Aridarum hebe

General description: 

Medium-sized clumping obligate rheophytes to 35 cm tall. Stem initially erect and somewhat condensed, later decumbent and rhizome-like, ca. 20 cm long 3 ca. 2.5 cm in diam., active portions obscured by the dense leaf bases, older parts naked with conspicuous scars and often producing reiterative shoots, stem copious strong roots. Leaves several together, petioles erect with blades very slightly arching; individual modules with 3–5 leaves, modules subtended by a linear-triangular 2-keeled prophyll to 5 cm long and ca. 5 mm wide, initially prophyll green, but soon decaying; petiole 11–17 cm long, very weakly D-shaped in cross section, ca. 3 mm wide x ca. 2.5 mm high, sheathing at the extreme base, dorsal edges slightly raised, semi glossy medium green; petiolar sheath with wings extended into a narrowly triangular ligular portion 5–9 cm long, ligule soon marcescent and shedding to leave a short collar of brown papery tissue; blade softly coriaceous, elliptic, 17–22 cm long x 4–5 cm wide, base cuneate, apex acute, apiculate for ca. 6 mm, adaxially semi glossy very dark green, glossy pale green abaxially; midrib abaxially prominent, somewhat paler than the rest of the blade, adaxially bluntly raised, with ca. 4 adaxially very weakly defined primary lateral veins on each side, these diverging at ca. 30° , adaxially primary lateral veins merging into the slightly raised marginal vein running ca. 2 mm in from the leaf margin; interprimary veins invisible adaxially, abaxially slightly darker than the primaries; secondary venation adaxially invisible. Inflorescence solitary, subtended by a ca. 5 cm long, very narrowly triangular somewhat leathery cataphyll; peduncle slender, spreading to slightly pendent, shorter than the petioles, up to 10 cm long, ca. 3 mm diam., terete, medium green, inserted slightly obliquely on the spathe; spathe narrowly ovate with a very long acuminate tip, not constricted, ca. 9.5 cm long, lower part campanuliform at anthesis, medium green, ultimately persistent through fruiting, long-apiculate for up to 1.5 cm, limb exterior glistening white, interior greenish in the lower 1/3, the remainder white, apicule distally green; limb inflating and then gaping at pistillate anthesis, prior to staminate anthesis limb caducous from the junction with the persistent lower part, limb falling more-or-less intact to leave the narrowly campanuliform persistent lower spathe, with a scarred regular rim. Spadix cylindric, ca. 4.5 cm long x ca. 6 mm in diam., less than half the length of the spathe, fertile almost to the tip, basally very shortly stipitate; stipe oblique, ca. 5 mm wide x ca. 2 mm long on longest side, white; pistillate flower zone weakly fusiform-cylindric, about the same width as the other fertile parts of the spadix, comprising about 1/5 of the spadix, ca. 7 mm long; pistils subglobose, truncate, c. 1.1 mm in diam., pale green; stigma sessile, capitate, papillose, almost as wide as the ovary medium green, producing a very conspicuous stigmatic droplet at anthesis, turning brownish as post anthesis; sterile interstice cylindric, equalling the pistillate zone in width, occasionally with the lowermost part irregularly naked, ca. 7 mm long, with 3–6 dense whorls of mainly oblong, excavated staminodes, these ca. 1.5 mm long x .75 mm wide, medium yellow, the cavity filling with an amber-yellow sticky fluid during staminate anthesis; staminate flower zone ca. 2/3 of total spadix length, ca. 2 cm long, lower part equalling the width of the sterile interstice, fertile to the tip, or the tip furnished with a few staminodes, glossy white; staminate flowers densely packed, each comprised of two stamens, ivory, 6 oblong, with the long sides parallel to the spadix axis, ca. 2 x 1 mm, connective excavated; thecae ellipsoid, embedded in the ends connective, each ca. 0.5 mm long; thecae horns two per stamen, ca. 0.5 mm long, peg-like, erect. Infructescence not observed.

Diagnostic description: 

Aridarum hebe differs from all other Aridarum species by the combination short, erect peg-like thecae horns, and the oblong, deeply excavated interstice staminodes filled with amber-yellow sticky fluid.


Aridarum hebe is known only from the Type locality, where it is decidedly rare. Riam Marthin Billa is a beguilingly aroid species-rich site, host to numerous new or just-described aroid species, notably Piptospatha pileata S. Y. Wong & P. C. Boyce.


Aridarum hebe is an obligate rheophyte on very hard shale waterfalls and river boulders under extremely wet hill forest at altitude of ca. 400 m asl.

Taxonomic Notes: 

The morphology and configuration of the staminate flowers of A. hebe are reminiscent of those of species of the NW Bornean Aridarum Borneense Group (A. borneense, A. nicolsonii Bogner, and A. crassum), although preliminary molecular analyses (Low, in prep.) does not support a close relationship between A. hebe and the Borneense Group.
The role of the liquid-filled interstice staminodes is unknown, but very probably is associated with pollination mechanics and pollinator management.

Taxonomic name: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith