Asplenium pinnatifidum

Asplenium pinnatifidum is a small fern with bright green, wrinkled, pinnatifid (lobed) fronds.[2][3] These form evergreen, perennial tufts.[3] Notable characteristics are the shiny stem, dark only at the base, and the long-tapering, variably lobed leaf blades.[2] The fronds are monomorphic, the sterile and fertile fronds appearing the same size and shape.[3]

The roots of A. pinnatifidum are not proliferous, so it appears as clusters of leaves springing from a single rhizome. The leaves are closely spaced on the rhizome, which is frequently branched.[2] The rhizome is about 1 millimeter (0.04 in) in diameter, covered with narrowly triangular scales which are dark reddish-brown or blackish in color, and strongly clathrate (bearing a lattice-like pattern).[2][4] The scales are 3 to 5 millimeters (0.1 to 0.2 in) long and 0.3 to 0.5 millimeters wide, with untoothed edges. The stipe (the stalk of the leaf, below the blade) is shiny and dark reddish brown at the base. The color fades to green in the upper one-third to one-half of the stipe. It is covered in narrowly triangular, dark reddish-brown scales at the base, which diminish into hairs in the upper part of the stipe.[2] It may show narrow wings from the base of the leaf to near the base of the stipe.[4] The stipe is 1 to 10 centimeters (0.4 to 4 in) long,[2] and may be from one-tenth to one and one-half times the length of the blade.[3]

The overall shape of the blade is narrowly triangular or lance-shaped, sometimes with an irregular outline. The blade tapers to a long point, the length of the taper being variable among specimens.[2][4] The blades are generally curled with downward-pointing tips.[3] The tip of the blade sometimes develops a swelling which may differentiate into a proliferous bud and, very rarely, into a plantlet, as in walking ferns.[5] Adventitious sporangia may appear around the buds when they form, even, unusually, on the upper surface of the leaf.[6] The base of the blade may be squared off or notched to a varying extent along the rachis (central axis of the leaf). The blade ranges from 2 to 17 centimeters (0.8 to 7 in) long, rarely to 20 centimeters (7.9 in), and 1 to 4 centimeters (0.4 to 2 in) in width, rarely to 13 centimeters (5.1 in), and is thick[2] and somewhat leathery.[3] Blades are either entirely pinnatifid (lobed but not completely cut), or cut to form a single pair of pinnae at the base. When they exist, the pinnae are roughly oval or triangle-shaped, sometimes narrow, and are from 5 to 20 millimeters (0.2 to 0.8 in) long, rarely to 90 millimeters (3.5 in), and 0.4 to 1 millimeters in width (rarely to 1.2 millimeters). The base of the pinnae may be squared off or taper to a point, while the edges are wrinkled to toothed. The tip can vary from rounded to pointed.[2] The lobes of the blade gradually diminish towards the tip, sometimes becoming simply wavy.[3] The rachis is green, sometimes turning tan when dry. The underside of the rachis and blade have a few scattered, small hairs.[2] Overall, the blades are quite morphologically variable;[3] in younger blades, the edges may be not at all lobed or may be wavy.[4] The veins are free and forking, only rarely anastomosing (rejoining one another to form nets).[2]

Underside of fertile leaf showing sori.

Each segment (pinna or lobe) of a fertile frond has one to six sori, sometimes more than forty in extreme cases. The sori usually fuse with one another as they age.[2] These are 1 to 2 millimeters (0.04 to 0.08 in) long They are covered with thin, whitish indusia with untoothed edges,[4] which are persistent.[3] Each sporangium holds 64 spores. The species has a chromosome number of 144 in the sporophyte, indicating an allotetraploid origin.[2]

Lobed spleenwort was first recognized by Henry Muhlenberg in 1813, who considered it a variety of Asplenium rhizophyllum, although he did not provide a description distinguishing the variety.[12] In 1818, Thomas Nuttall observed that it was always distinguishable from A. rhizophyllum, and described it as a species under the name of Asplenium pinnatifidum.[13] Alphonso Wood used the name Camptosorus pinnatifidus for the species in 1870,[14] but this was not widely accepted.

Oliver A. Farwell, observing an unusual specimen of A. pinnatifidum, was led to suggest that the species might be a hybrid between American walking fern, Camptosorus rhizophyllus (now A. rhizophyllum), and ebony spleenwort (A. platyneuron). Such a hybrid, Scott's spleenwort (A. × ebenoides) was already known, but Farwell thought it bore a greater affinity to A. platyneuron while A. pinnatifidum had a greater affinity to A. rhizophyllum.[15] He was correct in viewing A. pinnatifidum as a hybrid descendant of A. rhizophyllum, but incorrect in identifying the other parent, and his suggestion was not widely taken up in the literature. Nor did his later attempt at subdividing Asplenium, moving A. pinnatifidum to a new genus as Chamaefilix pinnatifida in 1931,[16] meet with much favor.

