The fern in the photo below is Lanceolate Spleenwort (Asplenium obovatum), a scarce species which is mainly found in coastal locations in southwest England and Wales, but which has a small population in the woods on the Oldbury Court estate (a location sometimes also referred to as “Glen Frome”), where it was first found in 1835.
There are several features which enable Lanceolate Spleenwort to be told apart from its more common relative Black Spleenwort (Asplenium adiantum-nigrum). To make this easier to follow, I need to introduce a few bits of fern terminology. The stipe of a fern is the main stalk of a fern frond. In most fern species, the frond is divided up: the main segments are called pinnae, and the lobes on each pinna are called pinnules.
Lanceolate Spleenwort has an elongated oval or oblong frond, with the pinnae nearest the base usually shorter than the ones further along, and these are often bent back towards the base of the plant (you can see this on some of the fronds in the photo if you look carefully). By contrast, in Black Spleenwort, these lowest pinnae are usually the longest, giving the frond a triangular shape. On Black Spleenwort the stipe is also usually much longer than on Lanceolate Spleenwort. I’ve included a photo of Black Spleenwort below, taken at a nearby location, and both of these features are obvious. There are other differences visible when you examine the fern close-up: the sori (spore-bearing bodies) on the underside of the leaf of Lanceolate Spleenwort are found only around the edges of the pinnules, whereas in Black Spleenwort they spread in a fan shape from the vein in the centre of the pinnule. Black Spleenwort gets its name from its solidly black stipe, whereas Lanceolate Spleenwort’s stipe is a green colour above, with a dark brown stripe below,
Lanceolate Spleenwort grows in sheltered fissures and crevices on acidic rocks, although in the warmer far southwest, it is also found on hedgebanks in lanes. The largest British populations are on sea cliffs in Devon, Cornwall and northwest Wales. The Oldbury Court site is the easternmost extant site in Britain for this species, and is isolated from other populations of Lanceolate Spleenwort by many tens of miles. In southwest England it is not found again until Exmoor, and in south Wales, not until the Gower peninsula, although there are old records from Beachley near Chepstow. Its northernmost locations, in Cumbria and southwest Scotland, are also very isolated. Globally its range extends south through France, Spain and Portugal to the Canary Islands, Madeira and the Azores, a distribution-type that is shared by a number of other fern species. Black Spleenwort is nowhere near as fussy, liking all rock types (loads of it grows on the limestone in the Avon Gorge, for example); I have even found it on walls in the city centre, near the Bristol Royal Infirmary and Old Market.
Here is the location of the plant I photographed (and a photo of the outcrop on which it was growing). If you want to visit it yourself, take care as it is on a steep muddy slope, high above the river. The best approach is to head up the zigzag footpath from the weir, and then take a right turn when you reach some railings. However, there are lots of similar-looking outcrops in the valley, from Eastville Park northeast to Winterbourne and it wouldn’t surprise me if some of these had their own mini-populations of Lanceolate Spleenwort too.