Flora of North America species comparison

The relationship of North American Asplenium ruta-muraria to its European counterparts is incompletely understood and bears further investigation. Based on features of the stems, M. L. Fernald (1928) segregated the North American taxon as A. cryptolepis, but most current authors agree that morphologic differentiation of North American and European material is too slight and inconsistent for recognition at the specific level. In Europe, two ploidy levels are treated as subspecies, diploid A. ruta-muraria subsp. dolomiticum Lovis & Reichstein and tetraploid A. ruta-muraria subsp. ruta-muraria, the latter representing the most compelling case for true autopolyploidy (i.e., based on chromosomal homology) known in ferns (G. Vida 1970). Chromosome counts of North American plants are consistently tetraploid; whether or not these plants are referable to subsp. ruta-muraria will remain unclear until additional evidence (e.g., isozymes) is obtained. Meanwhile, North American material should be designated simply as A. ruta-muraria, the convention used in most current manuals.

Although M. L. Fernald (1928) recognized Asplenium cryptolepis var. ohionis (= A. ruta-muraria var. subtenuifolium Christ), based on its acute rather than rounded segment apices, leaves assignable to this variety may occur on plants also bearing leaves more similar to those of the type variety (R. Cranfill 1980). The former is not recognized taxonomically here.

Numerous hybrids of Asplenium ruta-muraria with various taxa are known from Europe (T. Reichstein 1981), but only three are known from North America, all exceedingly rare. These are the hybrids with A. rhizophyllum (A. × inexpectatum), with A. trichomanes (A. × clermontae Syme), and with A. platyneuron (A. × morganii).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Members of this family can usually be identified by the combination of clathrate stem scales and indusiate linear sori. Supporting anatomic characteristics include the two vascular bundles in the petiole that unite distally in the petiole to form an X-shaped petiolar strand, and the single row of cells in the sporangial stalk. The scales consist of cells with dark, thick, radial walls and clear, thin, tangential walls, giving the scales a clathrate (latticelike) appearance reminiscent of lead moldings between plates of stained glass.

As construed here, Aspleniaceae comprise a single, huge, extremely diverse genus, Asplenium. A satisfactory taxonomic division into subgenera or satellite genera has not been possible because of the absence of any significant gaps. Various segregates have been proposed (e.g., Camptosorus, Phyllitis, Ceterach, Pleurosorus), but numerous "intergeneric" hybrids occur.

The members of Asplenium are popular with plant evolutionists, field naturalists, and fern gardeners, not only because of the interesting morphology of the plants but also because of their remarkable ability to form spectacular hybrids, often combining dramatically different leaf shapes. In North America, 23 diploid hybrids and allopolyploids have been recorded. At least two of these hybrid combinations occur as both sterile diploids and their fertile allotetraploid derivatives. Only those hybrids that are reproductively competent (through vigorous clone-forming by root proliferations or apogamy, or rarely through sexual reproduction) are treated in the key and fully described below.

Only about two-fifths of the reproductively competent species are believed to be cladistically divergent species; the other three-fifths are of hybrid origin (allopolyploids). For two of the allotetraploids, sterile diploids of the same parentage are also known. The most unusual allopolyploid phytogeographically is Asplenium adiantum-nigrum, the parents of which are known only in the Old World. These reticulate relationships are summarized in the reticulogram.

Polyploidy is widespread in Asplenium, and the chromosome numbers vary from 2 x to 6 x. Two species, Asplenium trichomanes and A. heterochroum, occur in different levels of polyploidy—2 x and 4 x, and 4 x and 6 x, respectively. The highest chromosome number known for Asplenium in North America is 2n = 216 (in A. trichomanes-dentatum and the hexaploid form of A. heterochroum). The only three apogamous taxa are A. monanthes (3 x), A. resiliens (3 x), and A. × heteroresiliens (5 x).

Genera 1, species ca. 700 (1 genus, 28 species, and 3 nothospecies in the flora).

(Discussion copyrighted by Flora of North America; reprinted with permission.)