oak fern

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of various ferns which grow on trees and walls, specifically common polypody, formerly thought to have medicinal properties when found growing on an oak.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun bright blue-green fern widely distributed especially in damp acid woodlands of temperate northern hemisphere