Asplenium platyneuron (syn. Asplenium ebeneum), commonly known as ebony spleenwort or brownstem spleenwort, is a fern native to North America east of the Rocky Mountains and to South Africa. It takes its common name from its dark, reddish-brown, glossy stipe and rachis (leaf stalk and midrib), which support a once-divided, pinnate leaf. The fertile fronds, which die off in the winter, are darker green and stand upright, while the sterile fronds are evergreen and lie flat on the ground. An auricle at the base of each pinna points towards the tip of the frond. The dimorphic fronds and alternate, rather than opposite, pinnae distinguish it from the similar black-stemmed spleenwort.
The species was first described in 1753 by Linnaeus as Acrostichum platyneuros, although Linnaeus' type drew on material from several other species as well. It was more commonly called Asplenium ebeneum, a name published by William Aiton in 1789, until the rediscovery and revival of the Linnaean epithet in the late nineteenth century. Several forms and varieties of the species have been described, but few are recognized today; in particular, larger and more fertile specimens, those with more or less toothed leaves, and those with proliferating buds are considered to fall within the natural range of variation of the species, and do not require taxonomic distinction. A. platyneuron f. hortonae, a sterile form with the pinnae cut to toothed pinnules, and f. furcatum, with forking fronds, are still recognized.
The formation of proliferating buds is one of several unusual adaptations for reproduction in this species. The buds form near the base of the stipe, and when covered with soil, can grow into new individuals as the frond that bore them dies. Ebony spleenwort is also well-adapted to propagate by spores: the upright sterile fronds help the spores enter the airstream for long-distance dispersal, and a low genetic load allows spores that have grown into a gametophyte to self-fertilize with a high degree of success. This dispersal ability seems to have helped the species spread rapidly in the Great Lakes region in the late 20th century. Long-distance dispersal may also explain its naturalized appearance in South Africa, and the existence of an isolated population found in Slovakia in 2009, its first known occurrence in Europe.
Ebony spleenwort has broad habitat preferences, growing both on rocks like many other North American spleenworts and in a variety of soils. Unlike many other spleenworts, it is not particularly sensitive to soil pH. It hybridizes with several other spleenworts, particularly mountain spleenwort and walking fern; these species, their sterile hybrid offspring, fertile allotetraploid hybrids, and backcrosses between allotetraploids and the parents are collectively known as the "Appalachian Asplenium complex". Two hybrids between A. platyneuron and spleenworts outside of this complex are also known.
Description Ebony spleenwort is a small fern with pinnate fronds, growing in tufts, with a shiny reddish-brown stipe and rachis. The fronds are dimorphic, with long, erect, dark green fertile fronds, which are deciduous, and shorter, spreading, lighter green sterile fronds, which are evergreen.
Roots and rhizomes This fern does not spread and form new plants via the roots. The rhizome, from or perhaps in diameter, bears a few narrowly linear-deltate scales, black to dark brown in color and strongly clathrate (bearing a lattice-like pattern). The scales are or long and wide, with margins entire (untoothed) or very nearly so.
Leaves The stipe (the stalk of the leaf, below the blade) is long, and comprises one-quarter to one-third of the length of the blade. It lacks wings, and is a shiny reddish-brown or blackish-brown throughout its length. Towards the base, it has a few threadlike scales similar in color to those of the rhizome. Starch granules are stored in tissue at the base of the stipe and, to a lesser extent, in the rhizome, giving the bases an enlarged appearance and a firm texture. The stipe bases are long-lived and may survive the disintegration of the rest of the stipe and the blade.
The leaf blade of ebony spleenwort is linear to narrowly oblanceolate (slightly wider in the upper half of the blade) in shape, measuring from long and from wide, sometimes as wide as . It is acute (pointed) at its tip and gradually tapers at its base. The blade is shiny and has a few scattered hairs, or lacks them entirely. The rachis (leaf axis), like the stipe, is reddish-, purplish- or blackish-brown, shiny and hairless.
The blade is cut into pinnae (leaflets) throughout its length, from 15 to 45 pairs per leaf. The pinnae are distinctly alternate along the rachis. They are oblong (roughly rectangular) or quadrangular in shape, sometimes somewhat falcate and angled towards the leaf base, those in the middle of the leaf blade measuring from in length and from in width. Each pinna has an obvious auricle at its base, pointing towards the tip of the blade and overlapping the rachis. Sometimes a small auricle pointing at the base of the blade is present as well The edges of the pinnae are crenate or serrulate (bearing rounded or small sharp teeth); they are more deeply cut in small forms. The tips of the pinnae may be acute or obtuse (blunt). On the underside of the blade, veins are clearly visible and free (they do not anastomose or rejoin each other). A few minute scales may also be scattered on the underside.
Sori and spores Pinnae of the fertile fronds carry from one to twelve, or even eighteen, pairs of sori on their underside, each long. Each pair of sori forms a chevron, pointing towards the base of the pinna along its midvein, on which they are centered. An indusium covers each sorus; these are whitish and translucent or silvery with a slightly toothed or erose (irregularly jagged) edge, soon withering to reveal the sori. Each sporangium in a sorus carries 64 spores. The diploid sporophyte has a chromosome number of 72.
