Mesophlebion (Holttum, 1971)

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 56-57

MESOPHLEBION

Mesophlebion Holttum, Blumea 19:29. 1971, a renaming of Mesoneuron Ching (1963), non Mezonevron Desf. (1818).

— TYPE: Mesophlebion crassifolium (Blume) Holttum [= Aspidium crassifolium Blume ]

For additional synonymy, see Holttum (1971, 1982).

Etymology.— Gr. meso, middle + phlebion, vein, in reference to basal basiscopic veins arising from costae between costules ( Fig. 9C ).

Plants terrestrial, mostly in wet forests, medium-sized; rhizomes short- to long-creeping, at apices with rather rigid (but never spine-like) dark scales bearing short acicular hairs; fronds mostly 35–150 cm tall, monomorphic to dimorphic ( M.oligodictyon ); stipes green when living, stramineous or reddish-flushed with age, scaly primarily at bases (except M.echinatum and a few other spp.), scales tan to brown, mostly 2–10 mm long, lanceolate, bearing short acicular hairs on margins and on surfaces; blades usually pinnate-pinnatifid with proximal pinnae not reduced, apices confluent and gradually tapering to subconform, or rarely, blades simple ( M. oligodictyon ); pinnae opposite or subopposite, often becoming alternate distally, shallowly to usually deeply lobed (to within 2 mm of costae); proximal pair narrowed at their bases, sessile to long-petiolulate (to 2 cm); rachises hairy to sometimes glabrescent, hairs acicular, sometimes with scales similar to those of stipes proximally, but scales shorter, blades lacking vegetative buds; veins free, rarely weakly united ( M. oligodictyon ), usually connivent at or just below the sinuses and touching sides of an often hairy ridge below sinus, simple in ultimate segments, ending at segment margins, basal basiscopic veins arising from costa midway between costules ( Fig. 9C ), near base of costule, or from costules (in smaller spp., e.g., M. arenicola ) of ultimate segments; costae adaxially prominent, grooved; aerophores sometimes swollen on living fronds, collapsing on drying, never peg-like; indument abaxially on axes (rachises, costae, costules, and sometimes veins) of acicular, unicellular hairs, laminar tissue between veins usually glabrous, lacking stipitate glands, sometimes with spherical or hemispherical glands, scales sometimes present abaxially along costae; indument adaxially only along rachis and costae, of acicular hairs; pustules absent; sori round to sometimes elongate along veins, medial, indusiate (except M. oligodictyon ), indusia round-reniform, thin, small to large, often caducous, glabrous or with short acicular setae; sporangia shortstalked, young ones often pale violet, glabrous or usually bearing a reddish or orangish stipitate spherical gland from the sporangial stalk, capsules lacking glands and hairs; spores dark brown, with long, continuous wings and a few crosswings, ridges, or minutely papillose; x = 36 (3 species counted, both diploids and tetraploids); no intergeneric or infrageneric hybrids are known.

Diagnosis.—The genus is generally relatively easy to recognize because of the deltate pinnate-pinnatifid blades that are widest at the base, dark green thick-herbaceous blades, creeping rhizomes, free veins connivent at or near sinuses, and presence of large, stipitate, reddish spherical glands on the sporangial stalks, a characteristic shared with its closest relatives, Cyclosorus s.s. and Ampelopteris . Mesophlebion is easily distinguished from the latter by the more deeply incised pinnae, and the non-proliferous fronds. From Cyclosorus , Mesophlebion is distinguished by the free veins, shorter-creeping rhizomes, and forest habitat (vs. fresh-water marshes and swamps, often in partial to full sun).

