Asplenium × Gravesii
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''Asplenium'' × ''gravesii'', commonly known as Graves' spleenwort, is a rare, sterile, hybrid fern, named for Edward Willis Graves (1882–1936). It is formed by the crossing of Bradley's spleenwort ( ''A. bradleyi'') with lobed spleenwort ( ''A. pinnatifidum''). It is only found where its parent species are both present; in practice, this proves to be a few scattered sites in the
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...
,
Shawnee Hills The Shawnee Hills are a region within the larger Interior Low Plateaus physiographic province located in southern Illinois, southern Indiana, and western Kentucky. In Illinois it rests mainly in an east–west arc roughly following the outline ...
, and
Ozarks The Ozarks, also known as the Ozark Mountains, Ozark Highlands or Ozark Plateau, is a physiographic region in the U.S. states of Missouri, Arkansas, and Oklahoma, as well as a small area in the southeastern corner of Kansas. The Ozarks cover ...
, reaching perhaps its greatest local abundance around
Natural Bridge State Resort Park Natural Bridge State Resort Park is a public recreation area located along the Middle Fork of the Red River, two miles south of the unincorporated community of Slade, in Powell and Wolfe counties, Kentucky, United States. The state park lies ...
. Like its parents, it prefers to grow in acid soil in the crevices of sandstone cliffs.


Description

''Asplenium'' × ''gravesii'' is a small fern, whose fronds grow in loosely bundled tufts. Its stem below the leaf blade is a shiny purplish-brown, while the green, narrowly triangular blades are cut into pinnae near the base, which diminish into lobes in the upper part. The fronds are monomorphic, showing little or no difference between sterile and fertile fronds. The fronds of ''A.'' × ''gravesii'', which are long, are closely spaced along an upward-curving, densely scale-covered
rhizome In botany and dendrology, a rhizome ( ) is a modified subterranean plant stem that sends out roots and Shoot (botany), shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from ...
in length. The scales are long and narrow, about long and 0.5 millimeters wide at the base, coming to a pointed tip. They are dark reddish-brown in color and strongly clathrate (bearing a lattice-like pattern). The stipe (the stalk of the leaf, below the blade) is typically long, shiny and purplish-brown, sometimes becoming green near the base of the leaf blade. It is rounded below and flat or grooved above. The overall shape of the blade is narrowly triangular, from long and wide. The leaf tissue is of a medium texture (neither delicate nor leathery), with clavate (club-shaped) hairs on the underside becoming gland-tipped, narrow scales on the veins. The lower part of the blade is cut into pinnae, diminishing to lobes in the upper part. The basal pinnae are triangular in shape, roughly equilateral, and nearly heart-shaped at their base; they are borne on short stalks. They are shallowly rounded or toothed around the edges. In larger specimens, the basal pinnae bear a pair of rounded lobes at their base. Successive pinnae above the base are narrower and less deeply cut, gradually diminishing into fused lobes or overlapping. The lobes continue to the pointed tip of the blade. The
rachis In biology, a rachis (from the [], "backbone, spine") is a main axis or "shaft". In zoology and microbiology In vertebrates, ''rachis'' can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the ''rachi ...
(central axis of the blade) is typically green and flat, with narrow
wings A wing is a type of fin that produces both lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform. Wing efficiency is expressed as lift-to-drag ratio, which compares the bene ...
, and a groove on the upper side near the base of the blade, where it becomes the stipe. Larger individuals may have a narrower, glossy-brown rachis. A few scales are present on stipe and rachis, becoming narrower and more twisted as they ascend from the rhizome. The abundant sori are dark brown and variable in shape, fusing with one another as they grow. The sori are covered with firm, white indusia. In wild specimens, they are found beneath the
costa Costa may refer to: Biology * Rib (Latin: ''costa''), in vertebrate anatomy * Costa (botany), the central strand of a plant leaf or thallus * Costa (coral), a stony rib, part of the skeleton of a coral * Costa (entomology), the leading edge of th ...
(midrib of the pinnae). As a sterile tetraploid hybrid, the
spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...
s are seen to be misshapen and abortive under microscopic examination. The species has a chromosome number of 144 (2 × 72) in the sporophyte. ''Asplenium'' × ''gravesii'' can potentially be confused with its parental species or with a number of the other ''Asplenium'' hybrids in the Appalachian ''Asplenium'' complex. Smaller specimens are most similar to ''A. pinnatifidum'', but can be distinguished from that species by their brown stipes (and often rachides), a smaller number of fronds in each tuft, and by the tip of their leaves (which is pointed, but not drawn out at length as in ''A. pinnatifidum''). ''A.'' × ''gravesii'' also has a slightly finer leaf texture, slightly sharper teeth on its leaves, and darker brown sori than ''A. pinnatifidum''. When compared to ''A. bradleyi'', larger ''A.'' × ''gravesii'' individuals are most similar, but the blades are not so deeply cut (''A. bradleyi'' being wholly pinnate) nor the pinnae toothed around the edges, the rachis shows at least some traces of winging, and more of the rachis tends to be green. The leaf texture is somewhat more delicate than the leathery blades of ''A. bradleyi''. ''Asplenium'' × ''gravesii'' differs from Trudell's spleenwort ( ''A. × trudellii''), another descendant of ''A. pinnatifidum'', by having a blade broadest at the middle or between the middle and the base, rather than at the base itself, and by the presence of brown color throughout the stipe and sometimes into the rachis. In contrast to Boydston's spleenwort ( ''A. × boydstoniae''), ''A.'' × ''gravesii'' has fewer than fifteen pairs of pinnae, which are not
sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...
, and when dark color is present in the rachis, it covers less than seven-eights of that structure. The most similar hybrid to ''A.'' × ''gravesii'' is probably Kentucky spleenwort ( ''A. × kentuckiense''), a hybrid of ''A. pinnatifidum'' and ebony spleenwort ( ''A. platyneuron''). In ''A. × kentuckiense'', the blade tapers at the base, the second and third pairs of pinnae being shorter than the fourth and fifth; in ''A.'' × ''gravesii'', all these pairs are approximately equal in size. ''A. × kentuckiense'' takes on a somewhat papery textures when dried, while ''A.'' × ''gravesii'' is more leathery. Finally, the
guard cells Guard cells are specialized cells in the epidermis of leaves, stems and other organs of land plants that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are larg ...
of the latter average 49 micrometers, slightly larger than the 46 micrometers of the former. As this character can only be examined by microscope, and the ranges of individual guard cell size overlap, some care is required in its use; 30 measurements from a single pinna were used to obtain an average length in previous studies. It is particularly useful in determining the identity of dried material.


