Hartstongue Fern, Phyllitis scolopendrium, is our only true fern with undivided fronds. This clump is growing in the bed of plants that my Mum made, years ago now, in the concrete base of an old greenhouse.
The hartstongue became established at the time the greenhouse stood here, growing around the edges under the staging in the humid atmosphere. Its jungly fronds added an exotic touch to my Dad's more down to earth greenhouse crops of tomatoes and his pots and trays of geranium cuttings.
Hartstongue is more common in the damper west of Britain (we're in a rainshadow, here on the eastern side of the Pennines) where it grows, sometimes lushly, on hedgebanks and old stone walls and in woodlands.