Trigonospora, Holttum, Holttum

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 89-91

TRIGONOSPORA

Trigonospora Holttum, Blumea 19:29. 1971.

— TYPE: Trigonospora ciliata (Wall. ex Benth.) Holttum [= Aspidium ciliatum Wall. ex Benth. ]— Cyclosorus subg. Trigonospora (Holttum) Panigrahi — Thelypteris subg. Trigonospora (Holttum) Fraser-Jenk. — Thelypteris sect. Trigonospora (Holttum) Fraser-Jenk.

Etymology.— Latin trigona, three-angled + spora, spore; the genus is unusual among eupolypod ferns in having trilete tetrahedral spores ( Holttum 1971).

Plants terrestrial, small to medium-sized (typically 20 cm –1+ m), rheophytes of rocky streambeds or of forest understories; rhizomes short, caudices erect, frequently with a tangle of thick roots that may serve as a ‘holdfast’; fronds dimorphic, fertile fronds with longer stipes and more deeply lobed pinnae, or monomorphic, pinnate-pinnatifid to pinnate-pinnatisect, arching to erect; stipes dull stramineous to brown; stipe scales brown, caducous, often appressed, glabrous or with marginal hairs; blades chartaceous, drying green to black, proximal pinnae gradually reduced (e.g., T. calcarata ) or only slightly reduced and reflexed ( T. ciliata ), never abruptly reduced to a series of hastate auricles; blade apex gradually reduced; proliferous buds absent; pinnae nearly symmetrical to asymmetrical and strongly oblique; margins subentire with dentate margins ( T. khamptorum , T. zeylanica ) to deeply pinnatisect ( T. calcarata ), pinna bases sessile to short-petiolulate, truncate to narrowly cuneate, acroscopic segment sometimes slightly enlarged or auriculate, in T. angustifrons basal segment free and enlarged, overlapping rachis; veins free, simple, ending at sinus, or with a single pair anastomosing and an excurrent vein running to shallow sinus ( T. khamptora, T. obtusiloba , and T. zeylanica ); aerophores inconspicuous or absent; indument abaxially nearly absent or with hyaline acicular hairs restricted to axes; indument adaxially of hyaline acicular hairs most frequently along costae; T. glandulosa with pale yellow subsessile glands; pustules absent; sori typically round and discrete, medial or inframedial, indusia large and persistent, glabrous to copiously hairy, rarely with glands ( T. glandulosa ); sporangia glabrous, or each with a stipitate gland arising from stalk; spores tan, trilete and tetrahedral, globose or bilateral, with echinulate or minutely papillose perine; x = 36, five species counted, both diploids and triploids known. Sledge (1981) suggested some of the taxonomic confusion of the T. calcarata group may be the result of infrageneric hybridization, and also suggested a potential intergeneric hybrid with Christella parasitica ( Sledge 1981; Sledge 752).

Diagnosis.— Trigonospora is unique within the Thelypteridaceae in having trilete spores. However, it can also usually be distinguished from Pseudocyclosorus by having an erect rhizome (vs. often creeping); proximal pinnae little reduced or gradually reduced (vs. never reduced to auricles along stipes); and aerophores at pinna bases lacking or inconspicuous (vs. swollen). There is limited overlap in range or habitat with other free-veined Thelypteridaceae with persistent indusia, but Plesioneuron , which is most diverse in Papuasia, extends into western Malesia. Trigonospora differs from Plesioneuron by fronds usually drying dark (vs. dull green or pale); pinna bases sometimes auricled (vs. usually rounded or cuneate); and stipe scales thin and deciduous (vs. thick and persistent). The amauropeltoid genera ( Amauropelta , Metathelypteris , and Coryphopteris ) usually occur at higher elevations or more northern latitudes, and tend to have much more delicate laminae.

Biogeography and ecology. —The 12 species of Trigonospora are restricted to southeast Asia, and the genus is most diverse in south India and the island of Sri Lanka, where seven species occur, four of which are endemic. The Sri Lankan species are morphologically distinctive and occur at different elevational ranges, from low country to 2200 m ( Sledge 1981). The genus extends through the Pan-Himalayan region into southern continental Asia, Java, Malaysia, and Sumatra, and possibly into Sulawesi ( T. koordersii ; Holttum 1974c, 1982). In China T. ciliata extends north to Guangxi and Guangdong. Throughout their range they frequently occur in rocky riverbeds and streamsides.

