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The grass genera of the world

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

Orthoclada P. Beauv.

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: studied by us - O. laxa Beauv.

Illustrations. • O. rarifolia: P. Beauv. (1812). • O. laxa: Kunth (1835). • O. laxa: Hitchcock (1936)

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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.’.