The grass genera of the world
Type species: O. compressa Edgew.
Habit, vegetative morphology. Perennial; stoloniferous and caespitose. Culms 10–40 cm high; herbaceous; branched above; tuberous. Plants unarmed. Leaves not basally aggregated; non-auriculate. Leaf blades linear-lanceolate; narrow; without abaxial multicellular glands; without cross venation; persistent. Ligule a fringed membrane.
Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.
Inflorescence. Inflorescence of spicate main branches; digitate. Primary inflorescence branches (2–)3–5. Inflorescence espatheate; not comprising partial inflorescences and foliar organs. Spikelet-bearing axes spikes; disarticulating; falling entire (i.e., the spikes falling entire). Spikelets solitary; secund (the spikes dorsiventral).
Female-fertile spikelets. Spikelets 4–6.5(–8) mm long; strongly compressed laterally; disarticulating above the glumes; not disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret. Hairy callus absent.
Glumes two; very unequal (G1 smaller); shorter than the spikelets; (the upper) shorter than the adjacent lemmas, or long relative to the adjacent lemmas; pointed; awned (G2, shortly), or awnless; carinate (the keel scabrid); very dissimilar (membranous, elliptic to oblanceolate, the upper tapering to a cuspidate or shortly-subulate tip and with a thickened, three-nerved keel). Lower glume much exceeding the lowest lemma; 1 nerved. Upper glume 3 nerved (in the keel). Spikelets with female-fertile florets only, or with incomplete florets. The incomplete florets if present, distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.
Female-fertile florets 3–6. Lemmas acute; not becoming indurated (membranous); entire; pointed; mucronate; hairy (villous below on the margins and keel); 3 nerved. Palea present; relatively long; entire (pointed); awnless, without apical setae (hairy); not indurated; 2-nerved; 2-keeled. Palea keels hairy (ciliate). Ovary apically glabrous. Styles free to their bases. Stigmas 2.
Fruit, embryo and seedling. Fruit small (1.5–2 mm long); ellipsoid. Hilum short. Pericarp free. Embryo large.
Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae over-arching the stomata; consisting of one symmetrical projection per cell to several per cell. Long-cells markedly different in shape costally and intercostally (the costals longer); of similar wall thickness costally and intercostally (medium walled). Intercostal zones with typical long-cells (or tending to be rather short). Mid-intercostal long-cells rectangular; having markedly sinuous walls (and conspicuous pits). Microhairs present; more or less spherical; clearly two-celled; chloridoid-type (with sunken basal cell). Microhair apical cell wall of similar thickness/rigidity to that of the basal cell. Microhairs (18–)21 microns long. Microhair basal cells 6–9 microns long. Microhairs 9.6–11.4–12 microns wide at the septum. Microhair total length/width at septum 1.6–2.2. Microhair apical cells (8.4–)9 microns long. Microhair apical cell/total length ratio 0.43–0.47. Stomata common; (21–)22.5–24(–25.5) microns long. Subsidiaries dome-shaped and triangular. Guard-cells overlapping to flush with the interstomatals (apart from the papillae). Intercostal short-cells common; not paired (solitary); not silicified. Intercostal silica bodies absent. Costal short-cells conspicuously in long rows. Costal silica bodies present in alternate cell files of the costal zones; exclusively saddle 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 interrupted; interrupted abaxially only, or interrupted both abaxially and adaxially. PCR sheath extensions present. Maximum number of extension cells 1–3. Mesophyll traversed by columns of colourless mesophyll cells. Leaf blade adaxially flat. Midrib conspicuous; having a conventional arc of bundles (a large median, with two smaller laterals on each side); with colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups; in simple fans and associated with colourless mesophyll cells to form deeply-penetrating fans (linking with traversing columns of colourless cells). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present (at least the primary laterals); forming figures (some of the primaries). Sclerenchyma all associated with vascular bundles. The lamina margins with fibres.
Classification. Watson & Dallwitz (1994): Chloridoideae; main chloridoid assemblage. Soreng et al. (2015): Chloridoideae; Cynodonteae; Eleusininae. 1 species (O. compressa Edgew.).
Distribution, phytogeography, ecology. North Africa, Iran to India.
Xerophytic; species of open habitats. Semidesert.
References, etc. Morphological/taxonomic: Hilu 1981. Leaf anatomical: studied by us.
Illustrations. • O. compressa, abaxial epidermis of leaf blade: 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’.