Variation needs further research. This species is widely distributed on outcrops in North America and other parts of the world. In Ky. it is generally occurs on sandstone cliffs, but it is also scattered on limestone along the Kentucky River Palisades (FAYE, GARR, MERC). It is likely that the Palisades plants are ssp. quadrivalens D.E. Meyer, which is distinguished by its larger spores (37-43 versus 27-32 micrometers) and higher chromosome number (2n = 144 versus 72). That taxon is known only on calcareous rocks, mostly in northeastern regions, but with a few disjunct occurrences further west (FNA 2).