Plesioneuron, Holttum, Holttum

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 65-67

publication ID	https://doi.org/ 10.17348/jbrit.v15.i2.1206
persistent identifier	https://treatment.plazi.org/id/03B787F6-FFA2-9B06-6216-78B6FB74FEEC
treatment provided by	Donat
scientific name	Plesioneuron
status

Treatment

PLESIONEURON View in CoL

Plesioneuron Holttum, Blumea View in CoL 22:232. 1975.

— TYPE: Plesioneuron tuberculatum (Ces.) Holttum View in CoL [= Nephrodium tuberculatum Ces. View in CoL ]

Mesophlebion subg. Plesioneuron Holttum View in CoL

For additional generic synonymy, see Holttum (1975, 1982, 1977b).

Etymology.— Gr. plesio, near + neuron, vein, the genus was segregated from Mesophlebion , from which it is distinguished by the basal basiscopic vein never arising far from its costule ( Fig. 6A View FIG ).

Plants terrestrial or epilithic, small to large (20–300 cm); rhizomes creeping to erect; fronds monomorphic, erect to pendent; stipes stramineous to dull brown, with ovate-lanceolate to linear, medium- to dark brown scales, these typically thin, sometimes thickened (e.g., P. costulisorum ) at the base of stipe and rhizome apex, sometimes essentially lacking; dark projecting spines sometimes present along stipes (e.g., P. dryopteroideum ); blades chartaceous to stiffly coriaceous, often drying dull-green or sometimes pale grayish, typically pinnate-pinnatisect, less often pinnate-pinnatifid, rarely pinnatifid ( P. fulgens ), proximal pinnae generally abruptly or subabruptly reduced, often deflexed, rarely gradually reduced (e.g., P. imbricatum ), apex conform or gradually reduced, proliferous buds lacking; pinnae straight or falcate, sessile or petiolulate, typically strongly asymmetrical at the base, excavate basiscopically (e.g., P. prenticei ), cuneate, rounded, or truncate, often with free basal lobe, but lacking expanded auricles; pinna apices frequently acuminate or caudate; sinus membranes often thickened, raised, and bearing hairs, forming a hairy ridge; veins generally free, running to the margin above the sinus or alongside sinus ridges, not forming areoles, basal basiscopic vein arising from or near costule; aerophores present as swollen discoloration, elongate in a few species (e.g., P. croftii ); indument abaxially of short, unicellular, hyaline acicular hairs, usually restricted to veins and costae, sometimes also on laminar tissue, rarely lacking (e.g., P. tuberculatum ), glands sometimes present, these yellow and spherical, sessile, or stipitate (e.g., P. subglabrum ), scales occasionally present on costae; indument adaxially typically restricted to veins and costae, of hyaline acicular hairs and/or antrorsely arching reddish hairs along rachises and costae; pustules sometimes present on laminar tissue adaxially or abaxially ( P.prenticei ); sori round, discrete, typically medial or inframedial, usually indusiate, indusia typically large, dark, and persistent, glabrous or with hairs or glands; sporangia unadorned, or with spherical amber or yellow glands (e.g., P.hopeanum , P.imbricatum , Fig. 2I View FIG ), or setulae; spores often black, often minutely spinulose (echinate) or occasionally winged; x = 36, all spp. diploid based on counts from four species, P.fulgens (n =36), P.caudatum (2 n = 72), P. keysserianum (n = 36), P.wantotense (n = 36). No hybrids are known.

