Arum costatum Wallich, Tent. Fl. Napal. 28. 1824.

Plants dioecious. Tuber brown outside, white inside, depressed globose, 3-5 cm in diam. Cataphylls 3, ca. 50 × 25 cm, membranous. Leaf solitary; petiole green, often slightly purplish, terete, 30-50 cm, proximal 1/4-1/3 sheathing into pseudostem, ca. 1 cm in diam.; leaf blade 3-foliolate; leaflets pale green abaxially, deep green adaxially, shortly petiolulate or subsessile; terminal elliptic or oblong, 16-30 × 9-16 cm, base auriculate and rounded, inner base narrowly cuneate, apex acuminate; lateral veins very numerous and running almost parallel, distinctly raised abaxially, impressed adaxially. Peduncle emerging from pseudostem, shorter than petioles, 25-45 cm. Spathe dark purple, with white longitudinal lines; tube cylindric, 4-8 cm, slightly costate inside, but in Xizang specimens almost smooth inside, throat margins slightly recurved; limb incurved, narrowly ovate-oblong, 3-4 cm, apex acuminate with a filiform tail. Spadix unisexual; female zone 2.4-3.5 cm; ovary green; style short; stigma white, disciform; appendix pendulous from spathe, dark purple, very long flagelliform, 20-50 cm, inflated toward base, base truncate, swollen disciform, with a stipe 3-5 mm, smooth throughout; male zone 2-3.7 cm; synandria of 3-5 stamens; connected filaments distinct; thecae dehiscing by a horseshoe-shaped slit. Fl. Jul.

Molecular phylogeny has revealed that Arisaema costatum is not closest to the other members of A. sect. Arisaema but is instead monophyletic with African species that have radiate leaves (Renner, Zhang & Murata, Amer. J. Bot. 91: 881-888. 2004).

Grasslands in forest areas; 2300-2400 m. S Xizang (Dinggyê, Nyalam) [Nepal].