| # The grass genera of the world |

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

### Commelinidium Stapf

~ Acroceras

Habit, vegetative morphology. Commelinaceous perennial; rhizomatous to stoloniferous, or decumbent. Culms 10–90 cm high; herbaceous. Culm nodes glabrous. Culm internodes solid. Leaves not basally aggregated; non-auriculate. Leaf blades ovate; broad; 15–40 mm wide; cordate, or not cordate, not sagittate; pseudopetiolate, or not pseudopetiolate; cross veined; disarticulating from the sheaths. Ligule a fringed membrane; short.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets.

Inflorescence. Inflorescence of spicate main branches (spiciform racemes), or paniculate; open; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary, or paired; somewhat secund; pedicellate.

Female-fertile spikelets. Spikelets 4–6 mm long; abaxial; compressed dorsiventrally; falling with the glumes; with a distinctly elongated rachilla internode between the glumes. Rachilla prolonged beyond the uppermost female-fertile floret (as a minute vestige), or terminated by a female-fertile floret; the rachilla extension when present, naked. Hairy callus absent.

Glumes two (separated by a marked internode); very unequal; (the longer) long relative to the adjacent lemmas; free; dorsiventral to the rachis; hairless; pointed (with a small apical callosity); awnless; carinate; similar (membranous-herbaceous). Lower glume 3–5 nerved. Upper glume 5 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets reduced. The proximal incomplete florets sterile. The proximal lemmas similar to the glumes; awnless; 5 nerved; more or less equalling the female-fertile lemmas to decidedly exceeding the female-fertile lemmas; less firm than the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas decidedly firmer than the glumes (leathery); smooth; not becoming indurated; entire; crested at the tip (cf. Acroceras, Cyrtococcum); awnless (but apiculate); hairless (shining); non-carinate; having the margins inrolled against the palea; with a clear germination flap; 7–9 nerved. Palea present; relatively long; entire; awnless, without apical setae; textured like the lemma; not indurated; 2-nerved; keel-less. Lodicules present; 2; free; fleshy; glabrous; not or scarcely vascularized. Stamens 3. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2; red pigmented.

Fruit, embryo and seedling. Fruit small. Hilum short.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Intercostal zones exhibiting many atypical long-cells. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type; (60–)63–78(–79.5) microns long; 6.3–9 microns wide at the septum. Microhair total length/width at septum 7–12.4. Microhair apical cells 27–36 microns long. Microhair apical cell/total length ratio 0.42–0.46. Stomata common; 27–33 microns long. Subsidiaries high dome-shaped and 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’.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with radiate chlorenchyma. Leaf blade adaxially flat. Midrib conspicuous; with one bundle only. Bulliforms not present in discrete, regular adaxial groups (constituting most of adaxial epidermis). All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Cytology. 2n = 36.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): cf. Panicoideae (as a synonym?); Panicodae; Paniceae; Boivinellinae. 3 species.

Distribution, phytogeography, ecology. West tropical Africa.

Shade species.

References, etc. Leaf anatomical: studied by us - C. gabunense (Hack.) Stapf.

Illustrations. • Commelinidium gabunense: Rose Innes, Ghana Grasses (1977). • Commelinidium gabunense (as C. nervosum): Jacques-Félix, 1962. • Fruiting floret of Commelinidium gabunense). Commelinidium gabunense. Thickened/inrolled lemma margins. • Germination flap of Commelinidium gabunense).

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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