From Latin, fenestratus, windowed, + Arum, hence windowed arum, coined to describe the unique translucent areas of the spathe limb.
Type species: Fenestratum culum P. C. Boyce & S. Y. Wong, sp. nov.
Small obligate rheophytes to ca. 15 cm tall. Stem initially compact, later elongating to ca 10 cm, sub-erect, ca. 3 cm in diam., basally with copious strong roots. Leaves many together, stiffly arching; petiole ca. 2 cm long, basal half broadly sheathing and swollen, ca. 5 mm wide, very pale green, remainder of petiole D-shaped in cross-section, ventrally pale matte green, dorsally very deep glossy green; petiolar sheath with wings extended into a narrowly triangular ligular caducous portion ca. 3 cm long; blade stiffly coriaceous, linear, 11–15 cm long × ca. 5 mm wide, base decurrent, apex acute and apiculate for ca. 4 mm, adaxially very deep glossy green, abaxially matte pale olive green; midrib abaxially and adaxially sharply prominent; primary lateral veins restricted to one on each side, arising from the base of the blade and remaining at the margin of the blade, meeting at the tip and there coalescing to form the apiculate point; all other veins invisible. Inflorescence up to three produced in sequence, alternating with membranous, later papery cataphylls; peduncle slender, somewhat exceeding the leaves, ca. 15 cm long × 1.5 – 2 mm in diam., pale green, uppermost part curving forward to hold the inflorescence at slightly less than 90° to the peduncle. Spathe ca. 3 cm long, ca. twice length of spadix; spathe limb broadly lanceolate with large transparent areas separated by opaque white veins, terminally specked reddish, and extended into a prominent reddish-stained rostrum ca. 4 mm long, limb opening at pistillate anthesis by a ventral elliptic fissure with incurving margins, limb caducous by deliquescence at junction with persistent lower part after staminate anthesis, falling in a single but deliquescing piece together with the spent parts of the spadix; lower spathe with fully fused margins, forming a tube, white, persistent after anthesis and forming a funnel-form structure subtending the developing fruits. Spadix sub-cylindric, 13 – 20 mm long × 5 – 7 mm wide; pistillate flower zone cylindric, comprised of ca 3 spirals of flowers, narrower than remainder of spadix, accounting for ca. ¼ of spadix, 3 – 6 mm long × 3 – 5 mm in diam., with single row squat polygonal glossy white staminodes at base; pistils crowded, subglobose, ca. 1 mm in diam., pale green; stigma sessile, discoid, almost as wide as pistil, , rather coarsely papillate, very pale green; sterile interstice 1 – 2 mm long × 5 – 7 mm in diam., composed of a single row of polygonal staminodes, these 1 – 2mm long × 0.5 – 1 mm wide, very pale creamy white; staminate flower zone slightly less than ½ of entire spadix length, 5 – 6 mm long × 5 – 7 mm in diam., very pale creamy white; staminate flowers large, spirally arranged, each comprised of two stamens, truncate, rhomboid from above, ca. 2 mm long × 1.5 mm wide; thecae set in deep pits on the top and bottom (with respect to the spadix axis) of each stamen separated by a conspicuous broad, slightly domed connective; appendix 4 – 5 mm long, slightly tapering, obtuse; appendix staminodes resembling stamens without thecae, very pale creamy white. Infructescences erect. Fruiting spathe campanuliform, ca. 1 cm long × ca. 1 cm wide, medium green with a scar along rim; fruits obpyriform, ca 3 mm long, stigmatic remain raised, darker green; seeds not seen.