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. 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.