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The grass genera of the world

L. Watson, T.D. Macfarlane, and M.J. Dallwitz

Hemarthria R.Br.

From the Greek hemi (half) and arthron (joint), referring to inflorescence ‘joints’ (internodes) which are hollowed out (halved) to take the sessile spikelet; alternatively (more likely, acknowledging Robert Brown’s perceptiveness), referring to a peculiar, ostensibly articulated but non-disarticulating rachis.

Including Lodicularia P. Beauv.

Habit, vegetative morphology. Perennial; stoloniferous, or caespitose, or decumbent. Culms 30–150 cm high; herbaceous; amply branched above. The branching simple. Culm nodes glabrous. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear-lanceolate (usually), or linear; narrow; flat; without cross venation; persistent; once-folded in bud. Ligule a fringed membrane.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality; overtly heteromorphic (the sunken, ‘sessile’ spikelets with dissimilar glumes, the non-sunken ‘pedicelled’ spikelets with similar glumes). Plants exposed-cleistogamous, or chasmogamous.

Inflorescence. Inflorescence of spicate main branches, or a single raceme (of flattened, dorsiventral spicate ‘racemes’, arising one or more from the sheaths of each of the upper leaves). Rachides hollowed (the sessile spikelet embedded). Inflorescence spatheate (usually); a complex of ‘partial inflorescences’ and intervening foliar organs. Spikelet-bearing axes spikelike (often curved); solitary, or clustered (fascicled); with substantial rachides; tardily disarticulating (the rachis initially tough); ultimately disarticulating at the joints. ‘Articles’ non-linear (clavate); with a basal callus-knob (rarely), or without a basal callus-knob (usually); not appendaged; disarticulating transversely to disarticulating obliquely; glabrous. Spikelets paired (each pair comprising a sessile spikelet and the ‘pedicelled’ member of the ‘pair’ below); secund (the sessile members in two alternating rows, more or less on one side of the rachis); consistently in ‘long-and-short’ combinations (the fused pedicels discernible). Pedicels of the ‘pedicellate’ spikelets discernible, but fused with the rachis. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets hermaphrodite.

Female-fertile spikelets. Spikelets 3–7 mm long; abaxial; compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent. Callus short (obtuse or truncate, in the sessile spikelet), or absent (in the pedicelled spikelet).

Glumes present; two; more or less equal; long relative to the adjacent lemmas; dorsiventral to the rachis; hairless; glabrous; awned (the lower biaristate in H. debilis, the G2 sometimes long-acuminate/aristate), or awnless; non-carinate; very 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 the back to flattened on the back; not pitted; relatively smooth; 5–7 nerved. Upper glume 5–7 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless; 2 nerved; decidedly exceeding the female-fertile lemmas; similar in texture to the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas not indented; less firm than the glumes (hyaline); not becoming indurated; entire; awnless; hairless; non-carinate; 0 nerved. Palea present; conspicuous but relatively short; entire; awnless, without apical setae; not indurated (hyaline); nerveless. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; white.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small; compressed dorsiventrally. Hilum short. Embryo large. Endosperm containing compound starch grains. Embryo with an elongated mesocotyl internode.

Seedling with a long mesocotyl.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals smaller and narrower than the unusually large intercostals, at least in H. uncinata); differing markedly in wall thickness costally and intercostally (the intercostals thicker walled, in H. uncinata). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; 48–84 microns long; 11.4–14.4 microns wide at the septum. Microhair total length/width at septum 4.8–6.5. Microhair apical cells 20–51 microns long. Microhair apical cell/total length ratio 0.46–0.65. Stomata common; 33–39 microns long. Subsidiaries mostly triangular (in H. uncinata). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified. Intercostal silica bodies mostly cross-shaped, or vertically elongated-nodular. Heavily pitted, thick walled prickle bases present in H. uncinata. Crown cells absent. Costal short-cells neither distinctly grouped into long rows nor predominantly paired (sic). Costal silica bodies ‘panicoid-type’; cross shaped and butterfly shaped, or dumb-bell shaped.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR cell chloroplasts with reduced grana; centrifugal/peripheral. Mesophyll with radiate chlorenchyma. Leaf blade adaxially flat. Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially (the colourless tissue extending across the adaxial part of the blade, contiguous with the epidermis). Bulliforms not present in discrete, regular adaxial groups (the epidermis extensively bulliform). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present (with the main bundles); forming ‘figures’, or nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Culm anatomy. Culm internode bundles in one or two rings.

Phytochemistry. Leaves without flavonoid sulphates (1 species).

Special diagnostic feature. Lower glume of female-fertile spikelet flattish, not pitted; ‘pedicellate’ spikelets similar to the female-fertile spikelets.

Cytology. Chromosome base number, x = 9, or 10. 2n = 18, or 20 (18+2B), or 36, or 54. 2, 4, and 6 ploid (and aneuploids). Chromosomes ‘small’.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Rottboelliinae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Rottboelliinae. 12 species.

Distribution, phytogeography, ecology. Tropical Africa, Madagascar, eastern Asia, Indomalayan region, Australia.

Commonly adventive. Hydrophytic to helophytic; species of open habitats; glycophytic. In water or in wet places.

Economic aspects. Significant weed species: H. altissima, H. compressa.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: Puccinia microspora, Puccinia levis, and ‘Uromycesclignyi. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca and Ustilago.

References, etc. Leaf anatomical: Metcalfe 1960; studied by us - H. uncinata R. Br.

Illustrations. • H. leoninum, as Avena: Kunth (1835). • H. uncinata: Gardner, 1952. • General aspect (H. altissima: Gibbs Russell et al., 1990. • Inflorescence detail of H. uncinata

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, and classifications. See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., Macfarlane, T.D., and Dallwitz, M.J. 1992 onwards. The grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references. Version: 11th December 2017.’.