Asplenium arcanum is a fern known from western Mexico and Nicaragua.
Asplenium arcanum has fibrous roots and an upright rhizome (underground stem). The rhizome bears narrow, blackish scales, 1to long and 0.1to wide, which are clathrate (bear a lattice-like pattern) and have untoothed margins. The fronds grow in clumps and are, including the stipe (the stalk of the leaf, below the blade), 7cmto20cmcm (03inchesto10inchescm) long. The stipes are dark purple in color at the base, becoming green towards the blade, lack wings, and have no hairs or scales except at the very base. They range from 1cmto12cmcm (00inchesto05inchescm) long and 0.3to wide, and typically make up one-half to two-thirds of the total length of the frond.
Each leaf bears from one to five pairs of pinnae (the first subdivisions of the leaf). The pair closest to the base is the largest.
The rachis (leaf axis) is green beneath, sometimes with streaks of purple at the base, smooth, and has small wings on the upper side, from 0.1to wide.
Fertile fronds bear from one to three pairs of sori per pinnule on both sides of the costule (pinnule midvein). They are covered by indusia 2to long and 0.5to wide, with jagged margins. The spores are kidney-shaped.
Specimens have in the past been identified as, or thought similar to, A. fragrans, A. minimum, A. pumilum, and A. tenerrimum. It differs from A. fragrans by the dark purple (rather than green) color at the base of its stipe, from A. pumilum by the lack of long whitish hairs both above and below the leaf, from A. tenerrimum by the enlargement of the side of the lowest pinna towards the leaf base, and from A. minimum by the deeply cut and toothed pinnae and pinnules (rather than rounded and lobed).
Material of this species was described by George E. Davenport in 1894 as a deeply cut variety of A. pumilum, var. laciniatum, based on material collected in Jalisco by Pringle. It was described as a species by Alan R. Smith in 2004 based on material from Nayarit. He gave it the epithet arcanum in reference to the unclear and mysterious relationship between this species and other spleenworts; it is roughly intermediate in form between A. tenerrimum and A. pumilum, but has well-formed spores, so it is not a first-generation hybrid.
Asplenium arcanum grows on stream banks and on wooded hillsides in the states of Nayarit and Sinaloa, at altitudes from 200to. It has also been collected at Apoyo Lagoon Natural Reserve in Nicaragua.[1]