Taxonomic and phylogenetic studies. —Initially included within Pseudocyclosorus by Ching (1963), Trigonospora was recognized by Holttum (1971) as a distinct genus on the basis of its trilete spores. As noted by Wagner (1974), this characteristic is unusual within what we now classify as eupolypod ferns. Although trilete spores have also been observed in Macrothelypteris torresiana ( Chandra 1973) , and species of grammitids (e.g., Alansmia) in Polypodiaceae ( Kessler et al. 2011), these may be spherical and/or with asymmetrical laesurae, and are unlike typical tetrahedral trilete spores. The same is sometimes true of spores in Trigonospora , which may be round or bilateral, occasionally occurring together with monolete spores ( Nayar & Chandra 1966). Based on our understanding of its phylogenetic position, the spore morphology of Trigonospora should be interpreted as a reversal ( Patel et al. 2019a), not as indication of a close relationship to other lineages with trilete spores, like Cyatheaceae, as Holttum (1971, 1974c) believed. Trilete spores are a diagnostic feature of the genus, however, and have been reported in several species, including T. ciliata , T. caudipinna , and T. khamptorum ( Holttum & Chandra 1971; Lin et al. 2013), and are verified here for T. calcarata, Gardette 559 ( UC); T.obtusifolia, Ballard 1393 ( UC); T. zeylanica, Ballard 1522 ( UC); and T. caudipinna, Mann s.n. ( UC).

He and Zhang (2012) and Patel et al. (2019a) inferred a close relationship between Trigonospora ciliata and Pseudocyclosorus , but with low support. The more densely sampled phylogeny of Fawcett et al. (in press.) recovers two Sri Lankan species of Trigonospora as sister to two morphologically distinctive taxa we determine as Pneumatopteris humbertii ( Holttum 1974a) and Pronephrium fidelei (Rakotondrainibe & Jouy 2012), both once-pinnate species of Madagascar. This clade is in turn sister to Pseudocyclosorus ( Fig. 1 ). As discussed in the Menisorus treatment, a combination of limited collections and geologically recent extinctions may have shaped our understanding of the Afro-Madagascan fern flora, sometimes leaving isolated lineages that are difficult to place taxonomically. In light of these difficulties, and our own limited sampling, we refrain from making new combinations for these non- Trigonospora pseudocyclosoroid taxa. Although our understanding of Trigonospora has benefited from excellent floristic treatments (e.g., Holttum 1974c, 1982; Sledge 1981), the genus has never been monographed throughout its range. A critical study of widespread and morphologically variable species such as T. ciliata and T. calcarata would be especially helpful for improving our understanding of the group.

Constituent species.— Trigonospora angustifrons Sledge ; T. calcarata (Blume) Holttum ; T. caudipinna (Ching) Holttum ; ** T. ciliata (Wall. ex Benth) Holttum ; T.glandulosa Sledge ; T.khamptorum (Holttum) Irudayaraj & Manickam ; T.loyalii Panigrahi & Sarn. Singh ; * T. obtusiloba Sledge ; T.sericea (J. Scott ex Bedd.) Holttum in Nayar and Kaur; T. subcaudipinna Sarn. Singh & Panigrahi ; T.tenera (Roxb.) Mazumdar ; * T. zeylanica Sledge.

Incertae sedis.— Trigonospora koordersii is a distinctive and unusual species, and known only from the type (Sulawesi), a considerable distance from the center of diversity of the lineage in Sri Lanka and southern India. It differs from all other Trigonospora species in having a conform terminal pinna, and is also distinctive in its elongate, nearly parallel-sided pinna segments or pinnules on an essentially bipinnate blade. This taxon was initially placed in Mesophlebion subg. Plesioneuron by Holttum (1971), though he later transferred it to Trigonospora ( Holttum 1974b) . Our concept of Plesioneuron is broader than Holttum’s, and we cannot rule out the possibility that T. koordersii belongs that genus.

For additional synonymy, see Holttum (1974c), Sledge (1981), Singh and Panigrahi (2005), and Lin et al. (2013).

Figures

FIG. 1. A synoptical treeillustrating relationships among genera of Thelypteridaceae based on Fawcett et al.(in press).