Aridarum surukense

General description: 

Small obligate clumping rheophytes ca. 18 cm tall. Stem condensed, suberect, ca. 1.5 cm diam, with copious strong roots. Leaves up to 15 together, petioles suberect with blades arching; petiole 5–7 cm long, laterally somewhat compressed, ca. 2 mm wide x ca. 3 mm high, distally weakly dorsally channelled, with the edges slightly raised and rounded, sheathing at the extreme base, very dark green, minutely roughened; petiolar sheath with wings extended into a narrowly triangular ligular portion ca. 4 cm long, ligule of short duration; blade coriaceous, elliptic, 8–10 cm long x 5–5.5 cm wide, base cuneate, apex acute to slightly acuminate, apiculate for ca. 5 mm, adaxially velvety very dark green, abaxially velvety pale olive green; midrib abaxially prominent, adaxially bluntly raised; primary lateral veins with ca. 4 on each side, diverging at c. 30°, adaxially prominent, abaxially very slightly sunken; interprimary veins almost equalling the primaries in appearance, abaxially defined by being darker than the surrounding tissue; secondary venation 6 invisible. Inflorescence solitary, subtended by a 3–4 cm long, very narrowly triangular somewhat leathery prophyll; peduncle very slender, arching with the inflorescence pendent, equalling the petioles, 6–8 cm long, 2–2.3 mm in diam., terete, dark green, inserted ventralobliquely on the spathe; spathe narrowly ovate, not constricted, barely opening at pistillate anthesis, ca. 5.5 cm long, lower part narrowly campanuliform at anthesis, weakly gibbous ventrally, dark green, ultimately persistent through fruiting; limb glistening white, apiculate for ca. 5 mm, apicule dark green; limb loosening at pistillate anthesis, prior to and during staminate anthesis deliquescent and crumbling from the junction of the spathe limb and the persistent lower part, limb tearing into jagged adherent strips that eventually fall to leave a brown slimy collar of decomposing tissue that this later rots and falls to leave the narrowly campanuliform persistent lower spathe, with a scarred irregular rim. Spadix subcylindric ca. 2.5 cm long x ca. 4.5 mm in diam. (widest part); pistillate flower zone slender cylindric, thinner than the rest of the spadix, comprising ca. 1/3 of the spadix, obliquely inserted on peduncle, zone ca. 3 mm in diam., ventral side ca. 1 cm long, dorsal side ca. 6mm long; pistils trapezoid-subglobose, truncate, ca. 0.9 mm in diam., pale green; stigma sessile, slightly impressed, discoid, papillose, less wide than ovary, cream; sterile interstice cylindric, truncate with the top of the pistillate zone, ca. 2 mm long, with one partial whorl of staminodes; interstice staminodes rather weakly differentiated from the staminate flowers, ca. 1.5 mm wide, greenish white, interpistillar zone equalling the staminate zone in width, later (at staminate anthesis) staminodes expanding laterally by extension of the filament until zone ca. 4 mm wide and blocking access to the pistils; staminate flower zone ca. 1/2 of total spadix length, ca. 1.2 mm long 3 ca. 4.5 mm wide, cylindrical, basally merging with the interstice and apically merging with the appendix; staminate flowers each comprised of a single stamen, ivory, 6 horseshoe-shaped in plan view and somewhat arching, with a conspicuous deep narrow suture between the thecae, ca. 1 x 1 mm, connective with the inner part slightly channelled, smooth, and the distal margins (with respect to spadix axis) verruculose; thecae ellipsoid, imperceptibly embedded in the ends of the ‘arms’ of the horseshoeshaped connective, each ca. 0.3 mm long, displaced to the proximal (with respect to the spadix axis) side of the stamen with distalpointing horns; thecae horns ca. 0.4 mm long, stiff, translucent except for the base, directed upwards; appendix ca. 1 cm long, usually comprising slightly more than 1/3 of the entire spadix, cylindrical, obtuse; appendix staminodes mostly comprised of very densely-packed irregular circular and partially coherent verruculose and pellucid-glandular brain-like staminodes, lowermost staminodes closely reminiscent of staminate flowers but lacking thecae horns, terminalmost few more laxly arranged, pale cream. Infructescences not observed.

Diagnostic description: 

Aridarum surukense may be readily distinguished from all other Aridarum species by the very dark green velvety elliptic leaf blades. Among the Rostratum Complex it has the shortest spathe (ca. 5.5 cm long), and spadix (ca. 2.5 cm long).

Distribution: 

Aridarum surukense is known only from the area of Nanga Suruk, on the western flank of the Muller Range.

Ecology: 

Aridarum surukense is an obligate rheophyte on exposed volcanic rocks along fast flowing streams under lowland perhumid forest at ca. 40 m asl.

Taxonomy
Taxonomic Notes: 

Aridarum surukense is a highly distinctive species, not only by virtue of the velvety very dark black-green leaves, but also in lacking staminodes at the base of the spadix (present in all other Aridarum species so far described), but also in the rather weak differentiation of the interstice staminodes from the staminate flowers.

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