The grass genera of the world
Habit, vegetative morphology. Glaucous, creeping perennial; rhizomatous. The flowering culms leafy. Culms 5–75 cm high; herbaceous; branched above, or unbranched above (O. paucinervis). The branching suffrutescent (O. mucronata). Plants conspicuously armed (leaf blades short, rigid and very pungent-tipped). Leaves not basally aggregated; conspicuously distichous; non-auriculate. The sheaths imbricate. Leaf blades narrow; 2–4 mm wide; flat and rolled (inrolled from the flat base); hard, woody, needle-like; without abaxial multicellular glands; without cross venation; disarticulating from the sheaths. Ligule a fringe of hairs. Contra-ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence paniculate (of short, crowded branches); fairly to very contracted; capitate to more or less ovoid, or more or less irregular; espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; somewhat secund, or not secund; pedicellate.
Female-fertile spikelets. Spikelets 5–9 mm long; adaxial; somewhat compressed laterally; disarticulating above the glumes; disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; hairy (to subglabrous); the rachilla extension with incomplete florets. Hairy callus present (with lateral hair tufts, not conspicuous). Callus short.
Glumes two; very unequal; shorter than the spikelets; shorter than the adjacent lemmas; hairless; glabrous; glume apices pointed or irregularly toothed; awnless (but sometimes with the nerve tip constituting a tiny mucro); carinate; similar (thinly membranous to hyaline). Lower glume 1 nerved. Upper glume 1 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.
Female-fertile florets 2–8. Lemmas similar in texture to the glumes to decidedly firmer than the glumes (membranous with scarious margins, or scarious); not becoming indurated; incised; not deeply cleft (bidenticulate); mucronate; hairy (with silky white hairs on the lower half of the mid-nerve, and along the laterals); somewhat carinate, or non-carinate; without a germination flap; 3 nerved. Palea present; relatively long; slightly apically notched; awnless, without apical setae, or with apical setae (the nerves sometimes slightly prolonged); textured like the lemma; not indurated; 2-nerved; 2-keeled. Palea keels silky hairy. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers 2 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2.
Fruit, embryo and seedling. Fruit small (1.1–1.5 mm long); ellipsoid; slightly concave on the hilar side; compressed laterally. Hilum short. Pericarp free. Embryo large (around 1/3 grain length).
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Mid-intercostal long-cells having markedly sinuous walls. Microhairs present (very large); elongated; clearly two-celled; chloridoid-type. Microhair apical cell wall thinner than that of the basal cell but not tending to collapse. Microhairs (39–)42–48(–51) microns long. Microhair basal cells 39 microns long. Microhairs (16.5–)20.4–21 microns wide at the septum. Microhair total length/width at septum 2–3.1. Microhair apical cells (15–)16.5–20(–21) microns long. Microhair apical cell/total length ratio 0.31–0.42. Stomata common; 21–22.5–24 microns long. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells not paired. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; panicoid-type; cross shaped, butterfly shaped, and dumb-bell shaped; not sharp-pointed.
Transverse section of leaf blade, physiology. Lamina mid-zone in transverse section open.
C4; XyMS+. PCR sheaths of the primary vascular bundles complete. PCR sheath extensions present. Maximum number of extension cells 1–3. PCR cell chloroplasts centripetal. Mesophyll not traversed by colourless columns (but deeply penetrating bulliform groups almost traversing the leaf). Leaf blade nodular in section (with deep adaxial and abaxial furrows, the abaxial ribs broader and flat-bottomed); with the ribs more or less constant in size. 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. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (including bundle sheath extensions); forming figures (Is with all bundles). Sclerenchyma all associated with vascular bundles.
Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Aeluropodinae. 2 species.
Distribution, phytogeography, ecology. Coastal Red Sea, tropical and southwest Africa.
Xerophytic; species of open habitats; halophytic. A sandbinder.
References, etc. Morphological/taxonomic: Stapf 1922. Leaf anatomical: this project.
Illustrations. • O. mucronata: Hooker’s Icones Plantarum 31 (1922). • General aspect (O. paucinervis): Gibbs Russell et al., 1990
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: 13th November 2017. delta-intkey.com/grass’.