Stem erect to decumbent, small or typically long-creeping and exceedingly slender, sometimes branched, protostelic, and variably indurated, usually bearing short trichomes. Leaves minute to small (2–5 mm), though rarely extending to 2 m; entire to pinnate, glabrous to pubescent, thin, and devoid of stomata; circinate in bud except in the smallest forms; petiole lacking stipules. Sporangia are positioned in marginal sori on short to elongate receptacles, enclosed by a bivalved to tubular indusium, borne on a short, approximately six-rowed stalk, with an oblique annulus uninterrupted by the stalk. Plants are homosporous, producing chlorophyllous (green) spores. The gametophyte is epigeal, photosynthetic, narrowly thalloid or filamentous, freely branched and sometimes furnished with gemmae. Archegonia occur on the lower surface or in clusters on filamentous branches; antheridia consist of roughly five to numerous cells, mostly on the lower surface or along filamentous branches.
Plants are epiphytic, terrestrial, or lithophytic. Stems are long-creeping, frequently threadlike and intertwined, or short-erect, protostelic, and clothed in brown hairs of one or two types. Roots are sparse or absent. Leaves are small (0.5–20 × 0.2–5 cm) and often form dense mats. The petiole is short, threadlike to wiry, often winged for part or all of its length. Blades are ovate, oblong, or lanceolate, simple to highly divided, typically one cell thick between veins (except in Trichomanes membranaceum), entire or dentate; scales or simple and/or stellate hairs are common along veins or margins. Veins are free and divergent, occasionally appearing as unattached “false” veins. Sori occur marginally at vein termini, enclosed in two-valved or conic involucres. Sporangia are borne on moundlike receptacles or elongate bristles, sessile or short-stalked, with an oblique annulus. Spores are green, globose, and trilete. Gametophytes are filamentous or ribbonlike, often a combination of both, much-branched (0.2–1 cm), persistent, gemmiferous, and capable of forming clonal colonies through vegetative propagation.
The family comprises six genera and approximately 650 species (two genera and 11 species within the flora), found across wet tropical and subtropical regions, with a few extending into temperate zones. Species occurring outside the flora display a remarkable breadth of morphology and habit, many substantially larger than their North American counterparts. Some authors have subdivided the Hymenophyllaceae into more than 30 genera; these subdivisions are here treated as subgenera and sections, following C. V. Morton (1968).
Although members of the Hymenophyllaceae can tolerate periodic desiccation and freezing, their delicate constitution constrains them to deeply sheltered habitats with consistently high moisture and humidity. This restriction likely explains their rarity in the flora and suggests that their current distribution may represent remnants of broader pre-Pleistocene ranges. Their persistence relies heavily—often entirely—on vegetative propagation by either the sporophyte or gametophyte. The extensive capacity for vegetative reproduction and gametophytic dispersal enables these gametophytes to persist indefinitely without completing their life cycle. Within the flora, several species survive exclusively as gametophytes, with sporophytes seldom or never produced.
The filmy ferns (Hymenophyllaceae) encompass more than 600 species within two major lineages largely corresponding to the traditional genera Hymenophyllum and Trichomanes (Schuettpelz & Pryer, 2006). They are predominantly tropical, though scattered occurrences appear in temperate regions. The family occupies a near-basal position in several phylogenetic analyses (Schneider et al., 2004). Based on extant taxa, the group is thought to have originated in the Paleotropics, perhaps in Asia (Dubuisson et al., 2003), although fossils are known from North America and China. Their fossil record is sparse, owing largely to the fragile, membranous fronds that fossilize poorly and the potential misidentification of superficially similar fossil ferns (e.g., the dicksoniaceous genus Coniopteris).
The most compelling Mesozoic evidence for the family comes from Upper Triassic specimens from North Carolina (Axsmith et al., 2001). Hopetedia praetermissa exhibits tripinnate fronds with lobed pinnules and funnel-shaped indusia, each harboring five to eight obovate sporangia. The annulus ranges from oblique to vertical; no spores were preserved. Eogonocormus cretaceum and E. linearifolium from the Lower Cretaceous of northeastern China are diminutive, thalloid plants with creeping rhizomes. Their marginal sori with in situ spores, borne on fan-shaped pinnule lobes, convincingly assign them to the Hymenophyllaceae (Deng, 1997, 2002).
POLYPODIOPSIDA — LEPTOSPORANGIATE FERNS
The Polypodiopsida, or leptosporangiate ferns (also known as Filiopsida or Filicales), comprise the most diverse monilophyte lineage, with estimates ranging from at least 8,800 to more than 12,000 species.
Leptosporangiate ferns employ a specialized terminology (see Chapter 9), distinct from that used for other vascular plants (see Lellinger 2002 for an authoritative, multilingual reference). Sporophytes are predominantly perennial herbs or trees, although some aquatic members (Salviniales) may be annual. Most possess horizontally oriented rhizomes (Figure 4.28A,B), which may grow subterraneously or at the surface (terrestrial), within rock crevices (epipetric), in water (aquatic), or upon other plants (epiphytic). Some taxa are arborescent, with tall, erect aerial stems—as in tree ferns (Figure 4.40), which may reach 20 m (66 ft). A few species are vines (Figure 4.37), bearing weak stems or elongate, vine-like leaves that sprawl across the substrate or climb vegetation. Stem anatomy is often diagnostic and may exhibit ectophloic or amphiphloic siphonosteles (Figure 4.22A,B), dictyosteles (Figure 4.22C), or protosteles (Figure 4.7).
Hymenophyllaceae, the filmy fern family (order Hymenophyllales), comprises seven or more genera and roughly 600 species, distributed chiefly across the world’s tropical regions, with limited representation in temperate zones.
Members are diminutive and delicate, typically epiphytic. The fronds range from entire to intricately dissected and are distinguished by laminae only one cell thick between veins. Sori occur at segment tips or margins and are encased in cup-shaped to narrowly conical indusia, which open toward the frond margin and may be deeply lobed distally. Sporangia are either borne on a short receptacle within the indusium or on a long stalk that extends beyond the indusial mouth. Spores are globose, tetrahedral, and unusual for being green and metabolically active at the time of dispersal.
Despite their evolutionary distinctiveness, the filmy ferns possess a highly fragmentary fossil record due to their extreme delicacy. The oldest unequivocal fossils belong to Hopetedia praetermissa from the Triassic of North Carolina. Traditionally, the two central modern genera (Hymenophyllum and Trichomanes) have been separated primarily by soral morphology. Additional genera—Cardiomanes, Cephalomanes, Crepidomanes, Hymenoglossum, and Serpyllopsis—have been accepted on the basis of subtle and often inconsistent morphological characters. Molecular data suggest that many of these groups are not monophyletic, rendering the taxonomy unsettled.
In the eastern United States, the filmy fern Trichomanes intricatum is remarkable in persisting solely as isolated colonies of independent gametophytes. These gametophytes, inhabiting cool, deeply shaded microsites beneath overhanging cliffs and boulders, appear to have lost the capacity to produce sporophytes entirely; none are known from any location worldwide. The plants survive as algal-like filaments and reproduce asexually via minute gemmae dispersed by air currents. Even more extraordinary is the unrelated Appalachian shoestring fern (Vittaria appalachiana, Pteridaceae), which occupies comparable habitats within a similar range and likewise appears incapable of completing sexual reproduction to form sporophytes. Its closest living relatives occur in Asia and the tropics.