Diagnosis.— Plesioneuron , which is predominantly found in New Guinea and the western Pacific, differs from the strictly Malesian Mesophlebion by the proximity of the origin of the basal basiscopic vein to the costule ( Fig. 6 View FIG ) (vs. arising far from it; Fig. 9C View FIG ), the presence of glandular or setulose sporangia (but never a reddish gland on sporangial stalks), and stiff, terete, spine-like scales along the stipe and rachis. These spines are often black and are also borne by species of Chingia , which differ from Plesioneuron in having less divided pinnae, veins generally anastomosing (vs. free), and by sori often inframedial or along costules (vs. medial to inframarginal; Holttum 1975). Plesioneuron may generally be distinguished from Pneumatopteris s.s., Reholttumia , and Sphaerostephanos by its pinnate-pinnatisect frond division, frequently asymmetrical pinna-bases, and free veins. Its larger stature and coarser laminae distinguish it from Coryphopteris and Old World Amauropelta , which also have free veins.

Biogeography and ecology.— Totalling 60 species, Plesioneuron reaches its greatest diversity in the mountains of Papua New Guinea. A few species extend north and west in Malesia, and ca. 12 spp. ( Holttum 1977b) occur in the Solomon Islands, Ponape, and northern Queensland to Melanesia ( Fiji) and Polynesia from Samoa, the Society Islands, and the Marquesas Islands. The genus appears to be absent from mainland Asia (including India, China, Thailand, and Vietnam), as well as from peninsular Malaysia, and western Malesia (Sumatra, Java), and has only a single widespread species in the Philippines ( P. savaiense ). Species are epipetric or terrestrial and occur at low to middle elevations, but occasionally reach ~ 3000 m ( Holttum 1982). They are found in forests, often along streams, and many species occur on limestone.

Taxonomic and phylogenetic studies.—Originally described as a subgenus of Mesophlebion ( Holttum 1971) , Plesioneuron was later elevated to generic status by Holttum (1975), who noted that it seemed to be most closely related to Chingia , an idea supported by morphological data and, now, molecular data.As treated by Holttum (1982), Plesioneuron comprised 49 species. We transfer one unusual species with bipinnate fronds, Plesioneuron marattioides , to Chingia based on morphology and molecular data (Fawcett et al. in press) and expand the genus to 60 by transferring thirteen species treated in Pneumatopteris by Holttum (1977b, 1982), and one by Takeuchi (2007), to Plesioneuron (listed below). These are almost entirely free-veined species,keyed by Holttum(1982:418) in the second half of his key to Pneumatopteris , couplet 1, and also in the second half of his key to Pneumatopteris ( Holttum 1973a) . A study of the pteridophyte flora of the east Indonesian islands of Ambon and Seram in the mid-1980’s yielded two additional species and a variety to the genus ( Kato 2007).

The eighteen samples, representing 15 of the 60 species of Plesioneuron (Fawcett et al. in press) are well-supported as monophyletic, and weakly supported as sister to Chingia . Together, these two genera form a clade with Menisciopsis , Grypothrix , and Mesopteris s.l. While each of these genera is well-supported as monophyletic, backbone support among these genera is weak.

Notes.— Plesioneuron is one of the least-studied genera of Thelypteridaceae , with many species known only from the type or a few collections. New Guinea, the center of diversity for the genus, remains poorly collected. A diverse and unusual group of diminutive epilithic calciphiles occurs at higher elevation karst terrains in Papua New Guinea and were treated within Pneumatopteris by Holttum (1982) (see incertae sedis under Pneumatopteris for further discussion, and an enumeration of these taxa), but may be most closely allied with Plesioneuron . We refrain from assigning these species to a genus, due to lack of material for both morphological and molecular study. Although several of these taxa share the asymmetrical pinna-bases, pinnatisect laminae, and free veins, some differ from typical Plesioneuron by proximal pinnae gradually reduced, and pinnae shallowly lobed, with a few anastomosing pairs of veins (vs. all veins free).

Incertae sedis.— Trigonospora koordersii (Christ) Holttum was initially included in Mesophlebion subg. Plesioneuron ( Holttum 1971) , but later Holttum (1974c) decided that this taxon shared a close resemblance to Trigonospora calcarata . This plant is known only from the type collection (L, image!), and it is unclear whether that specimen may yield spores that could verify its placement in Trigonospora . At the present time, we cannot rule out its placement in Plesioneuron .