soil samples at the roots of P. interjectum had a median value of 6.67, so that Koedam et al. (1992) classified it as 'neutrocline'.

P. interjectum grows in very much the same types of habitat as P. vulgare s.s., ie on rocks, cliffs, stony banks, mortared walls and also as an epiphyte on trees in damp woods.

Fermanagh occurrence

As with P. vulgare s.s. (Polypody), this perennial polypody is definitely still under-recorded in Fermanagh. Many of the existing records have been determined or verified by Paul Hackney at BEL. Intermediate Polypody is the second most common species of polypody in Fermanagh having been recorded so far in 90 tetrads, 17.1% of those in the VC. As the distribution map indicates, it is widely scattered throughout, but more frequent in the west of the county.

British and Irish occurrence

The Fern Atlas hectad map gave an early picture of the known distribution of P. interjectum in 1978, where the sub-Atlantic influence appeared quite strong in Britain, while the Irish distribution was then very much more evenly, or randomly scattered, although inland sites in NI appeared almost entirely absent (Jermy et al. 1978). The New Atlas hectad map now shows Intermediate Polypody is common and widespread throughout most of NI, but with apparent gaps in Cos Tyrone and Armagh (H36 and H37). The very obvious patchiness of the plotted distribution on both islands strongly suggests that recording of P. interjectum, although greatly improved in comparison with the Fern Atlas, remains incomplete.

Name

The Latin specific epithet 'interjectum' means 'intermediate in form', and of course refers to the fact that this species arose as a fertile hybrid (Gledhill 1985).

Threats

None.

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