| # The grass genera of the world |

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

### Oropetium Trin.

~ Tripogon

Including Kralikella Coss. and Dur.

Excluding Chaetostichium, Lepturella

Habit, vegetative morphology. Annual, or perennial; caespitose (dwarf, cushion-forming). Culms 2–15(–17) cm high; herbaceous; branched above, or unbranched above. Culm nodes glabrous. Culm internodes hollow. Plants unarmed. Young shoots intravaginal. Leaves mostly basal, or not basally aggregated; non-auriculate. Leaf blades linear; narrow; about 2 mm wide (to 4 cm long); setaceous, or not setaceous; flat, or folded, or rolled; exhibiting multicellular glands abaxially. The abaxial leaf blade glands intercostal and on the blade margins. Leaf blades without cross venation; disarticulating from the sheaths, or persistent. Ligule an unfringed membrane, or a fringed membrane; truncate; about 0.2 mm long. Contra-ligule absent.

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

Inflorescence. Inflorescence a single spike (straight, curved, sinuous or coiled). Rachides hollowed. Spikelets all partially embedded in the rachis. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes spikes; with substantial rachides (though these slender, herbaceous or spongy); persistent, or disarticulating; when fragile, disarticulating at the joints (or fracturing into segments of 1–4 spikelets). ‘Articles’ non-linear; disarticulating transversely; glabrous. Spikelets solitary; not secund; distichous; sessile.

Female-fertile spikelets. Spikelets 2.5–3.5 mm long; adaxial; compressed laterally; disarticulating above the glumes, or falling with the glumes (and with the joint); with conventional internode spacings. Rachilla prolonged beyond the uppermost female-fertile floret, or terminated by a female-fertile floret; the rachilla extension when present, with incomplete florets. Hairy callus present, or absent. Callus absent, or short; when present, blunt.

Glumes two, or one per spikelet (the G1 sometimes vestigial or missing); relatively large (G2); very unequal (except in terminal spikelets); exceeding the spikelets; (the upper) long relative to the adjacent lemmas; dorsiventral to the rachis; hairless; pointed; awnless; carinate, or non-carinate; very dissimilar (G1 reduced and scarious or missing, G2 covering the florets, hardened). Lower glume if present, much shorter than half length of lowest lemma; 0 nerved. Upper glume 1 nerved, or 3 nerved. Spikelets with female-fertile florets only, or with incomplete florets. The incomplete florets when present, distal to the female-fertile florets. The distal incomplete florets 1 (this male or sterile); merely underdeveloped.

Female-fertile florets 1 (or the second floret rarely also hermaphrodite?). Lemmas less firm than the glumes (hyaline); not becoming indurated; incised; 2 lobed (bidentate); not deeply cleft; mucronate, or awned. Awns when present, 1; (or mucro) from a sinus; non-geniculate; hairless (scabrous); much shorter than the body of the lemma; entered by one vein. Lemmas hairy (at the base or on the nerves), or hairless; carinate to non-carinate; without a germination flap; 3 nerved. Palea present; relatively long (oblong); entire, or apically notched, or deeply bifid; awnless, without apical setae; not indurated (hyaline); 2-nerved; 2-keeled. Palea keels wingless; glabrous. Lodicules present; 2; free; fleshy; glabrous. Stamens 3; with free filaments. Anthers about 0.7 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; brown.

Fruit, embryo and seedling. Fruit free from both lemma and palea (but included); small (about 1.5 mm long); fusiform; compressed laterally. Hilum short. Pericarp loosely adherent (removable when soaked). Embryo small (about 1/4 the length of the fruit).

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls (moderately thick, pitted). Microhairs present; more or less spherical; clearly two-celled; chloridoid-type. Microhair apical cell wall thinner than that of the basal cell but not tending to collapse. Microhairs (16–)19.5–21(–23) microns long. Microhair basal cells 12 microns long. Microhairs (9–)11.4–12(–12.6) microns wide at the septum. Microhair total length/width at septum 1.5–2. Microhair apical cells (6–)8.4–9(–10.5) microns long. Microhair apical cell/total length ratio 0.36–0.47. Stomata common; 22.5–24 microns long. Subsidiaries triangular. Intercostal short-cells absent or very rare. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; saddle shaped; not sharp-pointed.

Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.

C4; XyMS+. PCR sheath outlines even. PCR sheaths of the primary vascular bundles interrupted; interrupted abaxially only. PCR sheath extensions absent. PCR cell chloroplasts centripetal. Mesophyll with radiate chlorenchyma; without adaxial palisade; traversed by columns of colourless mesophyll cells (in places). Leaf blade with distinct, prominent adaxial ribs to ‘nodular’ in section; with the ribs more or less constant in size (rib apices rounded). Midrib not readily distinguishable; with one bundle only. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans (these sometimes linking with traversing columns of clourless cells). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (most bundles with anchors). Sclerenchyma all associated with vascular bundles.

Cytology. Chromosome base number, x = 10. 2n = 20.

Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Tripogoninae. 3–4 species.

Distribution, phytogeography, ecology. Arid subtropical Africa and mountains.

Mesophytic to xerophytic; species of open habitats; glycophytic. In shallow soil between or over rocks and in outwashes.

References, etc. Morphological/taxonomic: Phillips 1974. Leaf anatomical: Metcalfe 1960; studied by us - O. thomaeum (L. f.) Trin.; photos of O. capense provided by R.P. Ellis.

Illustrations. • Oropetium aristatum: Rose Innes, Ghana Grasses (1977). • General aspect (Oropetium capense): Gibbs Russell et al., 1990. • Oropetium thomaea: http://legacy.tropicos.org/Name/25520343.

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: 15th June 2025. delta-intkey.com’.

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