Taxonomy

The first specimens of the fern to be recognized were collected by Edward Willis Graves in 1917 at Sand Mountain near
Trenton, Georgia Trenton () is a city and the only incorporated municipality in Dade County, Georgia, United States—and as such, it serves as the county seat. The population was 2,195 at the 2020 census. Trenton is part of the Chattanooga, Tennessee–GA Me ...
. While Graves at first thought they might be a variant form of ''A. pinnatifidum'', further study in conjunction with William R. Maxon revealed them to be a hybrid between ''A. pinnatifidum'' and ''A. bradleyi'', both of which occurred nearby. Maxon published a description of the new species in 1918. Specimens in the herbarium of Harold W. Pretz, collected in 1913 along the lower
Susquehanna River The Susquehanna River ( ; Unami language, Lenape: ) is a major river located in the Mid-Atlantic (United States), Mid-Atlantic region of the United States, crossing three lower Northeastern United States, Northeast states (New York, Pennsylvani ...
and labeled as "''A. pinnatifidum''", were subsequently identified as the new species. Thomas Darling Jr., successfully crossed ''A. bradleyi'' with ''A. pinnatifidum'' in cultivation from 1954 to 1955 to produce ''A.'' × ''gravesii''. He supplied both the artificial crosses and live specimens collected at Sand Mountain to
Herb Wagner Warren Herbert Wagner Jr. (August 29, 1920 – January 8, 2000) was an eminent American botanist who was trained at Berkeley with E.B. Copeland and lived most of his professional career in Michigan. History Wagner was instructed in the ways o ...
for cytological studies. Wagner had previously used the size of
stomata In botany, a stoma (: stomata, from Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange between the internal air spa ...
in herbarium material to tentatively classify the species as a tetraploid. Wagner and Darling were able to grow the wild and artificial specimens together. While they were clearly the same species, the cultured specimens had pinnae less broad, particularly at the base, more widely spaced, and less deeply toothed; the sori were also halfway between the costae and the edge of the leaf, rather than beneath the costa. As the specimens had been cultured under the same conditions, they attributed this to genetic variation within the two parent strains. They were also able to show that about half of the chromosomes typically paired for
meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
. Since ''A. bradleyi'' is a hybrid of mountain spleenwort ( ''A. montanum'') and ''A. platyneuron'', while ''A. pinnatifidum'' is a hybrid of ''A. montanum'' and walking fern (''A. rhizophyllum''), half of the genetic material in ''A.'' × ''gravesii'' was ultimately contributed by ''A. montanum'' and should be able to pair, consistent with the results. Chromatographic experiments reported in 1963 showed that, like ''A.'' × ''kentuckiense'', chromatograms made from ''A.'' × ''gravesii'' contained all the compounds from the chromatograms of all three of its diploid ancestors: ''A. montanum'', ''A. platyneuron'', and ''A. rhizophyllum''. In 1974,
John Mickel John Mickel may refer to: * John Mickel (racing driver) (b. 1971) * John Mickel (politician) (b. 1953) * John Mickel (rugby union) * John T. Mickel (1934–2024), botanist {{hndis, Mickel, John ...
published ''Asplenosorus gravesii'' as a new combination for the species to allow the continued recognition of the genus ''Camptosorus'' for the walking ferns. Since then, phylogenetic studies have shown that ''Camptosorus'' nests within ''Asplenium'', and current treatments do not recognize it as a separate genus.


