wall fern

a small, hardy fern (Polypodium virginianum) of the most common family (Polypodiaceae) of ferns, with densely matted, creeping stems, found on cliffs and walls in E North America and often grown in gardens

See wall fern in American Heritage Dictionary 4

noun
A low-growing Eurasian fern (Polypodium vulgare) characterized by creeping stems that form dense mats.
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