1. Asplenium L. (spleenwort)

Plants perennial, homosporous. Rhizomes erect to short-creeping, compact, scaly, the scales clathrate (the endwalls colored, the centers transparent or nearly so). Leaves clustered, variously compound (simple or lobed elsewhere). Sporangia aggregated into sori on the undersurface of the leaf blades, extending along the veins, the indusium membranous, attached along 1 side. Spores 32 or 64 per sporangium, monolete, 25–60 mm long, black. Gametophytes green, flat, heart-shaped to kidney-shaped, often with stalked glands. About 700 species, worldwide.

Most of the Missouri species of this genus belong to a group known as the “Appalachian Asplenium Complex,” which is among the most intensively studied groups of ferns in the world. The genus is famous for its high levels of interspecific hybridization, with sterile, primary hybrids recovering fertility through a process of chromosomal doubling. Although relatively few of the primary hybrids have been reported from Missouri, they should be looked for in the field in areas where two or more species grow together. Hybrid spleenworts can usually be recognized by their unusual, irregular leaf division pattern, with adjacent pinnae or lobes unequal in size.