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From Oplismenus (another grass genus, q.v.) and the Greek opsis (appearance), denoting likeness.
Habit, vegetative morphology. Aquatic perennial; rhizomatous. Culms herbaceous; to 0.4–1.2 cm in diameter; branched above. Culm nodes glabrous. Culm internodes hollow. Leaves not basally aggregated; non-auriculate. Sheaths glabrous, with cross-veins. Leaf blades lanceolate to ovate; broad; 10–27 mm wide; cordate; flat, or rolled (convolute); shortly pseudopetiolate; without cross venation; persistent. Ligule a fringed membrane; truncate; 1–1.5 mm long. Contra-ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence of spicate main branches, or paniculate (of loose, non-digitate racemes); open; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; secund; biseriate; pedicellate.
Female-fertile spikelets. Spikelets about 9 mm long; lanceolate; adaxial; compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.
Glumes two; very unequal; (the upper) about equalling the spikelets (the lower about 1/2–1/3 its length); (the upper) long relative to the adjacent lemmas; dorsiventral to the rachis; hairless; pointed; awned (the upper caudate-aristate); non-carinate; herbaceous. Lower glume 3 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; paleate. Palea of the proximal incomplete florets fully developed. The proximal incomplete florets male. The proximal lemmas acuminate to shortly awned; awned, or awnless (but then attenuate-mucronate); 5 nerved; herbaceous.
Female-fertile florets 1. Lemmas ovate-lanceolate; papyraceous; smooth to striate; entire; not crested; awnless; hairless; glabrous (shining); non-carinate; with a clear germination flap; 5 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; 2-nerved; 2-keeled. Palea keels wingless. Lodicules present; 2; free; fleshy; glabrous; not or scarcely vascularized. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles basally fused. Stigmas 2; red pigmented.
Fruit, embryo and seedling. Fruit small to medium sized (about 4 mm long); ellipsoid; longitudinally grooved; compressed dorsiventrally (slightly). Hilum long-linear (nearly as long as the grain). Embryo large; not waisted. Endosperm hard.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Mid-intercostal long-cells having markedly sinuous walls. Microhairs present; panicoid-type. Stomata common. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; cross shaped (mostly), or nodular (few); not sharp-pointed.
Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma; Isachne-type. Leaf blade with distinct, prominent adaxial ribs to ‘nodular’ in section; with the ribs more or less constant in size. Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.
Cytology. 2n = 20.
Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paspaleae; Arthropogoninae. 1 species (O. najada).
Distribution, phytogeography, ecology. South America.
Hydrophytic, or helophytic.
Rusts and smuts. Smuts from Ustilaginaceae. Ustilaginaceae — Sorosporium.
References, etc. Morphological/taxonomic: Parodi 1937. Leaf anatomical: studied by us.
Illustrations. • Oplismenopsis najada: Parodi (1937, Notes del Museo de La Plata). • Oplismenopsis najada, abaxial epidermis of leaf blade: this project.
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: 25th January 2024. delta-intkey.com’.