Biogeography and ecology.— Mesophlebion comprises ca. 17 species, distributed from Myanmar and peninsular Thailand and Vietnam throughout Malesia, except eastern Java and the Lesser Sunda Islands ( Holttum 1982); it is absent from Melanesia and Polynesia, and represented in Australasia by three species in New Guinea. Nearly half of the species are rare in collections, and the greatest diversity is in Borneo, with 13 species. Species are generally found in low- to mid-elevation forests, or among rocks in riverbeds, from 0–1800 m. Species seldom occur in open habitats, except for M. arenicola and M. teuscheri , which grow on wet sandstone in exposed places ( Holttum 1982); as a likely consequence of this nutrient-deficient habitat, these two species are smaller (<50 cm) than others in the genus. One other small species, M. oligodictyon , differs from others in the genus in its dimorphic blades, lack of acicular hairs abaxially, and irregularly anastomosing veins, somewhat resembling species of the Sphaerostephanos beccarianus group ( Fig. 7A ). Holttum treated it in Mesophlebion based on the stipitate gland on sporangial stalk, and venation; its placement needs verification.

Taxonomic and phylogenetic studies.—The genus Mesoneuron was published by Ching (1963) in his treatment of mainland Asian Thelypteridaceae . Because of its similarity to Mezonevron Desv. (a synonym of Caesalpinia, Fabaceae), Holttum (1971) published the replacement name Mesophlebion . Initially, Holttum (1971) included the genus Plesioneuron as a subgenus of Mesophlebion , but later Holttum (1975) accorded Plesioneuron generic rank. Prior to that, Christensen (1929) recognized species of Mesophlebion as a distinct group, in Dryopteris , and compared them to the neotropical group Dryopteris subg. Steiropteris (herein recognized as Steiropteris ), which they superficially resemble in venation, blade architecture, and blade texture ( Fig. 9 ). Holttum (1982) commented on the pale violet-purple color of young sporangia (and sometimes indusia) of some species, a character observed by us in this genus and also in Steiropteris . The cause of this coloration, or its taxonomic importance, is unknown. Certain species complexes, especially M. crassifolium , M. chlamydophorum , M. motleyanum , and close allies, are quite variable, and circumscription of taxa requires re-examination and detailed field study, a situation noticed by Holttum (1982) and by us.

Sequence data show that Mesophlebion is most closely related to Cyclosorus s.s. and Ampelopteris , the three genera forming a clade that is sister to the neotropical genus Meniscium ( He & Zhang 2012; Almeida et al. 2016; Fawcett et al. in press). The relationship of Mesophlebion to Steiropteris and Plesioneuron , however, is more remote ( He & Zhang 2012; Almeida et al. 2016; Fawcett et al. in press).

Constituent species.— Mesophlebion arenicola Holttum ; M. auriculiferum (Alderw.) Holttum ; M. beccarianum (Ces.) Holttum ; M. caroli Holttum ; * M. chlamydophorum (Rosenst. ex C. Chr.) Holttum ; * M. crassifolium (Blume) Holttum ; M. dulitense Holttum ; * M. echinatum (Mett.) Holttum ; M. endertii (C. Chr.) Holttum ; M. falcatilobum Holttum ; M. hallieri (Christ) Holttum ; * M.motleyanum (Hook. ex Hook. & Baker) Holttum ; M. oligodictyon (Baker) Holttum ; * M.persquamiferum (Alderw.) Holttum ; * M. rufescens Holttum ; M. teuscheri (Alderw.) Holttum ; * M. trichopodum (C. Chr.) Holttum

Figures

FIG. 9.Steiropteris deltoidea,Fawcett 464 (VT) A. habit.B. pinna lobes,adaxial view.C.Mesophlebion sp., Karger 1796 (VT),pinna lobes,abaxial view.

FIG.7.A.Pneumatopteris parksii,Fawcett 645 (UC),pinna lobes,adaxial view.B.Reholttumia magnifica,Fawcett 676 (VT),pinna lobes,abaxial view.C.Pelazoneuron patens,Fawcett 744 (VT), abaxial pinna-lobes.D. P.serra,Fawcett 1034 (VT), adaxial pinna lobes.E. Pakau pennigera, Given 11842 (VT),pinna lobes,abaxial view. F.Pakau pennigera, Stuart s.n.(VT),pinna.