| # The grass genera of the world |

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

### Hydrothauma C.E. Hubb.

Habit, vegetative morphology. Aquatic annual. Culms herbaceous; branched above. Leaves not basally aggregated. Leaf blades linear (the emergent blades with sinuous longitudinal lamellae adaxially); narrow; pseudopetiolate (the blades of the lower leaves floating on their long ‘petioles’). Ligule an unfringed membrane.

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

Inflorescence. Inflorescence short, slender, a single raceme (with paired spikelets), or paniculate; spicate; non-digitate; espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets paired; secund; consistently in ‘long-and-short’ combinations. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets hermaphrodite.

Female-fertile spikelets. Spikelets abaxial; compressed dorsiventrally (asymmetrical in profile); falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes two; very unequal; (the upper) about equalling the spikelets; (the upper) long relative to the adjacent lemmas; dorsiventral to the rachis; hairless; awnless; non-carinate; very dissimilar (the lower short, orbicular-truncate, membranous-hyaline, the upper elongated, gibbous, equalling the spikelet, membranous-herbaceous). Lower glume 0 nerved. Upper glume distinctly saccate (cf. Sacciolepis); 7–9 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; paleate. Palea of the proximal incomplete florets reduced. The proximal incomplete florets male, or sterile. The proximal lemmas awnless; 7 nerved; decidedly exceeding the female-fertile lemmas (resembling G2); less firm than the female-fertile lemmas; not becoming indurated.

Female-fertile florets 1. Lemmas decidedly firmer than the glumes (leathery to crustaceous); becoming indurated; entire; awnless; hairless; non-carinate; obscurely nerved. Palea present; relatively long (somewhat leathery); tightly clasped by the lemma; entire (pointed); awnless, without apical setae; textured like the lemma; indurated. Lodicules present; 2; free; fleshy. Stamens 3. Anthers not penicillate; without an apically prolonged connective (anthers divaricate). Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae absent (though very abundant adaxially, both intercostally and over the lamellae). Long-cells of similar wall thickness costally and intercostally (fairly thin walled). Mid-intercostal long-cells rectangular; having markedly sinuous walls (the sinuosity coarse and irregular). Microhairs present; elongated; clearly two-celled; panicoid-type; without ‘partitioning membranes’ (the costals very much narrower). Stomata absent or very rare. Intercostal short-cells common (in places - exclusive of abundant microhair bases); not paired (seemingly solitary); not silicified. Costal short-cells conspicuously in long rows. Costal silica bodies present and well developed; ‘panicoid-type’; nearly all nodular.

Transverse section of leaf blade, physiology. Leaf blades ‘laminar’ (very unusual, with high densely papillate lamella-ribs running longitudinally in association with each vascular bundle).

C3; XyMS+. Mesophyll without adaxial palisade; not Isachne-type; without fusoids (but with rounded or angular lacunae in the abaxial part of the mesophyll). Leaf blade with distinct, prominent adaxial ribs (i.e., the lamellae). Combined sclerenchyma girders absent (the sclerenchyma very scanty). Sclerenchyma not all bundle-associated. The ‘extra’ sclerenchyma in adaxial groups (there being small groups of hypodermal fibres, and single large lignified hypodermal cells, in the lamellae).

Special diagnostic feature. The adaxial surface of the leaf blade raised into sinuous lamellae.

Classification. Watson & Dallwitz (1994): Panicoideae; Panicodae; Paniceae. Soreng et al. (2015): Panicoideae; Panicodae; Paniceae. 1 species (H. manicatum).

Distribution, phytogeography, ecology. Southern tropical Africa.

Hydrophytic (in shallow pools).

References, etc. Morphological/taxonomic: Hubbard 1947. Leaf anatomical: Metcalfe 1960, and studied by us.

Special comments. Worthy of detailed anatomical study based on better material than that seen. The presence of abundant microhairs on the submerged, astomatal abaxial blade surface is odd, and many of the vascular bundles are associated with an abaxial sclerenchyma group incorporating an extraordinarily large, lignified element. Fruit data wanting.

Illustrations. • Hydrothauma manicatum: Jacques-Félix, 1962. • Hydrothauma manicatum: Clayton, © Flora Zambesiaca 10 (1989).

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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