Polypodium interjectum × P. cambricum
(P. × shivasiae Rothm.), a hybrid polypody
Account Summary
Native, very rare, but probably still under-recorded.
1974; Hackney, P.; Boho Caves, voucher in BEL.
September and October.
Growth form and preferred habitats
P. × shivasiae is the rarest of the three Polypodium hybrids in Britain and Ireland. However, since the parent species are becoming quite frequently recorded in Fermanagh (particularly P. interjectum (Intermediate Polypody)), and as they are both weakly calcicole and undoubtedly overlap geographically and ecologically, this hybrid might be rather less rare in the VC than the current few records indicate. Having said that, as Roberts (1970) pointed out, survival of the relatively small, often sporadic populations of P. cambricum might be adversely affected by the formation of its hybrids with other species, since the offspring will compete with the diploid parent in a habitat in which it is already rather restricted. Under these circumstances any genetic barrier to hybridisation would tend to be strengthened.
P. × shivasiae is the most spectacular in appearance of the three sterile hybrid polypodies found in the British Isles, combining as it does large size with vigorous growth. As to be expected, it is intermediate in its morphological and physiological characteristics with respect to its parents. It grows on sheltered sections of limestone walls and on equally sheltered, shaded, parts of steep, wet, basic rock cliff faces, usually with both parent species nearby (Page 1997).
Fermanagh occurrence
All four Fermanagh records of this rare hybrid have been determined by Paul Hackney, the first record verified by R.H. Roberts. The details of the other three Fermanagh records, all collected by RHN, are as follows: Keenaghan Lough, Tievealough Td, September 1988; Levally House, 1 km SE of Roosky, October 1990; and W end of Lough Acrussel, 1988-90. This last record is marked "requires checking".
Name
The specific name 'shivasiae' commemorates the work of the British fern taxonomist M.G. Shivas, who contributed greatly to understanding of the genus in the 1960s and 1970s.