The grass genera of the world

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L. Watson and M. J. Dallwitz

Oplismenus P. Beauv.

From the Greek hoplismenos (armed), alluding to the awns.

Including Hekaterosachne Steud., Hippagrostis Kuntze, Orthopogon R. Br.

Habit, vegetative morphology. Annual, or perennial; decumbent. Culms 10–100 cm high; herbaceous; freely branched above. Culm nodes hairy. Culm internodes solid, or hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear to ovate; broad, or narrow; not setaceous; flat (thin); pseudopetiolate, or not pseudopetiolate; cross veined, or without cross venation; persistent; rolled in bud. Ligule a fringed membrane (very short), or a fringe of hairs.

Reproductive organization. Plants bisexual, with bisexual spikelets; with hermaphrodite florets.

Inflorescence. Inflorescence of spicate main branches (short unilateral racemes along a central axis); open. Primary inflorescence branches borne biseriately on one side of the main axis. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary, or paired (or the lower member reduced, or in clusters); secund; biseriate; shortly pedicellate; imbricate, or not imbricate; not in distinct ‘long-and-short’ combinations.

Female-fertile spikelets. Spikelets elliptic, or lanceolate, or ovate; abaxial; weakly compressed laterally, or not noticeably compressed to compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present.

Glumes two; more or less equal; shorter than the adjacent lemmas, or long relative to the adjacent lemmas; dorsiventral to the rachis; not pointed; awned (both or at least the lower, the awn of the lower always longer); carinate; similar (herbaceous, the awns often viscid). Lower glume 3–5 nerved. Upper glume 5 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets fully developed to reduced. The proximal incomplete florets male, or sterile. The proximal lemmas shortly awned, or awnless; 5–9 nerved; not becoming indurated.

Female-fertile florets 1. Lemmas dorsally compressed; similar in texture to the glumes to decidedly firmer than the glumes (papery to leathery); smooth (glossy); becoming indurated to not becoming indurated; white in fruit, or yellow in fruit; entire; indistinctly crested at the tip, or not crested; awnless; hairless; carinate to non-carinate; having the margins inrolled against the palea; with a clear germination flap; 3–5 nerved. Palea present; relatively long; entire (acute); awnless, without apical setae; textured like the lemma (smooth, glossy); indurated, or not indurated; 2-nerved. Lodicules present; 2; fleshy. Stamens 3. Anthers 1–2 mm long; not penicillate. Ovary glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit ellipsoid; longitudinally grooved (slightly), or not grooved; compressed dorsiventrally. Hilum short to long-linear (oblong, up to a half as long as the fruit). Embryo large. Endosperm hard; without lipid; containing only simple starch grains. Embryo without an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.

Seedling with a long mesocotyl. First seedling leaf with a well-developed lamina. The lamina broad; supine; 13–20 veined.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals narrow-rectangular); of similar wall thickness costally and intercostally (thin walled). Intercostal zones exhibiting many atypical long-cells, or without typical long-cells (incorporating or comprising squarish long-cells). Mid-intercostal long-cells rectangular (usually short, some irregular in shape); having markedly sinuous walls. Microhairs present; panicoid-type; (38–)46–59(–60) microns long; 4.5–6 microns wide at the septum. Microhair total length/width at septum 10–11.3. Microhair apical cells (20–)25–34.5(–36) microns long. Microhair apical cell/total length ratio 0.52–0.55. Stomata absent or very rare, or common; (34–)36–39(–42) microns long. Subsidiaries low dome-shaped, or triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common, or absent or very rare; not paired (mainly solitary); silicified, or not silicified. Intercostal silica bodies when present, cross-shaped. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; cross shaped to dumb-bell shaped, or nodular; not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma; Isachne-type to not Isachne-type. Leaf blade with distinct, prominent adaxial ribs, or adaxially flat; with the ribs more or less constant in size. Midrib conspicuous; with one bundle only. Bulliforms present in discrete, regular adaxial groups (these wide); in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

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

Phytochemistry. Leaves without flavonoid sulphates (O. hirtellus).

Cytology. Chromosome base number, x = 9, 10, and 11. 2n = 18, 36, 54, 72, and 90. Chromosomes ‘snall’.

Taxonomy. Panicoideae; Panicodae; Paniceae.

Distribution, ecology, phytogeography. 7 species; tropical and subtropical. Mesophytic; shade species; glycophytic. In forest.

Holarctic, Paleotropical, Neotropical, Cape, Australian, and Antarctic. Boreal and Tethyan. African, Madagascan, Indomalesian, Polynesian, and Neocaledonian. Euro-Siberian, Eastern Asian, and Atlantic North American. Mediterranean and Irano-Turanian. Saharo-Sindian, Sudano-Angolan, West African Rainforest, and Namib-Karoo. Indian, Indo-Chinese, Malesian, and Papuan. Fijian. Caribbean, Venezuela and Surinam, Amazon, Central Brazilian, Pampas, and Andean. North and East Australian. New Zealand. European. Southern Atlantic North American and Central Grasslands. Sahelo-Sudanian, Somalo-Ethiopian, South Tropical African, and Kalaharian. Tropical North and East Australian and Temperate and South-Eastern Australian.

Rusts and smuts. Rusts — Phakopsora and Puccinia. Smuts from Tilletiaceae and from Ustilaginaceae. Tilletiaceae — Tilletia. Ustilaginaceae — Ustilago.

Economic importance. Significant weed species: O. burmanii, O. compositus, O. hirtellus, O. undulatifolius. Important native pasture species: O. compositus.

References, etc. Leaf anatomical: Metcalfe 1960; this project.

Illustrations. • General aspect (O. hirtellus): Gibbs Russell et al., 1990. • Inflorescence of O. aemulus. • Inflorescence detail with spikelet close-up (O. aemulus). • Abaxial epidermis of leaf blade (O. aemulus)

The descriptions are offered for casual browsing only. We strongly advise against extracting comparative information from them. This is much more easily achieved using 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, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG).

Cite this publication as: ‘Watson, L., 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: 7th December 2015.’.