Know Your Natives – Ebony Spleenwort

Ebony Spleenwort (Asplenium platyneuron) of the Spleenwort Fern (Aspleniaceae) family is a small to medium size, once-cut, evergreen fern. The genus name is from Greek for “without a spleen” which may indicate that plants of this genus were historically thought to treat medical issues relating to the spleen. The specific epithet is from Greek words meaning wide or flattened veins. The common name refers to the dark, lustrous stipe (petiole) and rachis (midrib of blade). Its U.S. distribution is widespread, ranging from eastern Texas and Oklahoma to southeastern Minnesota, across to southern Maine, thence south along the Atlantic Coast and back to Texas. It occurs in additional scattered occurrences in the U.S. as far west as Arizona. Ebony Spleenwort is also found in subtropical and tropical southern Africa (a unique distribution not known for other North American ferns), eastern Canada, and in isolated populations in eastern Europe. In Arkansas, it is reported from all counties. Habitat preferences range from shady to partially sunny sites that may have deep, mesic to wet soils, in fields and open woodlands with a thin litter layer (terrestrial sites) to rock outcrops, rocky slopes and crevices of rocks and masonry walls. Across its broad range, variability of fronds has resulted in classification of a number of forms and varieties, but few such names are recognized today. Ebony Spleenwort hybridizes with several other Asplenium species; those occurring in Arkansas are listed at the end of this article.

Photo 1: Ebony Spleenwort growing in deep sandy soil of a flat woodland with Christmas Fern (upper right) and Blunt-Lobed Cliff Fern (Woodsia obtusa) (lower right). Photo – December 19
Photo 2: Ebony Spleenwort growing in a crevice between boulders in a deciduous woodland. Largest fertile frond is 15 inches long and 1½ inches wide. Photo – October 31.
Photo 3: Ebony Spleenwort growing naturally on the north side of a stacked-rock wall. Photo – November 25.
Photo 4: This tight clump of Ebony Spleenwort plants is on the north side of a woodland margin. Photo – December 6. (see related Photos 6 & 7)

Ferns have a two-phase life cycle: the gametophyte phase (a plant which has one set of chromosomes in each cell) and the sporophyte phase (which has two sets of chromosomes). The sporophyte phase is the most conspicuous. Most of us have probably never seen a fern in its gametophyte phase. During the sporophyte phase, a fern looks like, well, a fern. They bear clusters of sporangia called sori (aka fruit dots) on the underside of the frond or in separate fronds. Each sorus may be protected by an indusium (covering). The sporangia, attached on minute stems, produce the spores. When spores mature, the indusium, if present, shrinks back so that sporangia are exposed; they dry and split, releasing spores to the wind. When a spore lands in a favorable environment, it germinates to begin the gametophyte phase.  This plant, called a prothallus, contains both a structure called an archegonium, which makes an egg, and an antheridium, which makes sperm. Gametophytes are normally found in moist environments because the sperm need to swim to the eggs in the archegonia.  The sperm and egg (which each have one set of chromosomes, just like in humans) join to make the zygote, the first cell of the sporophyte, which actually begins growth directly on, and is briefly nourished by, the gametophyte. For some ferns, the sperm and egg necessary for new sporophyte plants to develop must be from different gametophytes (produced from two different spores). Ebony Spleenwort, however, is capable of self-fertilization whereby viable sporophytes can develop from gametes of a single gametophyte. For information specific to the sporophyte phase of Ebony Spleenwort, see below.

Photo 5: Beyond the indusia, the stemmed sporangia are exposed. Leaflets (pinnae or pinnules) are mostly glabrous with scattered, minute, filiform scales along the rachis and across undersides of leaflets. Photo – Nov 16

Ebony Spleenwort has the ability to reproduce vegetatively by “proliferating buds” that form at the base of fertile and sterile fronds. This is uncommon in ferns. These minute, spherical buds occur in limited numbers. In a favorable environment, while attached to the parent frond, a bud may develop into plantlets with its own rootlets and small fronds. Reportedly, buds that drop-off while lacking rootlets and fronds may still develop into plantlets. These clonal plants remain at the base of the parent plant so that tight clumps of variously aged plants may form. (Herein below, reference to “fertile” and “sterile” fronds relates to the presence or absence of sori, respectively.)

