7. Asplenium ruta-muraria L. (wall-rue) Pl. 1a,b; Map 7

A. cryptolepis Fernald

A. ruta-muraria var. cryptolepis (Fernald) Wherry

Leaves 2–10 cm long, monomorphic. Petioles green except sometimes brown at the very base, not shiny. Leaf blades 2–3 times pinnately compound, deltoid-ovate to obovate or lanceolate in outline. Pinnae alternate on the rachis, 7–30 mm long, triangular to obovate, the pinnules spatulate to nearly diamond-shaped and usually toothed to shallowly lobed at the tip, rarely deeply lobed at the base, the pinna bases not overlapping the rachis. Veins not anastomosing. Spores 64 per sporangium. 2n=144. May–September.

Scattered in the Ozark and Ozark Border Divisions (eastern U.S. and adjacent Canada; Europe, Asia). Pockets of dolomite and limestone bluffs, frequently in fairly exposed conditions; also on shaded dolomite boulders along streams.

The relationship of North American plants to those in Europe, where both diploid and tetraploid races of the species occur, are not well understood. American plants have at various times been treated as a separate species, a variety of the European species, or considered unworthy of taxonomic recognition. As there are no clear morphological distinctions between European and North American plants, they should continue to be treated as a single taxon until more detailed studies are completed. In Europe, the tetraploid cytotype of A. ruta-muraria is treated as ssp. ruta-muraria.

No hybrids involving wall-rue have been reported from Missouri. However, elsewhere it has been documented to hybridize with A. platyneuron (A. ¥morganii W.H. Wagner & F.S. Wagner), A. rhizophyllum (A. ¥inexpectatum E.L. Braun ex C.V. Morton), and A. trichomanes (A. ¥clermontae Syme). Of these, the last is the least likely to be found in Missouri, as wall-rue and maidenhair spleenwort do not grow in close proximity in the state.