Mesopteris

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 57-58

MESOPTERIS

Mesopteris Ching, Acta Phytotax. Sin. 16:22. 1978.

— TYPE: Mesopteris tonkinensis (C.Chr.) Ching [= Dryopteris tonkinensis C.Chr. ]

For additional synonymy, see Wang et al.(2015) and Holttum (1977a, 1982).

Etymology.— Gr. mesos, middle + pteris, fern. A genus endemic to China and neighboring northern Vietnam. Ching (1978) did not explicitly state why he chose Mesopteris as the name of his new genus, but in Mandarin the name for China is Zhōngguó (中國/中国), in English, Middle Kingdom.

Plants terrestrial, medium-sized (to ca. 2 m tall), of lowland tropical forests or disturbed sites, or on limestone; rhizomes long-creeping ( M. tonkinensis ) to erect ( M. paraphysophora ); fronds monomorphic, arching to erect, pinnate-pinnatifid; stipes dull brown to castaneous, rarely stramineous; stipe scales tan, glabrous, tortuous, linear-lanceolate; blades chartaceous to sub-coriaceous, often drying dark brown to reddish brown, darker adaxially and paler abaxially, apex conform to subconform, proximal pinnae not or little reduced, proliferous buds absent; pinnae short-petiolulate, with truncate to cuneate bases, proximal pinnae often narrowed towards their bases, not auriculate, margins dentate, with sinuses reaching ca. 1/3 towards costae, to deeply lobed to 1 mm from costae; veins often obscure adaxially except for swollen vein endings, free, connivent below sinus, or with one or more pairs anastomosing below it, veins terminating before reaching margins, or ending at margins, sinus membrane sometimes with tooth-like projection abaxially; aerophores absent; indument abaxially essentially lacking, or of short (<0.1 mm) hyaline acicular hairs on axes and laminae, and/or of minute spherical or stipitate glands, primarily restricted to axes, or red to amber resinous glands, which may be spherical and sessile, or viscid, sometimes abundant on laminae, these most easily observed on fresh material and not always apparent on dried specimens; indument adaxially essentially lacking, or of minute (<0.1 mm) hairs or minute colorless, golden brown glands primarily restricted to costae; pustules often present on laminar tissue abaxially; sori round, discrete or confluent ( M. attenuata ), medial, distributed along length of segments or mostly towards segment base (never restricted to segment apices), exindusiate or with small indusia, which may be glabrous, or obscured by abundant resinous glands ( M. attenuata , M. ceramica ); sporangia glabrous, or with glands on capsules or on stalks; spores dark brown to black, with broad, non-reticulating crests; x = 38, based on a count of M. tonkinensis provided by Wang et al. 2015. This is an anomalous and unexpected number within the family and needs verification; nearly all members of the cyclosoroid clade (which includes the greatest diversity in the family) share the synapomorphy of x = 36. No hybrids have been reported.

Diagnosis.— Mesopteris s.l. is distinguished from Amblovenatum s.s. by indusia small, absent, or obscured by copious resinous glands (vs. indusia conspicuous, and persistent); presence of red to amber-colored translucent resinous glands (vs. opaque, sulfur-colored to pale glands); indument on axes essentially lacking, or of short (<0.1 mm) stipitate-glandular and hyaline acicular hairs (vs. indument including long (0.5– 1 mm) hyaline acicular hairs); and laminae drying bicolorous, dark, often reddish (vs. concolorous green to pale olivaceous). Mesopteris differs from Grypothrix and Menisciopsis in having more deeply incised pinnae; it also lacks the stiff, thickened, sometimes spinelike scales often present on stipes in Chingia and Plesioneuron .

Biogeography and ecology.—The six species of Mesopteris , five treated in this genus here for the first time, occur in humid tropical forest to elevations of about 1000 m. In contrast to species of Amblovenatum s.s., several of which are extremely widespread, Mesopteris species tend to occupy more limited distributional ranges. The type species, M. tonkinensis , is restricted to the vicinity of the border area between China and Vietnam, where it occurs along roadsides and on limestone (Wang et al. 2015). The remaining species are primarily Malesian, with limited distributional ranges, extending as far east as the Solomon Islands.

Taxonomic and phylogenetic studies.—All species of Mesopteris were treated in Amphineuron (= Amblovenatum s.l.) by Holttum (1977a), a genus recognized by Ching but excluding A. tonkinensis , which he segregated in the monotypic genus Mesopteris in his classification of the ferns of China (1978). The Flora of China ( Lin et al. 2013) followed Ching (1978) in recognizing Mesopteris as monotypic, but Amphineuron s.l. was subsumed under Cyclosorus s.l.

The first molecular evidence that Amphineuron sensu Holttum (1977a, 1982) was not monophyletic was published by He and Zhang (2012). This result was corroborated by subsequent analyses, which further infer a close phylogenetic relationship between a Mesopteris tonkinensis collection from China and specimens of M. paraphysophora and M. pseudostenobasis from Malaysia and the Solomon Islands, respectively (Fawcett et al. in press). The three sampled species of Mesopteris are sister to a clade that includes the genera Grypothrix , Menisciopsis , Chingia , and Plesioneuron . See Amblovenatum for further discussion of taxonomy and phylogenetic relationships.