The grass genera of the world
Type species: Type: O. nodosa (Kunth) Dandy.
Including Hemigymnia Stapf
Habit, vegetative morphology. Perennial; decumbent (slender). Culms herbaceous; self-supporting; sometimes amply branched above. The branching simple. Culm nodes glabrous. Culm leaf sheaths keeled. Culm internodes solid, or hollow. Leaves not basally aggregated; non-auriculate. Leaf blades linear to lanceolate; broad, or narrow; flat (thin); pseudopetiolate, or not pseudopetiolate; cross veined, or without cross venation; tardily disarticulating from the sheaths; rolled in bud. Ligule an unfringed membrane to a fringed membrane.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence paniculate (the branches weakly unilateral, with appressed secondary racemelets), or a single raceme; open; with capillary branchlets; non-digitate. Primary inflorescence branches inserted all around the main axis. Inflorescence espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary, or paired; not secund; pedicellate (the pedicels widened upwards). Pedicel apices cupuliform.
Female-fertile spikelets. Spikelets 1.7–6.4 mm long; elliptic; abaxial or not orientated; compressed dorsiventrally; falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.
Glumes two; very unequal (O. grandiflora), or more or less equal; shorter than the spikelets (about 1/2 to 2/3 as long), or about equalling the spikelets (O. grandiflora); shorter than the adjacent lemmas (except in O. grandiflora); hairless (usually glabrous); pointed, or not pointed; awnless; similar (membranous). Lower glume 3–5 nerved. Upper glume 3–7 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; 5–7 nerved; more or less equalling the female-fertile lemmas; less firm than the female-fertile lemmas; not becoming indurated.
Female-fertile florets 1. Lemmas decidedly firmer than the glumes (subleathery); smooth to striate; becoming indurated to not becoming indurated; yellow in fruit; entire; pointed; awnless to mucronate (or rather, apiculate to mucronulate); hairless; non-carinate; having the margins lying flat on the palea (O. grandiflora), or having the margins inrolled against the palea; with a clear germination flap; 3 nerved, or 5 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; 2-nerved. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.
Fruit, embryo and seedling. Fruit small; compressed dorsiventrally (strongly). Hilum short. Embryo large; not waisted.
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 (the costals slightly thicker walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 39–69 microns long; 5–8.4 microns wide at the septum. Microhair total length/width at septum 6.5–10. Microhair apical cells 21–36 microns long. Microhair apical cell/total length ratio 0.48–0.62. Stomata common; 24–37.5 microns long. Subsidiaries dome-shaped. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs, or not paired (solitary); silicified, or not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies panicoid-type; not sharp-pointed.
Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma. Leaf blade nodular in section; with the ribs more or less constant in size. Midrib conspicuous, or 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.
Culm anatomy. Culm internode bundles in one or two rings.
Special diagnostic feature. Plants not as in Dichanthelium (q.v.).
Cytology. Chromosome base number, x = 9. 2n = 18. 2 ploid.
Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paniceae; Boivinellinae. 5 species.
Distribution, phytogeography, ecology. Africa, Indomalayan region, Australia.
Shade species and species of open habitats. Damp and shady places.
Economic aspects. Significant weed species: O. nodosa.
Rusts and smuts. Rusts Physopella and Puccinia. Taxonomically wide-ranging species: Physopella clemensiae, Puccinia orientalis, and Uromyces setariae-italicae.
References, etc. Leaf anatomical: studied by us - O. gracillima Hubbard.
Special comments. The morphological description incorporates corrections forwarded by J.F. Veldkamp (1997), who draws attention to peculiarities of the Malesian species, O. grandiflora. Illustrations. • O. arnottiana: Jacques-Félix, 1962. • O. gracillima, abaxial epidermis of leaf blade: this project. • O. gracillima, abaxial epidermis of leaf blade: this project. • TS leaf blade of O. gracillima: 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: 11th December 2017. delta-intkey.com/grass’.