Annual herb, 10-60 cm, stems erect to ascending or procumbent, branching ± widely, rooting, terete. Leaves 1-3 mm petiolate, 0.9-3 by 0.5-1(-1.6) cm, oblong or lanceolate-elliptic to obovate or subcircular, apex rounded or rarely obtuse, base cuneate or attenuate to rarely rounded, ± glaucous beneath; 3 main lateral veins, not or scarcely branched, uniting near margin and apex; reticulate venation obscure, rather dense to lax; laminar glands pale, very rarely with 1-2 black, large, usually ± prominent, ± dense, punctate; intramarginal glands black, ± dense or irregular. Inflorescence (l-)3-9-flowered, cymose, with sub-sidiary inflorescences on often long lateral branches, monochasial after 1st grade, lax; pedicels exceeding uppermost leaves, 4-13 mm in fruit. Flowers 5-7 mm ø, plane; buds ellipsoid, obtuse. Sepals 2.3-3.4 by 0.5-0.9 mm, free, not imbricate, linear to very narrowly oblong-lanceolate, entire; very acute (or rarely (Nepal) narrowly elliptic-oblong and subacute); laminar glands pale or rarely black, punctate to linear, variable in size and number; marginal glands black, few, sometimes one apical, or all absent. Petals yellow, 4-5½ by l-l¼ mm, oblanceolate, entire, persistent; apiculus small, acute, subapical; laminar glands absent or 1-2, pale, punctate; marginal glands black, 1-2 on or near the apiculus and occasionally also elsewhere. Stamens in 3 fascicles, 9-22, longest (2½-)3-4½ mm, c.¾ as long as the petals; anthers yellow, gland black. Ovary 1½ 2-mm, ± broadly ovoid to subglobose; styles 3(-4), 1-2.1 mm, ⅔- 1¼ times as long as the ovary, divergent; stigmas not or scarcely capitate; placentas 3(-4), axile. Capsule 3-5 by 3-5 mm, broadly ovoid to globose, longitudinally vittate. Seeds yellow-brown, 0.5-0.6 mm, cylindric, not carinate, densely shallowly reticulate-scalariform.
Asia-Temperate:, China South-Central (Yunnan ‒ present) Asia-Tropical:, Sumatera (Sumatera ‒ present) Burma: present Mt Kerintji: present Mt Kinabalu: present N. Borneo: present Nepal: present
Nepal to Burma and Yunnan, in Malesia: Sumatra (Mt Kerintji), N. Borneo (Mt Kinabalu). .
H. petiolulatum is variable, but the Malesian specimens fall within the range of variation of the type subspecies. On the other side of the Himalayan Range, in Yunnan and Szechuan, it is represented mostly by subsp. yunnanense (FRANCHET) N. ROBSON, in which the styles are longer than the ovary, the capsule is ± broadly ovoid, the leaf lamina is usually broadest at or below the middle and often larger than in subsp. petiolulatum, the branching is more regular, and the main inflorescence usually occupies 2-3 nodes.