Distribution

In principle, ''A.'' × ''gravesii'' might be found anywhere the ranges of the parent species overlap: throughout the mid- to southern
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...
and extending west through the
Shawnee Hills The Shawnee Hills are a region within the larger Interior Low Plateaus physiographic province located in southern Illinois, southern Indiana, and western Kentucky. In Illinois it rests mainly in an east–west arc roughly following the outline ...
into the
Ozarks The Ozarks, also known as the Ozark Mountains, Ozark Highlands or Ozark Plateau, is a physiographic region in the U.S. states of Missouri, Arkansas, and Oklahoma, as well as a small area in the southeastern corner of Kansas. The Ozarks cover ...
. In practice, its occurrences are highly scattered and rare, as the two parental species do not often occur adjacent to one another. It has been found in New Jersey, Pennsylvania, Maryland,
Hocking County, Ohio Hocking County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 28,050. Its county seat is Logan. The county was organized on March 1, 1818, from land previously included in Athens, Fairfield, and Ross ...
, Illinois, Kentucky, Tennessee,
Madison County, Missouri Madison County is a county located in the Lead Belt region of the U.S. state of Missouri. As of the 2020 census, the population was 12,626. Its county seat and largest city is Fredericktown. The county was officially organized on December 1 ...
, Arkansas,
Dade County, Georgia Dade County is a county in the U.S. state of Georgia. It occupies the northwest corner of Georgia, and the county's own northwest corner is the westernmost point in the state. As of the 2020 census, the population is 16,251. The county seat a ...
, and Alabama. An occurrence in West Virginia reported in 1926 and 1938 was subsequently found to be ''A.'' × ''trudellii'', and one from Virginia in 1944 to be ''A.'' × ''kentuckiense''. It has been noted as particularly abundant in the general vicinity of
Natural Bridge State Resort Park Natural Bridge State Resort Park is a public recreation area located along the Middle Fork of the Red River, two miles south of the unincorporated community of Slade, in Powell and Wolfe counties, Kentucky, United States. The state park lies ...
.


Ecology

Like the parental species, ''A.'' ''gravesii'' prefers acid soil; in fact, it may tolerate only ( pH 3.5–4.0) soils, while subacid (pH 4.5–5.0) soils are acceptable to both parents. It usually grows, like the parents, in exposed
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
cliff faces.


Cultivation

''A.'' ''gravesii'' was produced by artificial hybridization in 1954–1955 by Thomas Darling Jr., who provided a detailed account of the process. Spores of the two parental species were sown on damp
peat moss ''Sphagnum'' is a genus of approximately 380 accepted species of mosses, commonly known as sphagnum moss, also bog moss and quacker moss (although that term is also sometimes used for peat). Accumulations of ''Sphagnum'' can store water, since ...
and kept largely in the shade, except for a few hours of morning sun.
Gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...
s developed, and the young
sporophyte A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...
s that grew from them were large enough to remove in autumn. They were transplanted into a well-drained, gravelly
loam Loam (in geology and soil science) is soil composed mostly of sand (particle size > ), silt (particle size > ), and a smaller amount of clay (particle size < ). By weight, its mineral composition is about 40–40–20% concentration of sand–si ...
. They were raised in a humid terrarium exposed to morning sunlight, with a layer of tissue to shade them from direct sunlight. Of the ''A. gravesii'' in the mixed population, one reached maturity in August 1955; two others did not mature until June 1956. Difficulties were encountered due to
aphid Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...
infestation and various diseases promoted by excess moisture.


See also

* Asplenium hybrids


Notes and references


Notes


References


Works cited

* * * * * * * * * * * * * *


External links

*
NatureServe ExplorerAlan Cressler's comparison of fronds of ''A. × gravesii'' and its parent species
{{DEFAULTSORT:Asplenium gravesii gravesii Plant nothospecies Plants described in 1918 Ferns of the United States Endemic flora of the United States Flora of the Eastern United States