The light to dark brown spores are monolete, with a straight laesura with a margo (raised border). They are 41 to 52 microns long along their long axis. The perine (outermost layer of the spore) appears spiny, with rough ridges forming a netted pattern. The region of the perine between ridges has a smooth, rather than granular, texture.
Proliferating buds Ebony spleenwort is capable of proliferating by forming new plantlets from buds on the rachis at the base of the plant. These allow new individuals to form at different levels when prostrate fronds become buried in the leaf litter. Wagner & Johnson found proliferous buds in almost every site in the Great Lakes populations they studied; very rarely did more than one bud per plant occur. They occurred, on average, on 1 out of 6 plants, both on sterile and fertile fronds, and their position at the base of the lowest pinnae made them difficult to find among the cluster of fronds. The buds are button-like, pale in color, and appear on the upper side of the lowest pinnae. Each bud contains a shoot and one or sometimes two leaf primordia, covered in clathrate scales, occasionally with roots if the bud has begun to develop. Contact with soil at the plant base stimulates their development, and the decay of their connection with the parental leaf results in the development of a new plant. Similar buds have also been reported from the single-sorus spleenwort, Asplenium monanthes.
Identification The species most similar to Asplenium platyneuron is the black-stemmed spleenwort, A. resiliens. However, this stipe of this species is darker, and the pinnae are opposite, rather than alternate, along the rachis. Maidenhair spleenwort, A. trichomanes, is also pinnately cut with a dark, glossy rachis, but the pinnae are oval and somewhat rounded rather than broadly oblong, usually less than two times as long as wide. Neither of these two species, nor the other pinnate American spleenworts, have dimorphic fertile and sterile fronds. It is very similar to Boydston's spleenwort, Asplenium × boydstoniae, an extremely rare backcross with Tutwiler's spleenwort, Asplenium tutwilerae; however, Boydston's spleenwort has an elongated, acute frond tip similar to that of Tutwiler's spleenwort. Among similar South African spleenworts, A. lunulatum has proliferating frond tips, while A. monanthes has only one sorus on each pinna.
Among the larger ferns, A. platyneuron might be confused with a young Christmas fern, Polystichum acrostichoides, but that species is generally much larger and has a green, scaly stipe and rachis. Fishbone fern, Nephrolepis cordifolia, is likewise larger, with yellowish-green pinnae and a green rachis.
Taxonomy This species is commonly known as "ebony spleenwort" or "brownstem spleenwort" for the dark color of its stipe and rachis.
The basionym for the species is Acrostichum platyneuros, published by Linnaeus in Species Plantarum in 1753 (the official starting point of modern botanical nomenclature). Linnaeus' treatment came from a 1745 dissertation by his student J. B. Heiligtag, which in turn drew on the descriptions by several earlier authorities. The first description listed was that of Jan Gronovius, botanist and mentor of Linnaeus, in his Flora Virginica of 1743, based on a specimen collected by the collector John Clayton. The specific epithet platyneuros ("flat nerves") was drawn from a description by the botanist Leonard Plukenet in his Almagestum of 1696. Unfortunately, only Clayton's specimen, now the lectotype for the species, represents the species now known as Asplenium platyneuron; the specimens upon which the other descriptions are based, including that of Plukenet which yielded the specific epithet, are a mixture of Pleopeltis polypodioides and Polypodium virginianum sensu lato. In 1789, the species was independently described by William Aiton in Hortus Kewensis, and given the name Asplenium ebeneum, referring to the ebony color of its stalk. The species was also independently described by Olof Swartz in 1801, as Asplenium polypodioides, and by André Michaux in 1803 as Asplenium trichomanoides; both these names were reduced to synonymy with Asplenium ebeneum by Carl Ludwig Willdenow. The name Asplenium trichomanoides, however, had already been used by Linnaeus in 1767, making Michaux's name a nomen illegitimum.
It was Aiton's epithet, ebeneum, that was used by American botanists throughout most of the 19th century. The transfer of Linnaeus' specific epithet to form the name Asplenium platyneuron was mooted by D. C. Eaton in 1878, who attributed the combination to a pencil notation by William Oakes in the margin of a copy of Flora Virginica. Therefore, the combination has historically been attributed to Oakes or Oakes ex D.C.Eaton. However, in 1981, David B. Lellinger pointed out that Eaton had not, in fact, accepted the combination in 1878, preferring the more descriptive epithet ebeneum to the less accurate through senior platyneuron. Eaton's use, therefore, was to be treated as provisional, and not as a published combination by him. Eaton aside, the first valid publication of Asplenium platyneuron was that of Britton, Emerson E. Sterns, and Justus F. Poggenburg in 1888, which implicitly references Linnaeus's basionym. Despite the revival of Linnaeus' epithet, the name Asplenium ebeneum continued to be widely used through 1896, when Lucien Underwood, in Britton & Brown's Illustrated Flora, used the name Asplenium platyneuron with explicit reference to the basionym.
The species has twice been placed in genera segregated from Asplenium: as Chamaefilix platyneuros by Oliver A. Farwell in 1931, and as Tarachia platyneura by Sizuo Momose in 1960. Neither combination was widely accepted and current authorities do not recognize these genera.