| # The grass genera of the world |

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

### Olmeca Soderstrom

Named for the Olmec Indians of Mexico.

Habit, vegetative morphology. Arborescent perennial; rhizomatous (the stems solitary, the rhizome necks up to 8 m long). The flowering culms leafy. Culms 150 cm high; woody and persistent; to 5 cm in diameter; branched above. Buds from which the primary culm branches arise 1. Primary branches 1. The branching dendroid. Culm nodes glabrous. Culm leaf sheaths present; deciduous, or persistent; leaving a persisten girdle (rarely), or not leaving a persistent girdle; conspicuously auriculate, or not conspicuously auriculate. Culm leaves with conspicuous blades. Culm leaf blades linear, or lanceolate, or triangular. Culm internodes solid, or hollow. Unicaespitose. Rhizomes pachymorph. Plants unarmed. Leaves not basally aggregated; non-auriculate; with auricular setae. Leaf blades broad; pseudopetiolate, or not pseudopetiolate; without cross venation; rolled in bud. Ligule truncate, or not truncate; 1–4 mm long. Contra-ligule present, or absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. Not viviparous.

Inflorescence. Inflorescence paniculate; open; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes paniculate; with very slender rachides. ‘Articles’ glabrous. Spikelets pedicellate.

Female-fertile spikelets. Spikelets 30–40 mm long; linear, or lanceolate; not noticeably compressed; disarticulating above the glumes; disarticulating between the florets. Rachilla prolonged beyond the uppermost female-fertile floret; hairless; the rachilla extension with incomplete florets. Hairy callus absent.

Glumes two; very unequal; shorter than the adjacent lemmas; hairless; not pointed; awnless; non-carinate; similar. Lower glume longer than half length of lowest lemma. Spikelets with incomplete florets. The incomplete florets distal to the female-fertile florets. The distal incomplete florets merely underdeveloped.

Female-fertile florets 3–12 (variable in number). Lemmas not becoming indurated; entire; pointed; awnless, or mucronate (?); hairless; non-carinate; without a germination flap; 7–11 nerved. Palea present; relatively long; not convolute; apically notched; awnless, without apical setae; not indurated; 2-nerved, or several nerved (2–6); 2-keeled. Lodicules present; 3; free; membranous; ciliate; toothed, or not toothed; heavily vascularized. Stamens 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous; without a conspicuous apical appendage. Styles fused. Stigmas 2–3.

Fruit, embryo and seedling. Fruit large (1.5–2.5 cm in diameter); subglobose. Pericarp fleshy. Seed ‘non-endospermic’.

Seedling with a short mesocotyl. First seedling leaf without a lamina.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally (the costals rather smaller); of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 48–60 microns long; 6.9–9.6 microns wide at the septum. Microhair total length/width at septum 5.6–7.7. Microhair apical cells 24–33 microns long. Microhair apical cell/total length ratio 0.5–0.55. Stomata common; 25–30 microns long. Subsidiaries high dome-shaped (mostly), or triangular (some). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs; silicified, or not silicified. Intercostal silica bodies when present, narrowly saddle shaped. Costal short-cells predominantly paired. Costal silica bodies saddle shaped (a narrowish version predominating), or oryzoid (some of the saddles being almost interpretable as such); not sharp-pointed.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with adaxial palisade; with arm cells; with fusoids. The fusoids external to the PBS. Leaf blade adaxially flat. Midrib conspicuous; having complex vascularization. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups (a large group in each intercostal zone); in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’ (the primaries and some of the smaller bundles with large ‘anchors’ or I’s). Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Bambusoideae; Bambusodae; Bambuseae. Soreng et al. (2015): Bambusoideae; Bambusodae; Bambuseae; Guaduinae. 2 species.

Distribution, phytogeography, ecology. Mexico.

References, etc. Morphological/taxonomic: Soderstrom 1981e. Leaf anatomical: studied by us - O. recta Soderstrom.

Special comments. Fruit data wanting.

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: 15th June 2025. delta-intkey.com’.

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