| # The grass genera of the world |

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

### Hymenachne P. Beauv.

From the Greek hymen (a membrane) and achne (chaff), alluding to membranous glumes, lemmas and paleas.

Type species: Type: not designated. Lecto: H. amplexicaulis (Rudge) Chase, fide G.Panigrahi & A.K.Dubey, Taxon 35: 338–340 (1886).

Excluding Dallwatsonia

Habit, vegetative morphology. Perennial; decumbent aquatics. Culms 50–200 cm high; herbaceous; branched above, or unbranched above. The branching simple. Culm nodes glabrous. Culm leaf sheaths rounded. Culm internodes solid (but aerenchymatous). Leaves not basally aggregated; non-auriculate. Leaf blades linear to lanceolate; broad, or narrow; cordate, or not cordate, not sagittate; not pseudopetiolate; cross veined, or without cross venation; persistent. Ligule present; an unfringed membrane.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality.

Inflorescence. Inflorescence of spicate main branches, or paniculate (spiciform or contracted, often with appressed branches). Primary inflorescence branches inserted all around the main axis. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets secund (the branches unilateral). Pedicel apices truncate, or discoid. Spikelets not in distinct ‘long-and-short’ combinations.

Female-fertile spikelets. Spikelets narrowly lanceolate; abaxial; not noticeably compressed to compressed dorsiventrally; falling with the glumes; with a distinctly elongated rachilla internode between the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes present; two; very unequal; (the upper) shorter than the spikelets to about equalling the spikelets (G1 about half as long); shorter than the adjacent lemmas to long relative to the adjacent lemmas; free (widely separated); awnless; similar (hyaline, not gibbous). Lower glume about 0.5 times the length of the upper glume; 1–3 nerved. Upper glume 3–7 nerved (usually acuminate). Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas acute, acuminate or awnlike; more or less awned, or awnless; 3–5 nerved; more or less equalling the female-fertile lemmas to decidedly exceeding the female-fertile lemmas; less firm than the female-fertile lemmas to similar in texture to the female-fertile lemmas (hyaline, lanceolate, acuminate); not becoming indurated.

Female-fertile florets 1. Lemmas similar in texture to the glumes to decidedly firmer than the glumes (membranous to leathery); smooth; not becoming indurated; white in fruit; entire; pointed; awnless; hairless; non-carinate; having the margins lying flat on the palea (except at the base); without a germination flap; 2 nerved, or 3 nerved. Palea present; relatively long; gaping; entire; awnless, without apical setae; textured like the lemma; not indurated. Lodicules present; 2; free; fleshy; glabrous. Stamens 3. Anthers not penicillate. Ovary apically glabrous. Styles fused. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small; compressed dorsiventrally. Hilum short. Embryo large; waisted.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent. Long-cells similar in shape costally and intercostally; of similar wall thickness costally and intercostally. Mid-intercostal long-cells rectangular to fusiform; having markedly sinuous walls. Microhairs present; more or less chloridoid-type (though the apical cells fairly thin walled); 27–36 microns long; 7.5–9 microns wide at the septum. Microhair total length/width at septum 2–4.3. Microhair apical cells (12–)13.5–15(–18) microns long. Microhair apical cell/total length ratio 0.44–0.54. Stomata common; 30–34.5 microns long. Subsidiaries triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; in cork/silica-cell pairs, or not paired (solitary); silicified (when paired), or not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’.

Transverse section of leaf blade, physiology. C3; XyMS+. Mesophyll with non-radiate chlorenchyma. Leaf blade ‘nodular’ in section, or adaxially flat; with the ribs more or less constant in size. Midrib conspicuous; having a conventional arc of bundles (a small lateral on each side). Bulliforms present in discrete, regular adaxial groups; in simple fans. All the vascular bundles accompanied by sclerenchyma. Combined sclerenchyma girders present; nowhere forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Cytology. 2n = 24.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paspaleae; Otachyriinae. 5 species.

Distribution, phytogeography, ecology. Tropical.

Hydrophytic to helophytic (in swamps); glycophytic.

Economic aspects. Important native pasture species: H. acutigluma, in swamps.

References, etc. Leaf anatomical: studied by us - H. amplexicaulis (Rudge) Nees.

Illustrations. • Hymenachne amplexicaulis: Nicora & Rúgolo de Agrasar (1987). • Hymenachne amplexicaulis, abaxial epidermis of leaf blade: this project. • Hymenachne amplexicaulis, 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: 15th June 2025. delta-intkey.com’.

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