Bradley's Spleenwort

Bradley's Spleenwort

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Asplenium bradleyi, commonly known as Bradley's spleenwort or cliff spleenwort, is a rare epipetric fern of east-central North America. Named after Professor Frank Howe Bradley, who first collected it in Tennessee, it may be found infrequently throughout much of the Appalachian Mountains, the Ozarks, and the Ouachita Mountains, growing in small crevices on exposed sandstone cliffs. The species originated as a hybrid between mountain spleenwort (Asplenium montanum) and ebony spleenwort (Asplenium platyneuron); A. bradleyi originated when that sterile diploid hybrid underwent chromosome doubling to become a fertile tetraploid, a phenomenon known as allopolyploidy. Studies indicate that the present population of Bradley's spleenwort arose from several independent doublings of sterile diploid hybrids. A. bradleyi can also form sterile hybrids with several other spleenworts.

Variants of A. bradleyi have been reported. Specimens found growing in a very shaded environment, which lacked color in the rachis and were simply pinnate, have been mistaken for green spleenwort (A. viride). But even under such conditions, A. bradleyi has a more leathery leaf texture than A. viride, and their ranges do not overlap.[9] In 1923, Edgar T. Wherry described what he believed to be a new species of fern, Stotler's spleenwort (A. stotleri) (after T.C. Stotler, its discoverer). Wherry believed it to be the hybrid of lobed spleenwort (A. pinnatifidum) and A. platyneuron.[10] However, it was later shown to be simply a form of A. bradleyi with rounded, rather than sharp, teeth.[4] A dwarfed form of A. bradleyi, with fronds about 1 centimeter (0.4 in) long, was discovered in Illinois by Wallace R. Weber and Robert H. Mohlenbrock. This form lacked dark color in the stipe and rachis except for the very base; some slightly larger specimens, with a 2-centimeter (0.8 in) frond, retained the normal coloration of these structures.[5]

Among fertile species, A. bradleyi most closely resembles its parent species A. montanum. Several characteristics exit to distinguish them: the pinnae of A. bradleyi are toothed and less deeply lobed or cut than A. montanum (where the pinnae are often fully cut to pinnules), the dark color of the stipe extends into the rachis,[11] the upper pinnae lack stems,[6] and the overall shape of the leaf blade is parallel-sided, rather than lance-shaped. A. bradleyi also shows some resemblance to black spleenwort (A. adiantum-nigrum) (although their ranges do not overlap). The latter may be identified by its distinctly triangular-shaped leaf blade, more deeply cut leaves (the pinnules of its basal pinnae are lobed), and enlarged basiscopic, rather than acroscopic, pinnules.[11]

Asplenium bradleyi is similar to two hybrid species of which it is a parent, Graves' spleenwort (A. gravesii), a hybrid with A. pinnatifidum, and Wherry's spleenwort (A. wherryi), a backcross with A. montanum. In A. gravesii, the dark color of the stipe ends at the base of the leaf blade, the pinnae are more shallowly lobed and the enlargement of acroscopic lobes or pinnules is less distinct, and the apical portion of the blade forms a long, tapering tip with slight lobes (as in A. pinnatifidum), rather than being cut into pinnae.[12] In addition to the general reduction of the toothiness of A. bradleyi, A. gravesii also shows faint winging along the stipe.[13] Likewise, in A. wherryi, the dark color of the stipe again ends at the base of the leaf blade, the overall shape of the blade tends to be more distinctly lance-shaped, and the fronds are somewhat more deeply cut than A. bradleyi, progressing from bipinnate in the lower half to pinnate-pinnatifid and finally pinnate at the apex.[14] Finally, the diploid hybrid A. montanum platyneuron, from which A. bradleyi arose by chromosome doubling, is essentially identical in appearance to A. bradleyi. On close examination, its spores are found to be abortive, and the sori are smaller and not do not become fused with each other as they grow, as they do in fertile A. bradleyi.[15]

While both Asa Gray and Eaton identified A. bradleyi as a hybrid intermediate between A. montanum and A. platyneuron, the English botanist R. Morton Middleton proposed in 1892 that it was identical or closely related to A. viride. This conclusion was based on the examination of forms growing in shade on the Cumberland Plateau which lacked color in the rachis, and was endorsed by contemporary Tennessee botanists such as Augustin Gattinger and Kirby Smith.[17][9] This was rebutted in 1893 by Amos A. Heller, who pointed out that most collections of A. bradleyi had a dark stipe and that it possessed an auricle (the acroscopic pinnule) which A. viride lacked. Instead, Heller perceived in some of his specimens from the lower Susquehanna River affinities to A. montanum and A. pinnatifidum.[b][18] Middleton, nevertheless, continued to maintain his theory of an affinity with A. viride, and speculated that A. bradleyi was not a hybrid, but an \"intermediate\" between A. viride and lanceolate spleenwort (A. obovatum ssp. lanceolatum).[9]

