Sp. Pl. 2: 1082. 1753; Gen. Pl. ed. 5, 485, 1754.
Plants on rock, occasionally terrestrial or epiphytic. Stems creeping, usually branched, 3–15 mm diam., sometimes whitish pruinose; scales concolored to bicolored, lanceolate to ovate-acuminate, not clathrate to strongly clathrate, glabrous, margins entire to denticulate. Leaves monomorphic, closely spaced to distant, not conspicuously narrowed at tip, to 90 cm. Petiole articulate to stem, straw-colored, somewhat flattened or grooved to nearly terete, winged distally. Blade broadly ovate to deltate, pinnatifid to 1-pinnate at base, not pectinate, usually with fewer than 25 pairs of pinnae, not glaucous or conspicuously scaly; rachis sparsely scaly to glabrescent abaxially, puberulent to glabrous adaxially; scales ovate-lanceolate to linear, not peltate or clathrate. Segments linear to oblong; margins entire to serrate; apex rounded to attenuate. Venation free to anastomosing, if strongly anastomosing, then never with more than 1 included veinlet in fertile areoles. Sori often confined to distal region of leaf, discrete, circular to oval when immature, borne at tips of single veins, in 1–3 rows on either side of midrib; indument absent or of modified sporangia (sporangiasters), often bearing glandular hairs on bulbous head. Spores monolete, rugose to tuberculate. x = 37.
Some species traditionally included in Polypodium are treated here in other genera, for example, Pleopeltis and Pecluma.
Except for the tropical species Polypodium triseriale, North American Polypodium is a complex assemblage of interactive species. The North American species have ties to European taxa (e.g., P. vulgare sensu stricto, which probably originated by allopolyploidy between P. glycyrrhiza and P. sibiricum) but are quite distinct from them. Morphologic comparisons and continuing biochemical and molecular studies indicate that two groups of diploid species occur within the North American P. vulgare complex. One group includes P. glycyrrhiza and P. californicum; the second, P. amorphum, P. appalachianum, and P. sibiricum. Allopolyploid species have originated following hybridizations within a species group (i.e., P. calirhiza from P. glycyrrhiza × californicum, P. saximontanum from P. amorphum × sibiricum, and P. virginianum from P. appalachianum × sibiricum) as well as between members of the two groups (i.e., P. hesperium from P. amorphum × glycyrrhiza). These reticulate relationships are summarized in the reticulogram. We consider P. scouleri to be peripheral to the "core" diploids even though hybrids have been reported.
We have not included the European Polypodium cambricum Linnaeus [P. australe Fée], reported from San Clemente Island, California (R. M. Lloyd and J. E. Hohn 1969), in the North American flora because, since the single, original collection, efforts to relocate specimens in nature have failed (R. M. Lloyd et al. 1992).
Because taste is a characteristic used in the descriptions, the reader is cautioned to taste clean rhizomes from uncontaminated soils.
Species ca. 100 (11 in the flora).
|1||Leaves 1-pinnate at base; venation anastomosing, forming 2–3 or more rows of areoles on both sides of costae; sori in 2–3 rows on both sides of costae.||Polypodium triseriale|
|1||Leaves pinnatifid; venation free or, if anastomosing, only occasionally forming more than 1 row of areoles on both sides of costae; sori in 1 row on both sides of costae.||> 2|
|2||Mature blades leathery, stiff; sori more than 3 mm diam.; venation always anastomosing, forming 1 row of areoles; leaf segments more than 12 mm wide.||Polypodium scouleri|
|2||Mature blades herbaceous to leathery, rarely stiff; sori less than 3 mm diam.; venation free to anastomosing, occasionally forming 1 or more rows of areoles; leaf segments usually less than 12 mm wide.||> 3|
|3||Rachises and segment midribs puberulent adaxially; segment margins usually serrate.||> 4|
|3||Rachises and segment midribs essentially glabrous adaxially; segment margins entire or crenulate, but rarely serrate.||> 6|
|4||Scales on abaxial surface of rachises linear and hairlike, less than 3 cells wide; venation entirely free; stems intensely sweet, licorice-flavored.||Polypodium glycyrrhiza|
|4||Scales on abaxial surface of rachises narrowly lanceolate to ovate, usually more than 3 cells wide; venation often anastomosing; stems acrid, sweet, or bland, but not licorice-flavored.||> 5|
|5||Blade widest near base, proximal 1–3 pinnae equal to or longer than more distal pinnae; several areoles present on leaf segments; spores usually less than 58 µm; coastal California s of Humboldt County.||Polypodium californicum|
|5||Blade widest above base, proximal 1–3 pinnae shorter than more distal pinnae; areoles often absent from many leaf segments; spores usually more than 58 µm; California n of San Luis Obispo County to s Oregon.||Polypodium calirhiza|
|6||Sporangiasters absent (although occasional misshapen sporangia may be present); stem scales usually entire and symmetric; scales on abaxial surface of rachis linear-lanceolate, less than 6 cells wide; immature sori oval.||Polypodium hesperium|
|6||Sporangiasters (with bulbous heads with or without glandular hairs) present; stem scales usually coarsely toothed and contorted distally; scales on abaxial surface of rachis lanceolate-ovate, more than 6 cells wide; immature sori circular.||> 7|
|7||Sporangiasters bearing glandular hairs more than 40 per sorus; stem scales golden brown, nearly concolored; blades typically elongate-deltate, widest at or near base; tips of leaf segments acute to narrowly rounded.||Polypodium appalachianum|
|7||Sporangiasters (with or without glandular hairs) less common, fewer than 40 per sorus; at least a portion of each stem scale dark brown, often distinctly bicolored; blades typically oblong, widest near middle; tips of leaf segments rounded to obtuse.||> 8|
|8||Sporangiasters predominantly without glandular hairs; spores usually less than 52 µm; stem scales dark brown throughout or obscurely bicolored with lighter margins.||Polypodium sibiricum|
|8||Sporangiasters mostly bearing glandular hairs; spores usually more than 52 µm; some stem scales light brown or prominently bicolored.||> 9|
|9||Leaf blades more than 3.5 cm wide; e North America.||Polypodium virginianum|
|9||Leaf blades less than 3.5 cm wide; w North America.||> 10|
|10||Sporangiasters consistently bearing glandular hairs; spores rugose with small surface projections less than 3 µm tall; Cascade Ranges.||Polypodium amorphum|
|10||Sporangiasters bearing a few glandular hairs or occasionally lacking glands; spores with large tubercles, usually more than 3 µm tall; Rocky Mountains.||Polypodium saximontanum|