Photo 6: A proliferating bud on this “sterile” frond (white arrow) has developed rootlets and a frond. This bud has a width of 1/32 inch. Length of its frond is ⅜ inch. Red arrows point to a second plantlet that has become detached from the parent plant. Parent fern and plantlets are all from clump shown in Photo 4.
Photo 7: These young plants, probably clonal, came from the tight clump shown in Photo 4.

This non-rhizomatous fern may occur singly, in scattered groups, or clonal colonies (see Photos 1-4). Size of ferns varies considerably depending on moisture, soil depth and sunlight. A terete, mostly vertical, central rootstock elongates year-by-year as new growth emerges from the rootstock’s crown. The rootstock, base rounded, is roughened by the spiky bases of previous years’ fronds. Rootstocks are supported by a dense mass of long, tangled, equal-sized, well-spread, fibrous roots growing from all around the central rootstock.

Photo 8: This plant was in a shady, deep-soil site. It has a stubby central rootstock (⅜ inch wide) with a mass of fibrous roots to 6¾ inch long. Photo – November 9.
Photo 9: This plant was in a crevice between large rocks in a woodland. Central rootstock, shown with mass of rootlets removed, is 2 inches long (as measured between the 2 arrows) and ¼ inch wide. Photo – December 28.

Fertile and sterile fronds are different in size, shape and leaflet shape, thus a dimorphic fern. New fertile fronds emerge in spring as curled-up fiddleheads while the previous year’s sterile fronds remain viable. New sterile fronds emerge in early summer. Erect fertile fronds are to 16 inches long (plus a 2-inch stipe) and 2 inches wide. The smaller, low-arching to prostrate sterile fronds are to 6 inches long (plus a ½-inch stipe) and 1 inch wide. Both fertile and sterile fronds have a narrowing base and apex. Fertile fronds may have an overall equal width except for the apex (oblong-lanceolate) or be widened at mid- or upper-frond (elliptic to oblanceolate). Sterile fronds may be linear to oblanceolate and may be laterally curved. Initially green, the stipe and rachis quickly become a lustrous dark purplish to reddish brown. With winter temperatures, the brittle fertile fronds tend to become broken and mostly die off while the later growing, ground-hugging sterile fronds remain green into spring.

Photo 10: These fertile-frond fiddleheads are poised for springtime emergence. Length of central rootstock is ⅜ inch. Same plant shown in Photo 8.
Photo 11: This springtime plant has three new fertile fronds with apexes still in fiddleheads. Several erect to reclined, old fertile fronds remain along with several low-arching to prostrate old sterile fronds. Photo – April 18.

Fertile fonds may have up to 50 pairs of alternate leaflets while the shorter sterile fronds may have up to 30 pairs. Initially triangular, most leaflets of fertile fronds become oblong to linear-oblong with leaflets near the base remaining smaller and retaining a triangular shape. Leaflets of fertile fronds may be to about 1 inch long and ⅜ inch wide. Leaflets of sterile fronds are to about ¼ inch long and 3/16 inch wide.

The evenly spaced medium to dark green leaflets have stubby, green petiolules (stalks of leaflets) and straight midribs set perpendicular to the terete rachis. Leaflets of fertile and sterile fronds have an ear-like extension (auricle) on the distal side of leaflet base. Leaflets of sterile fronds are more rounded and more closely spaced than those of fertile fronds so that rachises of sterile fronds are mostly hidden, when viewed from above. At the apex, leaflets decrease in size and merge to form an acute to obtuse apex. Rachises are mostly glabrous except for a few scattered, small to minute filiform scales along the stipe and rachis, increasing somewhat toward the stipes’ bases, and continuing onto the undersides of leaflets. Pinnate veins are obscure on the upper leaflet surface and, on the lower surface, the flattened secondary veins are only slightly visible. Secondary veins, without noticeable branching, are free (they do not reach leaflet margins).

