Aridarum zygosetum

General description: 

Medium-sized clumping obligate rheophytes to 30 cm tall. Stem condensed, erect, ca. 5 cm long x ca. 2 cm in diam., mostly obscured by the dense leaf bases, with copious strong roots and often with stilt-roots emerging from between the leaf bases. Leaves many together, petioles suberect with blades slightly arching; individual modules with several leaves, modules subtended by a linear-triangular 2-keeled prophyll to 7 cm long and ca. 5 mm wide, initially prophyll green, but soon decaying; petiole 9–14 cm long, weakly D-shaped in cross section, ca. 3 mm wide 3 ca. 2.5 mm high, dorsal edges slightly raised, sheathing at the extreme base, medium green; petiolar sheath with wings extended into a narrowly triangular ligular portion 5–8 cm long, ligule soon marcescent and shedding to leave a short collar of papery tissue; blade softly coriaceous, elliptic, 14–22 cm long 3 4–5.5 cm wide, base cuneate, apex acute, apiculate for ca. 5 mm, adaxially semi glossy dark green, glossy pale abaxially; midrib abaxially prominent, somewhat paler than the rest of the blade, adaxially bluntly raised; primary lateral 4 or 5 adaxially weakly defined veins on each side, these diverging at ca. 30°; interprimary veins slightly darker than the primaries in appearance; secondary venation adaxially invisible. Inflorescence solitary or up to three together alternating with foliage leaves, subtended by a ca. 7 cm long, very narrowly triangular somewhat leathery cataphyll; peduncle slender, erect, equalling to slightly exceeding the petioles up to 14 cm long, ca. 3 mm diam., terete, medium green, inserted slightly obliquely on the spathe; spathe broadly ovate with a long acuminate tip, not constricted, 8–9 cm long, lower part campanuliform at anthesis, medium green, ultimately persistent through fruiting, limb glistening white, apiculate for up to 1 cm, apicule distally green; limb gaping at pistillate anthesis, caducous from the junction of the spathe limb and the persistent lower part, limb falling more-or-less intact to leave the narrowly campanuliform persistent lower spathe, with a scarred regular rim. Spadix tapering cylindric, ca. 4.5 cm long 3 ca. 7 mm in diam. (widest part), usually fertile to the tip; pistillate flower zone oblong cylindric, about the same width as the other fertile parts of the spadix, comprising slightly less than 1/4 of the spadix, ca. 1 cm long, 2–3 rows of trapezoidal to hexagonal glossy white staminodes at the base; pistils subglobose, truncate, c. 1.1 mm in diam., pale green; stigma sessile, slightly impressed, hexagonal, papillose, wider than ovary, pale green; sterile interstice cylindric, equalling the pistillate zone in width, ca. 1.3 cm long, with several dense whorls of hexagonal staminodes, these ca. 1.3 mm in diam., glossy white; staminate flower zone ca. 1/2 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 two or three staminodes, glossy white; staminate flowers densely packed, comprised of two stamens, ivory, ± oblong, with the long sides parallel to the spadix axis, ca. 2 x 1 mm, connective with the surface with a narrow longitudinal groove; thecae ellipsoid, imperceptibly embedded in the ends connective, each ca. 0.5 mm long; thecae horns two per stamen, ca. 3mm long, stiff, semi-translucent, appressed to the stamen during pistillate anthesis, becoming semi-erect and then erect at staminate anthesis, and then producing an irregular mass of pollen from the tip. Infructescence an erect splash-cup. Fruiting spathe campanuliform, ca. 1.5 cm long x c. 1.5 cm wide, medium green with a scar along the rim; immature fruits prismatic globose, c. 2 mm tall and in diam., dark green; stigmatic remains sunken, dark brown; ripe fruits deliquescing into a pale whitish slime; seeds ellipsoid, longitudinally very finely ridged, on one end with an elongate, curved micropylar appendage.

Diagnostic description: 

Uniquely Aridarum zygosetum shares with A. montanum having each stamen furnished with a pair of long setiform thecae horns with the tips of each horn pair overlapping, but is easily distinguished by the much larger, erect, elliptic, softly coriaceous leaf blades up to 22 cm long x 5 cm wide (vs. leaf blades linear-lanceolate, stiffly coriaceous, up to 10 cm long x 4.5 mm wide), the large spathe (ca. 9 cm vs. ca. 2 cm long), and longer spadix (ca. 4.5 cm vs. 1.5 cm long).


Aridarum zygosetum is known from two discrete populations ca. 150 km distant at Nanga Taman (Kalimantan Barat), and Kudangan (Kalimantan Tengah).


Aridarum zygosetum occurs as a rheophyte on exposed riverside granite boulders under lowland to hill perhumid forest; 90–350 m asl.

Taxonomic Notes: 

Despite sharing the characteristic  long setiform thecae horns with A. montanum in all other respects A. zygosetum is highly differentiated from A. montanum and it is not at all clear to which species A. zygosetum may be most closely related. Preliminary molecular analyses by the second author place A. zygosetum alone in an isolated clade.
Plants from Kudangan (Kalimantan Tengah, ca. 150 km SW of the Type locality at Nanga Taman) are indistinguishable in vegetative morphology from plants at Nanga Taman, but differ by the shorter sterile interstice separating the pistillate and staminate flower zone, the less robust thecae horns, and the shorter spathe limb inflating less at pistillate anthesis. Further analytical work is needed to determine if these two populations represent taxonomically significant units.
On several occasions inflorescences of A. zygosetum have been found in habitat with staphylinid beetles present at pistillate and staminate anthesis (Fig. 9E). Staphylinids have never previously been recorded for Aridarum, and they are not common pollinators in tropical Araceae in general. More investigations are needed.

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