This is a widespread, northeastern tetraploid (2n = 148) derived from crossing of two diploids: P. appalachianum (a largely Appalachian species) and P. sibiricum Siplivinsky (a boreal species). Without microscopic work, distinction from appalachianum is often difficult based on gross morphology (Cranfill 1980; FNA 2; W). Moreover, sterile triploid hybrids with appalachianum are occasionally found, with intermediate characters: as in colls. from MENI, MERC, PERR, ROWA, WOLF and elsewhere (at KY, EKY, etc.). Fronds of virginianum tend to be narrower (averaging 4.5 cm versus 5.8 cm), especially at the base (with oblong-lanceolate versus elongate-deltoid shape). Pinnae usually have blunt to rounded tips (versus acute to blunt); basal ones are unnotched on their lower side or just slightly (versus notched to 1-1.5 mm from the midrib). Rhizome scales tend to be narrower (averaging 1.1 mm versus 1.5 mm), and mostly with a darker brown central stripe (versus uniformly brown). P. virginianum is largely restricted to relatively mesic sites on rock outcrops; appalachianum often occurs on rocks but can also grow as an epiphyte. sometimes high on trees and overlooked by botanists (Keller et al. 2003; A. Risk & D. Dourson, pers. comm.).