Pellaea glabella is the smooth cliffbrake. For much of pteridological history, it was regarded as a reduced form or variety of Pellaea atropurpurea. P. glabella is known to exist in two cryptic species, one diploid and one tetraploid. The diploid reproduces sexually, while the tetraploid is normally apogamous. It is now known that the tetraploid form of the species is one of the parents of the original hybrid P. Xatropurpurea that became the apogamous species.
P. glabella is epipetric, normally growing on well-weathered limestone. It favors more exposed sites than P. atropurpurea.
This species is distinguished by its sessile or nearly sessile pinnae and smooth, not hairy, stipes.
Pellaea glabella subsp. glabella Mettenius ex Kuhn
Pellaea glabella subsp. missouriensis (G. J. Gastony) Windham
Pellaea glabella subsp. occidentalis (E. E. Nelson) Windham
Pellaea glabella subsp. simplex (Butters) A. Löve & D. Löve
P. glabella has been assigned a total of four subspecies:
The subspecies glabella and simplex are the tetraploids, while missouriensis and occidentalis are the diploids. Glabella and missouriensis have hairlike scales near the midrib, while simplex and occidentalis are completely glabrous.