Homalomeneae (Araceae) of Borneo XXVIII — Homalomena bengohensis [Chamaecladon Clade] a new species endemic to the Bengoh Range, Sarawak

From Firenze University Press Journal: Journal of Plant Taxonomy and Geography (Webbia)

Wong Sin Yeng, Institute of Biodiversity and Environmental Conservation, Universiti Malaysia

Peter C. Boyce, Dipartimento di Biologia, Centro Studi Erbario Tropicale (herbarium FT), University of Florence

Homalomena Chamaecladon Clade (sensu Wong et al. 2013) with major centres of species diversity in Peninsular Malaysia and especially Sumatera is relatively poorly represented on Borneo. The first species described for NW Borneo was Homalomena paucinervia Ridl. (Ridley 1905). Since then, spe-cies names for the clade on Borneo have been comprehensively misapplied and only recently has the taxonomy begun to be addressed (Boyce et al. 2010; Wong & Boyce 2020, 2021; Wong et al. 2020). Problems stem partly from the species tending to be outwardly rather similar in appearance, especially as preserved specimens, and partly from most of the key diagnostic data present in the usually tiny blooms being lost in herbarium specimens. A further fac-tor is that wild plants are most often encountered sterile or with the blooms post-anthesis, by which time the staminate florets and the staminodes asso-ciated with the pistillate florets, the most useful diagnostic portions of the spadix, have rotted. Consequently, the only practicable means to study these plants is by a combination of fieldwork and maintaining living plants.

At present we recognize four described species for the Chamaecladon Clade in NW Borneo, two meso-phytes in humid forest belonging to the Griffithii com-plex — Homalomena acuminata (Ridl.) S.Y.Wong & P.C.Boyce and H. ridleyiS.Y.Wong & P.C.Boyce (Wong & Boyce 2020); and two rheophytes: H. paucinervia and H. santubongensisS.Y.Wong & P.C.Boyce (Wong & Boyce 2 0 21).Here we describe a novelty for NW Sarawak, Homalomena bengohensis, that most closely resembles H. paucinervia and H. santubongensis in the form of the blooms but differs by characteristics of the leaf blades and spadix as well as by occurring on earth banks along small streams although not as a rheophyte. Geology in this paper is specified based on Hutchinson (1989, 2005) and Tate (2001).

Diagnosis

Homalomena bengohensis is unique among Bornean Chamaecladon Clade species in the broadly elliptic, oblong-spathulate or spathulate leaf blades. The spadix most closely resembles that of H. santubongensis in the presence of well-developed staminodes but differs by the truncate base to the spathe and fully sessile spadix.

Description

Small aromatic (terpenoids) terrestrial mesophytes up to c. 15 cm tall. Stem epigeal, erect with internodes somewhat elongated, rooting from the lower-most nodes and through the petiole bases; roots c. 1–3 mm diam., tough, flexuous, medium brown, velvety. Leaves up to c. 10 together per shoot, petioles erect to spreading; petiole 4–10 cm long, c. 2 mm diam. midway, dorsally very nar-rowly channelled, dull medium green, glabrous; petiolar sheath 1.5–3 cm long, extending c. 1/4 length of the peti-ole, clasping at the base, width between both margins c. 1 mm, wings persistent; leaf blade broadly elliptic, oblong-spathulate or spathulate, 6–12 cm long by 2–3 cm wide, thinly coriaceous, semi-glossy, glabrous, medium green adaxially, abaxially paler matte green with the higher order veins darker-translucent, base cuneate, apex acute to acuminate with a brief (c. 2 mm long) tubular mucro, margins smooth; midrib adaxially slightly impressed, abaxially prominent; primary lateral veins 5–7 on each side of midrib, adaxially slightly impressed, abaxially alternating with much fainter much more numerous interprimaries, diverging at c. 40°–60° from the midrib; secondary and tertiary venation obscure; all veins run-ning into a very slightly thickened intramarginal vein. Blooms up to 3 together, produced sequentially in a sim-ple synflorescence; peduncle terete, slender, 4–5 cm long by c. 1 mm diam., coloured as for petiole; spathe ca 1.5 cm long, ca 5 mm wide across the truncate-gibbous base, limb dorsally incurved towards the tip, ventrally straight, not constricted, ventrally semiglossy pale green, dor-sally medium green, interior shiny pale green, apex with a terminal short brown mucro to 1.5 mm long, spathe inflating at anthesis and opening by a broad slit with the margins recurving and hyaline, later closing and enclos-ing the spadix, persisting until basal dehiscence at fruit dispersal. Spadix slightly exceeding spathe limb open-ing at anthesis, c. 1.6 cm long by c. 5 mm diam., sessile and obliquely inserted onto spathe; pistillate florets in two spirals, ovaries compressed-globose very slightly nar-rowed below the stigma; stigmas about half as wide as ovary, ca. 0.3 mm diam., disc-like, sessile; associated sta-minodes globose on a very slender stipe, sessile, cream, less than half as tall as the pistil; staminate zone c. 1 cm long, closely resembling the outline of the spathe with a straight ventral side and a curved dorsal side, apex acute; staminate florets each consisting of two stamens, anthers rounded, c. 0.5 mm tall, 1–1.5 mm long by 0.5–0.8 mm wide, creamy white with white tips; post anthesis blooms pendulous by bending of the peduncle. Infructescence ripening within the persistent spathe, ripe fruit and seed not observed.

Etymology

From Bengoh, plus Latin -ensis, to indicate originating from.

Distribution

Known only from the type locality where popula-tions are scattered.

Ecology

Banks of muddy streamlets leading into larger streams in rather open perhumid riverine forest on Palaeogene sandstones.

DOI: https://doi.org/10.36253/jopt-14154

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