The first "new" British fern to be discovered for over 50 years has been spotted by an amateur naturalist.
The differences between species are often subtle
Civil engineer Matt Stribley found what turned out to be the diaphanous bladder fern by the banks of a Cornish river.
His discovery, on the Camel Trail near Wadebridge, was identified by experts from the Natural History Museum.
Common in southern Europe, the fern is thought to have thrived unnoticed in the UK for up to 5,000 years, mistaken for another widespread British species.
That plant was the brittle bladder fern, which looks very similar.
But when local botanical recorder Rosaline Murphy sent a specimen to the South Kensington museum, botanists there correctly identified it as Cystopteris diaphana.
"Britain is arguably the best biologically known piece of land in the world, so the discovery of a new native species is an increasingly rare and very exciting event," says Fred Rumsey, the museum expert who identified the plant.
"It is amazing to think this plant could have been here waiting to be discovered since just after the last ice age. It just goes to show how important amateur naturalists are in helping us to discover more about British biodiversity."
Bladder ferns are small ferns, which favour shady habitats.
Their common name "bladder" refers to the inflated cells of the protective spore covering.
Different species of bladder ferns are distinguished by their spores.
The last "new" British fern to be discovered was the least adders tongue (Ophioglossum lusitanicum) found in 1950 in the Scilly Islands.