Hemarthria Prodr. 207 (1810).
Derivation:. From Greek hemi (half) and arthron (jointed) i.e. half-jointed, alluding to the resistance of the raceme internodes to breaking up.
Key references (keys and floras):. G.Bentham, Flora Australiensis 7: 510–511 (1878); C.A.Gardner, Flora of Western Australia 1 Gramineae 304 (1952); J.W.Vickery, Flora of New South Wales, Gramineae 19: 17–18 (1961); E.E.Henty, Manual Grasses New Guinea 108 (1969); M.Lazarides, Tropical Grasses S.E. Asia 44–45 (1980); J.C.Tothill and J.B.Hacker, Grasses of Southern Queensland 256–257(1983); J.P.Jessop, Flora of South Australia 4: 1989 (1986); B.K.Simon, Key to Australian Grasses 123 (1993); S.W.L.Jacobs and K.L.McClay, Flora of New South Wales 4: 428 (1990); N.G.Walsh, Flora of Victoria 2: 627 (1994); D.I.Morris, Student's Flora of Tasmania 4B: 358–359 (1994); D.Sharp and B.K.Simon, AusGrass (2002); J.P.Jessop, Grasses of South Australia 520 (2006); S.W.L.Jacobs, R.D.B.Whalley & D.J.B.Wheeler, Grasses of New South Wales, 4th ed, 272 (2008).
W.D.Clayton & S.A.Renvoize, Genera Graminum (1986), genus (629).
Native. 12 species, from tropical Africa, Madagascar, eastern Asia, Indomalayan region and Australia. 1 species in Australia, WA, NT, SA, Qld, NSW, Vic, and Tas. Also New Guinea and Malesia.
Habit. Perennial, stoloniferous or tufted or decumbent. Leaf blades narrow. Ligule a fringed membrane.
Inflorescence. Inflorescence of spicate main branches or paniculate (of flattened, dorsiventral spicate racemes), a single raceme or spike, spatheate (usually), a compound pseudo-inflorescence. Spikelet-bearing axes spikelike (often curved), with heteromorphic spikelets (the sunken, sessile spikelets with dissimilar glumes, the non-sunken pedicelled spikelets with similar glumes), solitary or clustered (fascicled), with spikelets in pairs, one sessile, the other pedicelled, ultimately disarticulating at joints. Internodes with a basal callus-knob (rarely) or without a basal callus-knob (usually), disarticulating transversely to disarticulating obliquely, glabrous.
Spikelets. Spikelets all partially embedded in rachis, dorsally compressed, 2 flowered, with 1 fertile floret, paired, in pedicelled/sessile combinations (pedicels fused with rachis); sessile spikelet with lower incomplete floret. Fertile spikelets abaxial (with lower glume on side away from rachis), falling with glumes.
Glumes. Glumes more or less equal, long relative to adjacent lemmas, hairless, glabrous, awned or awnless, non-keeled, dissimilar (in the embedded spikelet, the outer tough, the inner membranous) or similar (both tough in the pedicelled spikelets). Lower glume two-keeled (the keels not winged), convex on back to flattened on back, relatively smooth, 5–7 nerved. Upper glume 5–7 nerved.
Florets. Lower incomplete floret(s) sterile. Lemmas awnless, 2 nerved, exceeding fertile lemmas, similar in texture to fertile lemmas, not becoming indurated. Fertile florets 1. Lemmas not indented, less firm than glumes (hyaline), not becoming indurated, entire at apex, muticous, 0 nerved, glabrous. Palea conspicuous and relatively short, entire, nerveless. Callus short (obtuse or truncate, in the sessile spikelet) or absent (in the pedicelled spikelet). Lodicules 2. Stamens 3. Grain small, compressed dorsiventrally. Hilum short. Embryo large. Pedicels discernible, but fused with rachis. Pedicelled spikelets present, similar in shape to sessile spikelet, male.
Kranz Anatomy. C4.
2n = 18 or 20 (18+2B) or 36 or 54, 2, 4, and 6 ploid (and aneuploids), commonly adventive.
Habitat. Hydrophytic to helophytic. In water or in wet places. Species of open habitats.
Classification. Panicoideae; Andropogoneae.
Types Species. H. compressa (L.f.) R.Br.
Biogeographic Element. Clifford & Simon 1981, Simon & Jacobs 1990: Gondwanan.