Reholttumia

Fawcett, Susan & Smith, Alan R., 2021, A Generic Classification of the Thelypteridaceae, Fort Worth, Texas, USA: BRIT Press : 76-77

REHOLTTUMIA

Reholttumia S.E. Fawc. & A.R.Sm. , gen. nov. — TYPE: Reholttumia nitidula (C.Presl) S.E. Fawc.& A.R.Sm. [= Nephrodium nitidulum C.Presl ]

For species synonymy, see Holttum (1973a, 1974a, 1977b, 1982).

Etymology.— The name honors Richard Eric Holttum (1895–1990), whose keen insights, perseverance, and fundamental contributions to our knowledge of Thelypteridaceae enabled the present work. Holttum’s focus was especially on species in the Old World tropics where the family is most diverse and least known. His careful examination of type material in numerous major herbaria resolved many problems that had plagued the taxonomy of the group for more than a century.

Plants terrestrial, medium-sized to large (to 2 m tall); rhizomes ascending, erect, short-creeping or rarely long-creeping, rhizome scales brown, often caducous; fronds monomorphic, erect to arching, rarely pendant, pinnate-pinnatifid; stipes stramineous, less often brown; stipe scales thick, brown, broadly ovate to linear-lanceolate, glabrous or with marginal setae, apparently caducous, rarely extending to rachis ( R. rodigasiana ); blades membranaceous to chartaceous, drying green, sometimes brown-olivaceous, but never reddish, blade apex gradually reduced, proximal pinnae not reduced, or gradually to subabruptly reduced to small, sometimes auriculate pinnae, the smallest typically at least 1 cm long, rarely less than 5 mm, proliferous buds absent; pinnae sessile to subsessile, pinna-bases truncate, margins usually incised 1/3–3/5 to costae, segment apices typically rounded, sometimes truncate ( R. truncata ), rarely acute ( R.costata ), proximal pinnae gradually tapered towards bases or with acroscopic auricles; veins of one to three pairs anastomosing to form an excurrent veinlet running to sinus membrane, or ending below it ( R. ecallosa ), subsequent veins ending at margin above sinus; aerophores absent, or present as swelling or discoloration at pinna-base, never peg-like; indument abaxially of short, erect hairs sometimes present on axes, laminar tissue typically glabrous, but erect acicular hairs sometimes present ( R. ecallosa ); indument adaxially often restricted to antrorsely arching hyaline or reddish hairs along rachis and costae, but short-erect hairs sometimes present on axes; pustules sometimes present on adaxial laminae, these large and irregular or minute, colorless, and glandular ( R. ecallosa ); sori round, discrete, typically medial on veins, sometimes inframedial, almost always indusiate (except R. costata sometimes exindusiate), though shriveled indusia in mature sori may be obscure, indusia dark brown, glabrous or with acicular hairs; sporangia glabrous, or with stipitate glands on capsules, these usually colorless, rarely bearing setulae (except R. oxyoura ); spores tan to brown, perine with tubercles or robust echinae ( Patel et al. 2019a; Tryon & Lugardon 1991); x = 36, based on five species counts, mostly diploid, but R. truncata has both diploid and tetraploid cytotypes. No interspecific or intergeneric hybrids are known.

Diagnosis.— Reholttumia is distinguished from Pneumatopteris s.s. by having inconspicuous or swollen (vs. peglike) aerophores; segment apices typically rounded or truncate (vs. broadly acute); sporangia sometimes bearing stipitate glands, but rarely setulose; proximal pinnae gradually or subabruptly (vs. abruptly) reduced, rarely reduced to less than 5 mm long; and laminae drying green, or brown-olivaceous (vs. sometimes reddish). The pinna segments of Reholttumia typically have about 7 veins per cm (vs. about 11 veins per cm in Pneumatopteris s.s.), but species with smaller proportions, and narrower pinnae (e.g., R. macroptera , R. kerintjiensis ) may have as many as 11 veins per cm. Another species, R. laevis , is atypical in the genus in having oblique, asymmetric pinnae, small stature, and a creeping rhizome; it is also the earliest-diverging species in our sample. Christella differs by generally having abundant hairs on axes and laminae, and often by having characteristic pear-shaped orangish laminar and sporangial glands; it also lacks laminar pustules. Sphaerostephanos differs in sometimes having yellow, sessile spherical glands. Plesioneuron differs by having dark (vs. tan) spores, and pinnae usually incised nearly to the costae.

Biogeography and ecology.—The 30 species of Reholttumia occur in forests and openings, often beside streams, generally at lower elevations, but some species reach about 2300 m. The greatest species diversity is in Malesia, but a dozen species occur in the Pacific and Australasia ( Holttum 1977b), with one species reaching the Hawaiian Islands ( R. hudsoniana ). The western range of the genus nearly corresponds to that of its most widespread and highly variable species, R. truncata , which occurs in Australia, China, India, Japan, Laos, Malaysia, Malesia, Myanmar, Sri Lanka, Thailand, and Vietnam. The genus is represented on Madagascar by at least one species, R. remotipinna .

Taxonomic and phylogenetic studies.—All constituent species of Reholttumia were treated within Pneumatopteris by Holttum (1971, 1973a, 1974a, 1977b, 1982). However, his taxonomic concept of Pneumatopteris has proven to be extremely broad and heterogeneous, as it encompasses species from ten distinct lineages, which we treat herein within existing genera or in newly described genera (see Pneumatopteris for further discussion). Because the type of Pneumatopteris , P.callosa , is in a small clade that is sister to Sphaerostephanos (a genus of about 200 species; Fig. 1 ), we recognize Pneumatopteris in a greatly restricted sense, and segregate Reholttumia —a morphologically coherent and large clade that is sister to Pneumatopteris s.s. plus Sphaerostephanos ( Fig. 1 ; Fawcett et al. in press). This new genus represents the largest segregate of Pneumatopteris sensu Holttum. In our phylogenetic analyses, Reholttumia is represented by 25 accessions, including 13 of 30 currently recognized species, and several undescribed species (Fawcett et al. in press).

Figures

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