The grass genera of the world
Habit, vegetative morphology. Perennial; caespitose. Culms 20–80 cm high; herbaceous; unbranched above. Culm nodes glabrous (seemingly fragile). Culm internodes solid. Young shoots intravaginal. Leaves mostly basal; non-auriculate. Leaf blades linear; narrow (filiform, conduplicate-involute, rigid); about 0.5–0.7 mm wide; setaceous; folded, or rolled; without cross venation; persistent; rolled in bud. Ligule a fringed membrane, or a fringe of hairs (short); truncate; 0.2–0.3 mm long. Contra-ligule absent.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence a single spike (narrow, cylindrical, terminating the culm). Rachides hollowed. Inflorescence espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes cylindrical spikes; solitary; with substantial rachides; disarticulating; disarticulating at the joints. Articles linear; without a basal callus-knob; slightly appendaged (at the upper end); disarticulating obliquely (very much so); glabrous. Spikelets solitary (i.e. only theoretically in pairs, the pedicel fused with and indistinguishable from the rachis); not secund (the sessile spikelets in two opposite rows); distichous; sessile; not imbricate.
Female-fertile spikelets. Spikelets 4–6 mm long; abaxial; compressed dorsiventrally; planoconvex; falling with the glumes (and with the adjacent joint); with conventional internode spacings. Rachilla terminated by a female-fertile floret. Hairy callus absent. Callus blunt (and thick).
Glumes two; more or less equal; exceeding the spikelets; long relative to the adjacent lemmas; dorsiventral to the rachis; hairless; glabrous; not pointed (the tips rounded); awnless; non-carinate (rounded on back); very dissimilar (G1 obtuse, leathery, G2 apically notched or entire, membranous-hyaline). Lower glume much exceeding the lowest lemma; not two-keeled (and wingless); convex on the back; not pitted; relatively smooth; 6–7 nerved. Upper glume 2 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas awnless (obtuse); 0 nerved, or 2 nerved; more or less equalling the female-fertile lemmas to decidedly exceeding the female-fertile lemmas; similar in texture to the female-fertile lemmas (hyaline); not becoming indurated.
Female-fertile florets 1. Lemmas less firm than the glumes; entire; blunt (truncate-obtuse); awnless; hairless; glabrous; non-carinate; without a germination flap; 2 nerved. Palea present, or absent; when present, very reduced (adherent to the lodicules); entire (truncate), or apically notched (emarginate or bilobed); awnless, without apical setae; textured like the lemma; not indurated (hyaline); nerveless; keel-less. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers about 2.5 mm long; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; brown.
Fruit, embryo and seedling. Fruit compressed dorsiventrally. Hilum short. Embryo large.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous to lacking (the epidermis very featureless, but the costal zones - especially over the midrib - with narrower long-cells). Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally (fairly thick walled). Intercostal zones mainly with typical long-cells (with a few short to almost circular). Mid-intercostal long-cells with shortly rounded ends; having markedly sinuous walls (and conspicuous pits). Microhairs absent. Stomata absent or very rare. Intercostal short-cells common; mostly in cork/silica-cell pairs (mostly somewhat superposed); silicified. Costal short-cells also predominantly paired (the short-cell configurations and silica bodies not noticeably different from those of the intercostal zones). Costal silica bodies all small, intergrading rounded, tall-and-narrow, and crescentic; not sharp-pointed.
Transverse section of leaf blade, physiology. C4; XyMS. PCR sheath outlines even. PCR sheath extensions absent. Mesophyll with radiate chlorenchyma. Leaf blade with distinct, prominent adaxial ribs (the rather slight ribs emphasised by tufts of large, inflated macrohairs, cf. Hygrochloa); with the ribs more or less constant in size. Midrib conspicuous (by its position, the narrowing of the lamina, the smaller abaxial epidermal cells and smaller abaxial sclerenchyma group associated with it, and by the smaller macrohairs above it); with one bundle only. The lamina symmetrical on either side of the midrib. Bulliforms bulliforms indistinct. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (with the main laterals); forming figures (the main laterals with anchors). Sclerenchyma not all bundle-associated. The extra sclerenchyma in a continuous abaxial layer (there being a conspicuous, lignified, large-celled hypodermis abaxially between the main lateral bundles).
Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Rottboelliinae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Rottboelliinae. 1 species (O. gracillima).
Distribution, phytogeography, ecology. Tropical Africa, Madagascar.
Helophytic; species of open habitats; glycophytic. Upland streamsides and marshy places.
References, etc. Leaf anatomical: studied by us.
Illustrations. • General aspect (O. gracillima): 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: 11th December 2017. delta-intkey.com/grass’.