Killarney fern
Vandenboschia speciosa
Family · Hymenophyllaceae · Harmless
The fern whose frond is one cell thick and therefore translucent like frosted glass. It lives only where sun never reaches and the air is saturated, behind waterfalls and at cave mouths.
Streams and ditches · Oak woods · Walls and rocks
Also known asbristle fern, Killarney bristle fern
How to tell it
Fronds ten to fifty centimetres, dark green and visibly transparent against the light, because the blade is one cell thick and has no pores. Much divided and delicate in appearance. The sori sit on the margin inside a trumpet-shaped tube with a bristle protruding. A black hairy rhizome pressed to the rock.
What it is confused with
No dangerous lookalikes known in the Iberian Peninsula.
What it is for
None. It was collected en masse in nineteenth-century Europe for fern collections under glass domes, and that drove it to extinction across much of its range. It is today among the most protected ferns on the mainland and the islands, and drying out without permanent humidity kills it within hours.
The law says
A strictly protected species under the Habitats Directive and the Bern Convention. Do not touch it, do not collect it and do not shine strong light on it.