Polypodium appalachianum
Polypodium appalachianum is a fern species native to eastern North America. Sometimes called the Appalachian polypody or Appalachian rockcap fern, it is very similar in appearance to "Polypodium virginianum". For years, "P. virginianum"—long considered a variety of the British "Polypodium vulgare"—was recognized as having cryptic races, with diploid, triploid, and tetraploid representatives. Since the triploid specimens bore abortive spores, it was apparently the hybrid between the diploid and tetraploid groups. In 1991, it was resolved that the type of "P. virginianum" was the tetraploid series, and that it is an allotetraploid species of hybrid origin, with the diploid  species as one parent. The diploid species was then named "P. appalachianum". The other parent of "P. virginianum" was found to be "Polypodium sibiricum". The tetraploid of hybrid derivation tolerates warmer climates than either parent. "Polypodium sibiricum" is also now known to be one of the parents of the allotetraploid "Polypodium vulgare", along with "Polypodium glycyrrhiza". "P. appalachianum" is an epipetric plant, preferring sandstone or other hard, noncalcareous rocks. However, it is known to grow as an epiphyte in the Smoky Mountains. Throughout much of its range, it will be found to grow in places more sheltered from the sun  and heat than is "P. virginianum". Both species, and their hybrid, can form large clonal colonies, forming dense mats that hold organic matter in place on rock shelves and surfaces.