Strophocaulon, S.E. Fawc. & A.R. Sm., S. E. Fawc. & A. R. Sm.

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 87-88

STROPHOCAULON

Strophocaulon S.E. Fawc. & A.R. Sm. , gen. nov.

— TYPE: Strophocaulon unitum (L.) S. E. Fawc.& A. R.Sm. [= Polypodium unitum L.]

For additional species synonymy, see Holttum (1974a, 1977b, 1978, 1982).

Etymology.— Gr. strophos, twisted cord + caulon, stem, in reference to the tortuous, long-creeping subterranean rhizomes ( Fig. 10B ), which distinguish it from Sphaerostephanos .

Plants terrestrial, forming colonies in open sites, medium to large, to> 1.2 m tall; rhizomes thick (5+ mm), black, long-creeping, and subterranean, with internodes to 7+ cm long ( Fig. 10B ), bearing stramineous to brown, lanceolate, sometimes tortuous scales with setose margins; fronds arching to erect, monomorphic, pinnate-pinnatifid; stipes dull brown to dull stramineous; stipe scales lanceolate, stramineous, sometimes tortuous, bearing marginal setae; blades chartaceous to coriaceous, often bicolorous (drying paler below), often drying reddish-black adaxially, frond apex attenuate ( S. invisum ) or conform ( S. unitum , Fig. 10C ), frond base with pinnae abruptly reduced ( S. unitum ), with many pairs of pinnae reduced to auricles (sometimes only a few mm long), each subtended by an aerophore (auricles rarely lacking; Hayashi 2018), or blades truncate ( S. invisum ), proliferous buds absent; pinnae sessile to short-petiolulate, pinna bases truncate or broadly cuneate, sometimes slightly dilated; margins sharply dentate to incised to 1/3 towards costae; veins with at least one pair anastomosing at an angle <90 degrees, with an excurrent veinlet running to the sinus, with several pairs of veins forming areoles, running along a deep, narrow sinus membrane, lowest veins sometimes arising from costae, not from costules ( Fig. 10A ); aerophores inconspicuous; indument adaxially of hyaline, spreading, acicular hairs, these restricted to axes, or also present on laminar tissue; indument abaxially of hyaline acicular hairs present on all axes ( Fig. 10A ), frequently abundant on laminar tissue, often forming a tangled mat of whitish hairs along rachises, spherical yellow or brown glands present on laminar surface, restricted to veins, or glands absent; pustules absent; sori round, discrete or somewhat coalescent with age, supramedial to costular, indusia stramineous to dark brown, persistent, bearing superficial hairs; sporangia bearing spherical yellow glands ( S. unitum ) or setulae ( S. invisum ) on capsules; spores brown, perine of low knobby ridges (Patel et. al. 2019a); x = 36, only diploids known based on eight counts, representing both species. No intra- or intergeneric hybrids have been reported.

Diagnosis.—Although Holttum (1978) treated the two species of Strophocaulon in Sphaerostephanos , he noted that they were unique in three respects. Unlike most Sphaerostephanos species, which are narrowly restricted, both have broad geographic ranges, occur in open habitats (vs. forested habitats), and have long-creeping rhizomes (vs. generally short-creeping, suberect, or erect rhizomes). This last character is the most reliable for distinguishing Strophocaulon from Sphaerostephanos . Although at least three species of Sphaerostephanos , S. scandens Holttum , S. austerus (Brause) Holttum , and S.mundus (Rosenst.) Holttum , may have long internodes, they have scandent, not subterranean rhizomes. As noted by Holttum (1977b), three other species of Thelypteridaceae share the coarse, long-creeping rhizomes, preference for open habitats, and tendency to form large colonies. These species have often been confused, because of their similar habit, widespread overlapping distributions, and problematic nomenclatural history. Among these, Cyclosorus interruptus can be distinguished by scales on the abaxial costae of laminae, and Christella arida (D. Don) Holttum and Christella harveyi (Mett.) Holttum both bear elongate or pear-shaped (vs. spherical) glands on abaxial laminae and often on sporangial stalks (characteristic of many species of Christella s.s.), and sporangial capsules lacking both glands and setulae (vs. glands present in Strophocaulon unitum , and setulae present in Strophocaulon invisum ). As in S. unitum , bicolorous laminae abruptly reduced to many pairs of auriculate pinnae is a characteristic shared by species of Pneumatopteris s.s., but these species frequently have pustulate laminae, always bear peg-like aerophores, and, when glands are present, they are colorless, not yellow or brown. Both yellowish glands and abruptly reduced, auriculate pinnae are commonly shared by Sphaerostephanos s.s., but the rhizome morphology remains the most reliable character to distinguish them, illustrating the importance of complete herbarium specimens.

Biogeography and ecology.—Both species of Strophocaulon are widespread in the Paleotropics, and often occur in wet open areas, in partial sun, at low elevations; they frequently form extensive colonies near rivers or in disturbed sites, although S. unitum reaches elevations of 2000 m in New Guinea. Strophocaulon unitum is distributed in East Africa, in India, throughout Malesia, and into the Pacific, including the Mariana Islands, the Solomon Islands, New Caledonia, Fiji, and Samoa. Strophocaulon invisum has a partially overlapping distribution. It is widespread in the Pacific, and reaches its western range limit in Sulawesi, extending southward into New Guinea and Queensland ( Holttum 1977b).

Taxonomic and phylogenetic studies.—Both species of Strophocaulon were recognized in Sphaerostephanos by Holttum (1974a, 1977b, 1978, 1982), a genus with which they share several morphological features. Based on recent molecular analyses ( Patel et al. 2019a; Fawcett et al. in press), Strophocaulon is distantly related to Sphaerostephanos s.s., but has no close relatives; its two constituent species are on a long branch, with their position unresolved within the large christelloid clade. This clade includes Strophocaulon , plus two subclades, the pseudocyclosoroids ( Abacopteris , Amblovenatum s.s., Christella s.s., and Pseudocyclosorus ) and the sphaerostephanoids ( Pronephrium s.s., Pneumatopteris s.s., Reholttumia , and Sphaerostephanos ).

Notes.—Four varieties of Sphaerostephanos unitus have been described, but we make no combinations for them in Strophocaulon , pending more detailed study of the variation among these plants throughout their wide range. These include the unlocalized type, var. unitus , which exhibits brownish glands on and between veins, var. mucronatus (Christ) Holttum , which exhibits yellow glands restricted to veins, var. papilliferus Holttum , which has no glands, but a papillose lamina, and is restricted to higher elevations of New Guinea, and lastly, var. dimorphophyllus T. Hayashi et al. , which includes plants with fertile fronds bearing peg-like (non-laminar) aerophores, known from a single location in Borneo ( Hayashi et al. 2018). An unusual specimen from Singapore, M. Tan 2011174 ( VT), has a subconform terminal pinna, pinnae incised 2/5, large indusia, and ruby red glands on abaxial laminae.

Figures

FIG. 10.Strophocaulon unitum, Fawcett 641 (VT),A.pinna lobes,abaxial view.B. rhizome.C.laminar apex.