Medium-sized obligate clumping rheophytes to ca. 20 cm tall. Stem condensed, suberect, ca. 2 cm diam., with copious strong roots. Leaves many together, arching; petiole 5–12 cm long, weakly Dshaped, c. 2 mm wide x c. 3 mm high, sheathing at the extreme base, minutely scabridulous matte medium green; petiolar sheath with wings extended into a narrowly triangular ligular portion up to 8 cm long, soon deliquescing; blade softly coriaceous, elongate-elliptic, 17–20 cm long x 5–6 cm wide, base cuneate, apex acute to slightly acuminate, apiculate for c. 5 mm, adaxially semi glossy medium, pale grey green and minutely punctate abaxially; midrib abaxially prominent, adaxially bluntly raised; primary lateral veins ca. 6 per on each side, weakly raised abaxially, slightly impressed adaxially, diverging at ca. 30° ; interprimary veins weaker than primaries; secondary venation adaxially invisible, abaxially nearly invisible. Inflorescence solitary, subtended by a ca. 8 cm long, narrowly triangular somewhat leathery prophyll; peduncle very slender, arching with the inflorescence pendent, shorter to equalling the leaves, ca. 10 cm long, ca. 2 mm in diam., terete, matte medium green, inserted ventral-obliquely on the spathe; spathe broadly ovate, not constricted, barely opening at pistillate anthesis, 6–7 cm long, lower part narrowly campanuliform at anthesis, weakly gibbous ventrally, matte medium green, ultimately persistent through fruiting; limb glistening white, apiculate for up to 7 mm, apicule distally 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. 3.5 cm long x c. 8 mm in diam. (widest part); pistillate flower zone slender cylindric, markedly thinner than the rest of the spadix, comprising slightly more than 1/4 of the spadix, slightly obliquely inserted on peduncle, zone ca. 4 mm in diam., ventral side 1.1 cm mm long, dorsal side ca. 8 mm long; interpistillar staminodes confined to a single row at the base of the pistillate zone, ligular, ca. 1 mm long, glossy white; pistils trapezoid-subglobose, truncate, c. 1.1 mm in diam., greenish white; stigma sessile, slightly impressed, discoid, papillose, somewhat less wide than ovary, cream; sterile interstice cylindric, abruptly truncate with the top of the pistillate zone, ca. 4 mm long, with several dense whorls of staminodes; interstice staminodes well developed, ligular-clavate, glossy white ca. 1.5 mm wide, initially interpistillar zone equalling the staminate zone in width, later (at staminate anthesis) staminodes expanding laterally by extension of the filament until zone ca. 6.5 mm wide and blocking access to the pistils; staminate flower zone ca. ½ of total spadix length, ca. 1.5 cm long x ca. 8 mm wide, cylindrical, basally merging with the interstice and apically merging with the appendix; staminate flowers each comprised of a single stamen, pale orange, 6 horseshoeshaped in plan view, with a deep narrow suture between the thecae, this more conspicuous in alcohol-preserved specimens, ca. 1.5 x 1.5 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 horseshoe-shaped connective, each c. 0.3 mm long, displaced to the proximal (with respect to the spadix axis) side of the stamen with distal-pointing horns; thecae horns c. 0.5 mm long, stiff, translucent except for the base, directed upwards; appendix 10–15 mm long, usually comprising slightly more than ½ of the entire spadix, cylindrical, obtuse; appendix staminodes mostly comprised of very densely-packed cylindrical-globose occasionally partially coherent, lowermost staminodes closely reminiscent of staminate flowers but lacking thecae horns, terminalmost few more laxly arranged, pale orange. Infructescences with the lower persistent spathe somewhat narrowly funnelform, ca. 2 cm long 3 1 cm wide across the mouth; fruits (fully developed but not ripe) obpyriform, semi glossy pale green with the stylar region somewhat laterally thickened; seeds not observed.
Aridarum alatense differs from all other species allied to A. rostratum by the combination of narrowly elliptic leaf blades with weakly undulate and minutely crispulate margins, and the matte medium green petioles and peduncles. In overall appearance A. alatense most closely approaches A. hippocrepis but differs ca. twice the number of rows (6 vs. ca. 3) of pale orange (vs. white) staminate flowers.
Aridarum alatense is so far known only from the middle altitudinal range of Gunung Alat, Schwaner Mountains.
Aridarum alatense occurs as a rheophyte on riverside metamorphic rocks and boulders under somewhat open moist upper hill forest at ca. 900 m asl.