Studies on Homalomeneae (Araceae) of Borneo XXVII: A new Homalomena [Chamaecladon Clade] endemic to the Santubong Peninsula

A new species of Homalomena Chamaecladon Clade is described from the Santubong Peninsula, Kuching Division, Sarawak, to where it is endemic, and compared with H. paucinervia from the nearby Matang Massif, and with H. atrox from Sri Aman, the two other most similar described species from NW Borneo. All three species are illustrated from living plants, and keyed-out.


INTRODUCTION
Homalomena is one of the largest genera of aroids occurring on Borneo, with 64 accepted species, and with at least twice that number yet to be described (Boyce & Croat 2011). Species of the Homalomena Chamaecladon clade (sensu Wong et al. 2013) are distinctive by the spathe lacking a constriction and by having pistillate florets wherein the staminode is much shorter than the pistil. Currently the clade consists about 140 published names, of which about 40 are of unresolved status and 19 are confirmed synonyms. The clade occurs from Sumatera though to New Guinea, and as far north as southern Indochina. The principle areas of species' richness and diversity are Peninsular Malaysia and, especially, Sumatera wherein the diversity of form, especially epidermis ornamentation reaches extraordinary extremes . It is puzzling that Borneo, an island with a staggeringly rich aroid flora, appears to be much less provided for in species and with much lower diversity when it comes to the Chamaecladon clade.
Species of the Homalomena Chamaecladon clade (sensu Wong et al. 2013) occurring in NW Borneo pose taxonomic problems in part owing to misapplication of West Malaysian species' names to undescribed Bornean species and in part because species' boundaries remain poorly understood. The first species for the clade described from NW Borneo was H. paucinervia Ridl. (Ridley 1905), described from riverside rocks of the Matang Massif near Kuching. After that, save Furtado's stumbling attempt to clarify the taxonomy for Homalomena (Furtado 1939), no further taxonomic progress was made until the publication of H. atrox P.C. Boyce et al. (Boyce et al. 2010 As has been noted in previous papers (e.g., Hay 1998, Kartini et al. 2015) the extraordinarily complex surface geology of the island of Borneo is enormously important in the separation of species in aroid genera such as Homalomena, Schismatoglottis, and Alocasia, among others. Geology in this paper is specified based on Hutchinson (1989Hutchinson ( , 2005 and Tate

Diagnosis
Homalomena santubognensis is most similar to H. paucinervia but readily distinguished by the overall stouter spadix (spadix width × length 4 mm × 1.9 cm [1:4.5] vs 3 mm × 1.95 mm [1:6.5]), by a stigma about half as wide as the ovary (vs equaling the ovary in width), by pistillate florets each with an oblong staminode (vs pistillate florets lacking staminodes), by petioles entirely green not stained deep red in the lower half, and by leaf blades abaxially semi-glossy pale green (not slightly glaucous). From H. atrox, the only other species in NW Borneo with lanceolate leaf blades, H. santubongensis is immediately distinct in being entirely glossyglabrous (not microscopically pubescent), and by the smooth (not corrugated petiolar sheath).

Description
Small strongly aromatic (terpenoids) Steensian rheophytes (Boyce and Wong 2019: 505 et seq.) up to c. 13 cm tall. Stem epigeal, erect and congested, in older plants occasionally becoming somewhat elongated and decumbent with the active tip ascending, rooting from the lower-most nodes and through the petiole bases; roots c. 1-3 mm diam., tough, flexuous, pale brown, slightly velvety, highly adhesive to substrate. Leaves up to c. 15 together per shoot, petioles erect to spreading; petiole 4-10 cm long, c. 2 mm diam. midway, dorsally very narrowly channelled, bright medium green, glossy glabrous; petiolar sheath 1.5-3 cm long, extending c. 1/4 length of the petiole, clasping at the base, width between both margins c. 1 mm, wings persistent; leaf blade narrowly elliptic to oblong-lanceolate, 6-12 cm long by 2-3 cm wide, thinly coriaceous, glossy, glabrous, medium green adaxially, abaxially paler green with the higher order veins darker- translucent, base cuneate, apex acute to acuminate with a brief (c. 1.5 mm long) tubular mucro, margins smooth; midrib adaxially slightly impressed, abaxially prominent; primary lateral veins 4-6 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 running into a very slightly thickened intramarginal vein. Blooms up to 5 together, produced sequentially in a simple synflorescence; peduncle terete, slender, 2-5 cm long by c. 1 mm diam., coloured as for petiole; spathe ellipsoid, not constricted, at anthesis 2.2 cm long by 5-8 mm wide, with a terminal short mucro to 1-2 mm long, exterior glossy dark green with faint paler speckling, interior glossy medium green, spathe gaping at anthesis with the margins recurving and opening to expose the pistillate florets, closing post staminate anthesis and persisting until basal dehiscence at fruit dispersal. Spadix slightly exceeding spathe limb opening at anthesis, c. 1.9 cm long by c. 5 mm diam., shortly stipitate, stipe stout, c. 3 mm diam.; pistillate florets in two or three (incomplete) spirals, ovaries oblong-globose; stigmas about half as wide as ovary, ca. 0.3 mm diam., disc-like, sessile; associated staminodes oblong, sessile, cream, about tall as the ovary; staminate zone c. 1.5 cm long, 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, dirty white with the tips pale; post anthesis blooms pendulous by bending of the peduncle. Infructescence ripening within the persistent spathe, exposed by the spathe shedding; fruit rather squat with the tops flattened by pressing on the spathe interior during development and the stigma impressed, whitish green with the stigma remnants brown, crushed fruits smelling faintly of overripe fruit/butyric acid; seed, very small, c. 1 mm long, oblong-ellipsoid, testa longitudinally ribbed.

Etymology
The species epithet is derived from the name of the type locality, plus the Latin suffix -ensis, indicating 'originating from'.

Distribution
Known only from the Santubong Peninsular where it is locally abundant.

Ecology
Rheophytic in the flood zone of relatively exposed Paleogene sandstone waterfalls and streambanks under humid lowland between 40-240 m asl.

Notes
With the addition of Homalomena santubongensis the described species of lanceolate-leaved Homalomena Chamaecladon clade now number three for NW Borneo. Although Homalomena paucinervia (Figure 3) is highly similar in overall appearance it is separated not only by the characteristics described here but also geographically and to an extent ecologically, with H. santubongensis restricted to the more open and somewhat drier Paleogene sandstones of the Santubong Peninsula, while H. paucinervia occurs on the much wetter Cretaceous sandstones of the Matang Massif. Homalomena atrox (Figure 4), from riverine shales on the Batang Ai drainages is distinct by the microscopically pubescent leaves and petioles.
The three species may be keyed out as follows: