Perennial or annual herb, (2½-)5-72 cm, with stems erect or decumbent, branching strictly from the base (rarely elsewhere) or unbranched below the inflorescence, not rooting, 4-lined. Leaves sessile, 0.4-2.5 by 0.12-0.8 cm, lanceolate to linear or oblong or rarely ovate-lanceolate, apex obtuse to rounded, base cordate to rounded or sometimes cuneate, amplexicaul, ± glaucous beneath; 1-3 main basal veins, not or scarcely branched, with 1 or more lateral veins often visible; reticulate venation absent; laminar glands pale, not prominent, small and dense above, larger and laxer beneath; intramarginal glands absent. Inflorescence (l-2-)3-c. 30-flowered, cymose and regularly dichasial to monochasial or wholly monochasial, or with branches in uppermost leaf axil, or mixed, sometimes with subsidiary inflorescences from upper 1-3 leaf pairs, lax; pedicels usually exceeding uppermost leaves, 2-30 mm in fruit. Flowers (5-)6-12(-15) mm ø, plane; buds ellipsoid, subacute. Sepals 2.8-7.5(-9) by 0.8-2 mm, free, imbricate, lanceolate to narrowly elliptic, entire, acute to subacute, 3-5-nerved, often with ± prominent midrib, especially in fruit; laminar glands pale, linear towards the base, becoming punctate distally; marginal glands absent. Petals pale or bright yellow or orange, 5-10 by 2-5 mm, c. 1.3 times as long as the sepals, obovate to oblanceolate, entire, persistent; apiculus small, mucronate to rounded, lateral; laminar and marginal glands absent. Stamens not in apparent fascicles, c. 30-50, longest 2½-4mm, c. ¾ as long as petals; anthers yellow, gland amber. Ovary 1½-2.3 mm, narrowly ovoid-conic; styles 3,0.7-1.8 mm,½ -9\10 as long as ovary, ± divergent; stigmas capitate; placentas 3, parietal. Capsule 2½-8 by l-3½(-4) mm, narrowly ovoid to cylindric, not vittate. Seeds yellow-brown, ½ mm, cylindric, not carinate, longitudinally ribbed with fine transverse striae.
Asia-Temperate:, China South-Central (Yunnan ‒ present); Taiwan (Taiwan ‒ present) Asia-Tropical:, Assam (Assam ‒ present); India ‒ present; New Guinea ‒ present; Vietnam (Vietnam ‒ present) Australasia: Bhutan (?): present E., W. & S. Highlands: present Morobe: present New Caledonia: present New Zealand: present Western Distr: present
Vietnam, Taiwan, China (Yunnan), India (Assam), Bhutan (?), in Malesia: New Guinea (Morobe, E., W. & S. Highlands, Western Distr.), Australia, New Zealand, New Caledonia.
H. gramineum appears to remain quite distinct from H. japonicum except in Bhutan, where hybrid (?) intermediates occur. It can be distinguished from H. japonicum by its larger flowers and floral parts, longer petals relative to sepals, more numerous stamens and, usually, its narrower leaves and lanceolate to narrowly elliptic acute sepals. In H. japonicum the sepals rarely show this combination of characters. In addition, H. gramineum never has the straggling to prostrate habit often found in H. japonicum.
DYER - in E. & P., Nat. Pfl. Fam., ed. 2, 21. 1925: 181
HAND.-MAZZ. - in Symb. Sin. 1931: 404
CHOISY - in DC., Prod. 1. 1824: 549
BAILEY - in Rep. Austr. Ass. Adv. Sc. 1898: 429
R. KELLER - in Bull. Herb. Boiss. 1908: 187
G. FORSTER - in Queensl. Fl. 1899: 100
LEV. - in Bull. Soc. Bot. Fr. 1908: 589, 593
KANJILAL & DAS - in Fl. Assam. 1935: 100
R. KELLER - in E. & P., Nat. Pfl. Fam., ed. 2, 21. 1925: 183
HOCHR. - in Candollea. 1925: 437
GUILLAUMIN, Fl. Nouv. Caled. 1948: 217
VAHL - in Symb. Bot. 1791: 86