On the lower sides of fertile frond leaflets are a double row of straight, elongate-oblong sori (the clusters of sporangia), to 1/16 inch long. Sori are positioned obliquely to either side of the central vein. Sori, aligned with the secondary-vein pattern, are positioned between the leaflet’s central vein and the low-points of the serrate to dentate margins. The number of sori on any particular leaflet varies from few to about 15 alternate-pairs. Sori are initially covered by a translucent to silvery indusium attached to the leaflet along one side – the side away from the central vein. Within each sorus, minute, spherical sporangia are atop minute stalks. In late summer into fall, indusia split to expose these tannish sporangia so that, upon drying, the dust-like spores can be released and dispersed by wind. With spores having been released, the sporangia have a frazzled appearance.

Photo 12: Ferns are dimorphic. A 15½-inch-long fertile frond on left (in 2 sections) is displayed to show upper and lower surfaces. Sterile fronds on right are displayed to show upper surface. Photo – December 19.
Photo 13: Display shows lower sides of sterile fronds. Along with sterile fronds being smaller, prostrate, and sometimes laterally curved, leaflets are more round and more closely spaced. Frond at lower left is 2 inches long. Photo – December 6.
Photo 14: Sori are positioned diagonally between the central vein and the low-points of the serrate to dentate leaflet margins. Sori are protected by translucent covers, called indusia (see arrow). Photo – November 16.
Photo 15: A pair of fertile fronds – lower side shown on left and upper side on right. Auricles are on the distal margin of leaflets; arrows indicate direction of frond apex. Photo – November 9.
Photo 16: With spores dispersed, sporangia have a frazzled appearance. Photo – December 15.

Ebony Spleenwort hybridizes naturally with several other species of the genus – some recorded in Arkansas. A sterile hybrid with Walking Fern (A. rhizophyllum) is Scott’s Spleenwort (A. x ebenoides). A sterile hybrid with Lobed Spleenwort (A. pinnatifidum) is Kentucky Spleenwort (A. x kentuckiense). Both hybrids are of conservation concern. Hybridization between Ebony Spleenwort and Mountain Spleenwort (A. montanum), along with a back-cross, has resulted in a fertile species – Bradley’s Spleenwort (A. bradleyi).

This small to medium sized, evergreen fern has delicate fronds which add fine-detail to a garden setting. It contrasts nicely with broad-leaf plants as well as with other ferns. This non-aggressive fern is suitable for fern gardens, rock gardens, and natural areas in sandy to loamy soils as well as rocky areas. Partial sun to dappled shade is acceptable where soils are typically moist and drainage is good. It may occur naturally in stacked-rock and masonry walls.

As many as nine additional species and four hybrids of Asplenium have been reported to occur in Arkansas.* These other species and hybrids are mostly limited to rocky outcrops; whereas Ebony Spleenwort is also found in terrestrial habitats. Additionally, characteristics that separate Ebony Spleenwort from the other Arkansas spleenworts include: 1) More obvious dimorphic fronds, 2) Alternate leaflets with basal auricles, 3) Leaflets of sterile fronds rather oval-shaped and closely spaced and 4) Dark purplish to reddish brown stipe/rachis throughout. It is most similar to Black Stem Spleenwort (aka Little Ebony Spleenwort) (Aspelnium resiliens). However, both the fertile and sterile fronds of the smaller Black Stem Spleenwort are erect and about the same size and shape, and its leaflets are opposite.

*Spleenwort species and hybrids reported to occur in Arkansas:

Bradley’s Spleenwort – Asplenium bradleyi
Scott’s / Dragon Tail Spleenwort – Asplenium x ebenoides
Graves’ Spleenwort – Asplenium x gravesii
Kentucky Spleenwort – Asplenium x kentuckiense
Mountain Spleenwort – Asplenium montanum
Lobed Spleenwort – Asplenium pinnatifidum
Ebony Spleenwort – Asplenium platyneuron
Black Stem / Little Ebony Spleenwort – Asplenium resiliens
Walking Fern – Asplenium rhizophyllum
Wall-rue – Asplenium ruta-muraria
Northern / Forked Spleenwort – Asplenium septentrionale
Maidenhair Spleenwort – Asplenium trichomanes
Trudell’s Spleenwort – Asplenium x trudellii

Article and photographs by ANPS member Sid Vogelpohl

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