I suppose I'd better explain:
In Britain and Ireland there are three species of, so-called, 'filmy ferns'. These are Wilson's Filmy Fern (Hymenophyllum wilsonii), the Tunbridge Filmy Fern (H. tunbridgense) and the Killarney Bristle Fern (Trichomanes speciosum). These grow in wet, sheltered sites on rock faces - usually in areas with high rainfall. They all have translucent leaves which are only one cell thick (hence the 'filmy fern' name). In the 19th century colonies of the handsomest of these species, T. speciosum, were practically destroyed by fern collectors, and they have never really recovered. At least the colonies of the adult form of T. speciosum - i.e. the spore-bearing or 'sporophyte' stage - have never recovered but, relatively recently, an extraordinary discovery was made. Ferns have a complicated life cycle and before they reach the sporophyte stage go through a 'gametophyte' stage in which a female element is fertilised by a male element. Typical fern gametophytes are usually tiny, heart-shaped scales clinging to rocks or soil.
Around 30 or 40 years ago Donald Farrar, a professor of botany at Iowa State University, discovered that certain American fern species, including filmy ferns, produce 'independent gametophytes' - tiny plantlets that never produce adult sporophytes; they exist in a state of permanently arrested development. He discovered colonies of these independent gametophytes by examing the interiors of rock crevices, fissures and other shady places with a torch (OK - a 'flashlight'!). Unlike typical fern gametophytes, the independent gametophytes of the American filmy fern species, Trichomanes intricatum, are like green mats of thread-like filaments - sometimes covering several square metres. They reproduce via tiny, specialised buds, called 'gemmae', which become detached and dispersed to new locations.
In 1989 Prof. Farrar was on sabbatical leave in Britain and found colonies of the independent gametophyte of T. speciosum here. This discovery prompted further research and such colonies have now been found in several areas of Britain and elsewhere in Europe.
The big question, of course, is how did this phenomenon arise? The American fern expert, Robbin C. Moran, has put forward an extraordinary explanation: Most filmy ferns are tropical species found clinging to trees in rain and cloud forests. Until the middle of the Tertiary period, 35 million years ago, such forests existed in America and Europe. Then the climate began to cool and become more seasonal, culminating in the last Ice Age. A few species hung on as sporophytes in wetter, somewhat warmer areas (like western regions of Britain and Ireland) but in other drier, colder areas they 'toughed out' the climate change by confining themselves to the stable micro-climates in rock crevices and similar places, and arresting their development to their gametophyte stages. So, and I hardly dare write this, those little green patches that we saw on Saturday, and which were not noticed until just over 20 years ago, have probably existed in Britain for millions of years!
The colonies of T. speciosum gametophytes which we found on Saturday looked like thin layers of bright green cotton wool pasted to the rock and we had to use torches to see them (and because of the cramped and awkward conditions my photo is rubbish - well, that's my excuse!).
All stages of T. speciosum are protected by law in Britain and Ireland (it's a Red Data Book species) - which is why I can't tell you where the site is. A passerby asked us what we were looking at and, I'm ashamed to say, we lied and told him that we were looking for mosses. On the bright side, though, by lying we didn't have to kill him and he didn't have to endure my explanation of what a gametophyte is!
Dave Bishop, September 2012
Moran, R.C., 'A Natural History of Ferns', Timber Press, 2004
Rich, T.C.G. & Jermy, A.C., 'Plant Crib 1998', BSBI, 1998