The grass genera of the world
Habit, vegetative morphology. Perennial; rhizomatous, or caespitose. The flowering culms leafy. Culms 80–180 cm high; herbaceous; to 0.4 cm in diameter; branched above. Culm nodes glabrous. Culm internodes hollow. Plants unarmed. Leaves not basally aggregated; auriculate; with auricular setae. Leaf blades lanceolate to elliptic; broad; pseudopetiolate; cross veined; rolled in bud. Ligule present; an unfringed membrane (minutely ciliolate); truncate; 0.5–1 mm long. Contra-ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence paniculate, or of spicate main branches (then the primary branches reduced to open racemes); open; with capillary branchlets; espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; pedicellate.
Female-fertile spikelets. Spikelets 4–12 mm long; compressed laterally; falling with the glumes. Rachilla prolonged beyond the uppermost female-fertile floret (adherent to the lower back of the palea); hairy (minutely); the rachilla extension with incomplete florets. Hairy callus absent.
Glumes two; very unequal (G1 about 2/3 the length of G2); shorter than the adjacent lemmas; free; hairless (sparsely scabrid); pointed; awnless; carinate (somewhat); similar (lanceolate, herbaceous-membranous). Lower glume 3–5 nerved. Upper glume 3–5 nerved. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped. Spikelets without proximal incomplete florets.
Female-fertile florets 1–5 (decreasing in size acropetally). Lemmas acuminate; similar in texture to the glumes to decidedly firmer than the glumes; not becoming indurated; entire; pointed; awnless; hairless (sparsely scabrid above); slightly carinate; 5–6 nerved. Palea present; relatively long; apically notched (minutely); awnless, without apical setae; textured like the lemma; not indurated; 2-nerved (but folded, bringing the keels together); 2-keeled (the keels joined to the lower part of the internode above). Palea keels minutely winged (above, and the internode above situated in the groove between them). Lodicules present; 2; free; fleshy; glabrous; not toothed; heavily vascularized. Stamens 2 (O. laxa), or 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2.
Fruit, embryo and seedling. Fruit free from both lemma and palea; small; compressed laterally. Hilum short. Embryo large; with an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells markedly different in shape costally and intercostally (the costals relatively much longer); of similar wall thickness costally and intercostally (thin walled). Intercostal zones with typical long-cells (though with some rather short). Mid-intercostal long-cells rectangular; having markedly sinuous walls (very deeply so). Microhairs present; panicoid-type; (46.5–)57–60 microns long; 3–3.6 microns wide at the septum. Microhair total length/width at septum 12.9–20. Microhair apical cells (28.5–)34.5–37.5(–39) microns long. Microhair apical cell/total length ratio 0.58–0.66. Stomata common; (30–)36–39 microns long. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare. Costal short-cells conspicuously in long rows. Costal silica bodies panicoid-type; mostly cross shaped, butterfly shaped, and dumb-bell shaped; not sharp-pointed.
Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with adaxial palisade; seemingly without arm cells (but material seen poor); with fusoids (shortish). The fusoids an integral part of the PBS. Leaf blade with the ribs more or less constant in size. Midrib conspicuous; having a conventional arc of bundles (one large bundle, two small laterals); with colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups (these large); in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders absent (adaxial strand-abaxial girder combinations only). Sclerenchyma all associated with vascular bundles.
Cytology. Chromosome base number, x = 12. 2n = 24. 2 ploid.
Classification. Watson & Dallwitz (1994): Centothecoideae; Centotheceae. Soreng et al. (2015): Panicoideae; Zeugiteae. 2 species.
Distribution, phytogeography, ecology. 1 in southern Mexico to tropical South America and West Indies; 1 in southeastern tropical Africa.
Shade species; glycophytic. In forests.
References, etc. Leaf anatomical: this project.
Illustrations. • O. rarifolia: P. Beauv. (1812). • O. laxa: Kunth (1835). • O. laxa: Hitchcock (1936)
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’.