Edgar T. Wherry speculated at length on the hybrid origins of A. bradleyi and other Appalachian spleenworts in 1925, but the scheme he proposed was later found to be untenable, although he did recognize the contribution of A. platyneuron to its ancestry.[10][c] Herb Wagner, in 1953, suggested instead that it was the hybrid of A. montanum and A. platyneuron, noting that Eaton and W. N. Clute had already made tentative suggestions along those lines.[19][d] His cytological studies the following year showed that A. bradleyi was an allotetraploid, the product of hybridization between A. montanum and A. platyneuron to form a sterile diploid, followed by chromosome doubling that restored fertility.[20]

In addition to its parental species, A. bradleyi hybridizes with several other spleenworts. Its hybrid with A. pinnatifidum was recognized as such by William R. Maxon in 1918. He named it A. gravesii for its discoverer, Edward W. Graves.[13] It can also backcross with its parental species. Wherry collected specimens of A. bradleyi montanum from a cliff near Blairstown, New Jersey in 1935.[27] It is not thought to have been collected again until 1961, when it was described and named in Wherry's honor.[14] Specimens believed to be A. bradleyi platyneuron were collected at an early date at McCall's Ferry, along the Susquehanna River. The site of collection was submerged by the building of the Holtwood Dam.[28] A preliminary report of both diploid A. bradleyi and A. bradleyi platyneuron from Sequatchie County, Tennessee was made in 1989.[29]

One of the \"Appalachian spleenworts\", A. bradleyi can be found along the Appalachian Mountains from northern New Jersey and Pennsylvania southwest to Georgia and Alabama, and occasionally along the Ohio Valley to the Ozarks and Ouachitas, where it is found in Missouri, Arkansas, and eastern Oklahoma. Populations in Maryland and New York are considered historical.[30] Populations are generally scattered in the Appalachians, but more frequent in the Ozarks and Ouachitas.[2]

Bradley's spleenwort (Asplenium bradleyi) has a very spotty distribution over much of its range which includes the Appalachians and Ozark regions. The fern occurs from New Jersey and southern Pennsylvania, west to Illinois, and south to Oklahoma, Louisiana, and Georgia. Historically known from the Shawangunk Mountains of New York.

Bradley's spleenwort (Asplenium bradleyi) very closely resembles mountain spleenwort (A. montanum) and may reflect a hybridization of A. montanum and A. platyneuron. Bradley's spleenwort will have a blade that is oblong with nearly parallel sides. The blade of mountain spleenwort is much more triangular in shape, tapering to the apex.

Asplenium bradleyi, commonly known as Bradley's spleenwort or cliff spleenwort, is a rare epipetric fern of east-central North America. Named after Professor Frank Howe Bradley, who first collected it in Tennessee, it may be found infrequently throughout much of the Appalachian Mountains, the Ozarks, and the Ouachita Mountains, growing in small crevices on exposed sandstone cliffs. The species originated as a hybrid between mountain spleenwort (Asplenium montanum) and ebony spleenwort (Asplenium platyneuron); A. bradleyi originated when that...

Bradley's spleenwort (Asplenium bradleyi) is a small fern that grows on rocks. The leaves are generally lance-shaped, 2-17cm long, and divided into 5-15 pairs of leaflets, or pinnae. The pinnae are roughly triangular in shape and have toothed or lobed margins. Pinnae are largest at the base of the stem and gradually decrease in size as they approach the top of the stem. The lower half of the leaf stem is maroon-colored and the upper half is green. As in most ferns, the spores are produced on the underside of the leaves.

The PA Biological Survey (PABS) considers Bradley's spleenwort to be a species of special concern, based on the few occurrences that have been recently confirmed and its specialized habitat. It has a PA legal rarity status of Threatened and has been assigned a suggested rarity status of Endangered by PABS. Fewer than 10 populations, all with few individuals, are currently known from the state.

Because of its small populations in the state, Bradley's spleenwort is vulnerable even without such threats as indiscriminate spraying of herbicide, quarrying, invasive species, and forest fragmentation. 59ce067264

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