As a member of the "Appalachian Asplenium complex", A. pinnatifidum readily acts as the progenitor of hybrids, as well. A. × gravesii was recognized as a hybrid of A. pinnatifidum and A. bradleyi by W. R. Maxon in 1918.[9] Edgar T. Wherry noted the similarities between A. montanum, A. pinnatifidum, and A. × trudellii in 1925,[10] and in 1936 concluded that Trudell's spleenwort was a hybrid between the first two.[17] That same year, A. kentuckiense was described by Thomas McCoy; Wherry identified it as a hybrid between A. pinnatifidum and A. platyneuron.[17] In 1951, Herb Wagner, while reviewing Irene Manton's Problems of Cytology and Evolution in the Pteridophyta, suggested in passing that A. pinnatifidum itself might represent a hybrid between A. montanum and A. rhizophyllum.[18]

In 1953, he made chromosome counts of A. × trudellii, which had been classified by some simply as a variety of A. pinnatifidum. As A. pinnatifidum proved to be a tetraploid while A. montanum was a diploid, a hybrid between them would be a triploid, and Wagner showed that this was in fact the case for A. × trudellii.[19] His further experiments, published the following year, strongly suggested that A. pinnatifidum is an allotetraploid, the product of hybridization between A. montanum and A. rhizophyllum to form a sterile diploid, followed by chromosome doubling that restored fertility.[20] However, the hypothesized sterile diploid has never been found.[2][lower-alpha 1] Partial pairing of homologous chromosomes in A. × gravesii and A. × trudellii confirmed A. montanum parentage for A. pinnatifidum,[21] while an artificial hybrid between A. pinnatifidum and Tutwiler's spleenwort (A. tutwilerae) helped confirm their shared A. rhizophyllum parentage.[22]

Wagner's conclusions as to the parentage of A. pinnatifidum were supported by later chromatographic analyses, in which the chromatograms of A. pinnatifidum contained all the compounds detected in the chromatograms of both parents.[23] In 1985, an allozyme analysis confirmed the hybrid parentage of the species,[24] and revealed that A. pinnatifidum had probably originated independently through chromosome doubling at more than one locality.[25]

In 1956, C. V. Morton pointed out that, as A. pinnatifidum had been shown to arise from hybridization between walking fern and mountain spleenwort, it would constitute an intergeneric hybrid if walking fern was placed in the genus Camptosorus (as Camptosorus rhizophyllus). The hybrid genus ×Asplenosorus had been published, but Morton noted that it lacked a Latin diagnosis and was therefore invalid under the International Code of Botanical Nomenclature; he preferred to continue recognizing Asplenium pinnatifidum in Asplenium.[26] The ICBN's rules were relaxed in 1972, and in 1974, John Mickel published Asplenosorus pinnatifidus as a new combination for the species to allow the continued recognition of Camptosorus.[27] Since then, phylogenetic studies have shown that Camptosorus nests within Asplenium,[28][29] and current treatments do not recognize it as a separate genus.[2]

In addition to A. × gravesii, A. × trudellii, and A. × kentuckiense, A. pinnatifidum is known to be the parent of several other hybrid species. A sterile triploid hybrid, formed by the crossing of A. pinnatifidum with a diploid cytotype of maidenhair spleenwort (A. trichomanes ssp. trichomanes) was discovered in 1969[30] and named A. × herb-wagneri in 1977.[31][lower-alpha 2] Finally, several unnamed hybrids have been grown in culture. The hybrid between A. pinnatifidum and A. tutwilerae showed a "nondescript" morphology intermediate between the two parents, with slightly narrower lobes and more dimorphic fronds than A. pinnatifidum, overall resembling A. × kentuckiense but with more regular lobes.[22] Another, between A. pinnatifidum and the tetraploid American hart's-tongue fern (A. scolopendrium var. americanum) yielded peculiar specimens with a long blade, similar in texture and doubled indusia to the hart's-tongue fern, but lengthened and tapering to a point, and not lobed except for two surprisingly large auricles at the base.[32]

Native to eastern North America, A. pinnatifidum occurs in the middle and southern Appalachian Mountains, from Pennsylvania and New Jersey southwest to Alabama and the northeastern corner of Mississippi. It is also found in the Shawnee Hills and to some extent in the Ozarks, with outlying occurrences in southeastern Oklahoma and in Iowa County, Wisconsin.[33] Early reports from New England proved to be variants of Scott's spleenwort, Asplenium × ebenoides.[34]

Clump of A. pinnatifidum in its typical cliff crevice habitat.

A. pinnatifidum can be found on acidic rocks, often in steep habitats, from altitudes of 0 to 1,000 meters (0 to 3,281 ft).[2] Sandstone is its usual substrate.[4] The soil formed when these rocks weather must be subacid (pH 4.5–5.0) to mediacid (pH 3.5–4.0) to support A. pinnatifidum.[35]

The species is considered apparently secure globally (G4), but is endangered in many parts of its range. NatureServe considers it to be critically imperiled (S1) in Illinois, Maryland, Mississippi, New Jersey, Oklahoma, South Carolina, and Wisconsin, imperiled (S2) in North Carolina, and vulnerable (S3) in Arkansas, Georgia, Pennsylvania, and Virginia. It is threatened by changes in land use, habitat fragmentation, and certain forest management practices.[1]

A. pinnatifidum can be cultivated in rock gardens and terraria. It prefers medium light and will grow on a moist soil or potting mixture.[36] Some authorities recommend adding sandstone chips to the soil.[4]

• Asplenium hybrids