oak fern

noun3

1. •
   A fern of genus Gymnocarpium (syn. Thelypteris) which grows on trees, especially Gymnocarpium dryopteris.
2. •
   A bat's wing fern (Histiopteris incisa)
3. •
   Synonym of common polypody (Polypodium vulgare), a fern formerly thought to have medicinal properties when found growing on an oak.