Menisorus, Alston, Alston

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 55-56

MENISORUS

Menisorus Alston, Bol. Soc. Brot. , sér. 2, 30:20. 1956.

— TYPE: Menisorus pauciflorus (Hook.) Alston [= Meniscium pauciflorum Hook. ]

For complete synonymy, see Holttum (1974a).

Etymology.— Gr. mene, moon + sorus, sorus, in reference to the crescent-moon-shaped, or lunulate, sori on the crossveins, similar to those of Meniscium .

Plants small to medium-sized, terrestrial, rheophytes of rocky streambeds in lowland tropical forests, or forest understory plants to ~ 1m tall; rhizomes erect, ascending, or short-creeping ( M.blastophorus ), rhizome scales caducous; fronds monomorphic, ascending or arching, pinnate to pinnate-pinnatifid; stipes stramineous; stipe scales appressed, sparse, glabrous, brown, ovate-deltate, to slightly elongate, apparently caducous; blades drying dark, sometimes grayish white abaxially in M. pauciflorus , proximal pinnae not or little-reduced, apex conform to subconform, frequently with proliferous buds, or persistent plantlets arising from distal rachis, just below frond apex; pinnae with crenate margins, obliquely dentate or shallowly lobed, bases truncate to narrowly cuneate; veins with one to several pairs anastomosing, these forming a zig-zag excurrent vein to sinus, with subsequent veins running along deep sinusmembrane; aerophores absent ( M. pauciflorus ) or elongate ( M. unitus ); indument abaxially and adaxially essentially lacking, or of short, erect hairs restricted to axes and occasional small scales on costae abaxially; pustules present in M. unitus ; sori exindusiate, round to elongate, discrete or confluent along arching cross-veins (meniscioid sori); sporangia glabrous; spores papillose or winged ( Holttum 1974a; Tryon & Lugardon 1991); x =36, diploid, based on a count of M. unitus . No hybrids are known.

Diagnosis.— Menisorus may be recognized by the combination of exindusiate sori, at least some of which are elongate along veins, and subapical proliferous buds. It shares these characteristics with Ampelopteris prolifera , but that monotypic genus may be distinguished by its forked hairs, indeterminate growth of frond apices, and stipitate glands on sporangial stalks.

Biogeography and ecology.— Menisorus is the only genus within Thelypteridaceae that is restricted to Africa. Its three constituent species are widespread in lowland and montane equatorial forests, up to about 2500 m, reaching Madagascar and the Comoros, and the Atlantic island of Bioko. Menisorus pauciflorus is quite distinctive and bears the narrow elongate pinnae with oblique costules characteristic of some other rheophytic, but distantly related, thelypteroids (e.g., Sphaerostephanos cataractorum , Menisciopsis boydiae , Fig. 5B ); its typical habitat is rocky, forested streambeds ( Lovett & Thomas 1986).

Taxonomic and phylogenetic studies.— Menisorus , segregated by Alston (1956) from Meniscium , was treated as a monotypic genus based solely on the highly distinctive M. pauciflorus . Until recently (Fawcett et al. in press) Menisorus remained one of only two of 337 genera of ferns (as recognized by PPG I 2016) yet to be included in a phylogenetic study—the last is Thysanosoria (Lomariopsidaceae) ( Chen et al. 2018). Based on molecular evidence, Pneumatopteris unita (Kunze) Holttum is sister to Menisorus pauciflorus , and the two are well-supported on a long branch with no close relatives; therefore, we treat them as congeners. Based on morphological similarity, we expand Menisorus to also include M. blastophorus , recognizing the genus to include three species. The phylogenetic position of Menisorus is along a grade of cyclosoroid genera, after the divergence of Pakau , a monotypic genus endemic to New Zealand and Australia (described herein) and before the divergence of Pelazoneuron , a New World genus of 16 species (elevated to generic status herein) ( Fig. 1 ).

Notes.— Thelypteridaceae is represented in Africa by only about 55 species ( Holttum 1974a), far fewer than in the Neotropics or southeastern Asia; however, these species represent more than a dozen major lineages within the family (Fawcett et al. in press), a pattern that may have been greatly accentuated by recent extinctions ( Aldasoro et al. 2004). The genus Pneumatopteris sensu Holttum has proven to be the most polyphyletic of his taxonomic concepts, resolving in 10 distinct lineages (see Pneumatopteris for further discussion). African species treated in Pneumatopteris by Holttum (1974a) resolve in clades corresponding to Abacopteris , Christella , Menisorus , Pseudocyclosorus , or Reholttumia . While we have made new combinations for some of these, we remain uncertain about the placement of others, pending further study, and we treat two species which may have close affinities to Menisorus as incertae sedis.

Constituent species.— Menisorus blastophorus (Alston) S.E. Fawc. & A.R. Sm. ; * M. pauciflorus (Hook.) Alston ; * M. unitus (Kunze) S.E. Fawc. & A.R. Sm.

Incertae sedis.—We suspect the following species may be most appropriately treated in Menisorus but refrain from making combinations pending further study; the Madagascar and Comoros endemic Pneumatopteris subpennigera (C. Chr.) Holttum , and P. oppositifolia (Hook.) Holttum , an endemic to the islands of Bioko, São Tomé, and Annobón, both of which resemble M. unita , but lack proliferous buds.

Figures

FIG.5.A.Menisciopsiscyatheoides,Takeuchi1719 (UC),pinna lobes,abaxial view.B.M.boydiae,Baldwin116 (US),habit.C.Menisciopsisrubrinervis,Fawcett716 (VT), pinna,abaxial view.Abacopteris aspera,James 1693 (VT).D. pinna.E.pinna,abaxial view.

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