| # The grass genera of the world |

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

### Hygroryza Nees

From the Greek hygros (wet, moist) and Oryza (a related genus).

Including Potamochloa Griff.

Habit, vegetative morphology. Aquatic perennial; stoloniferous (with adventitious roots). The flowering culms leafy. Culms herbaceous; unbranched above. Culm nodes glabrous. Leaves not basally aggregated; non-auriculate; without auricular setae. Sheath margins free. Sheaths inflated, tessellate-veined. Leaf blades ovate-lanceolate to elliptic; broad; to 20 mm wide; somewhat cordate; flat; pseudopetiolate (floating); cross veined; persistent. Ligule an unfringed membrane; truncate; 0.8 mm long. Contra-ligule absent.

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

Inflorescence. Inflorescence relatively few spikeleted (partly enclosed in the upper leaf sheath); paniculate; open; espatheate (but the uppermost leaf quite reduced); not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes persistent. Spikelets solitary; not secund; imbricate.

Female-fertile spikelets. Spikelets unconventional (by virtue of the vestigial glumes); lanceolate; compressed laterally; falling with the glumes (at least, disarticulating well below the annular rim which presumably represents them: the floret falling on its long, slender stipe). Rachilla terminated by a female-fertile floret. Hairy callus absent.

Glumes more or less absent (being represented at the most by an annular rim). Spikelets with female-fertile florets only.

Female-fertile florets 1. Lemmas attenuate; not becoming indurated (papery or firmly membranous); entire; pointed; awned. Awns 1; median; apical; non-geniculate; hairless; about as long as the body of the lemma to much longer than the body of the lemma. Lemmas hairy (hispid, especially on the veins); carinate; without a germination flap; 5 nerved (the laterals close to the margins); with the nerves confluent towards the tip. Palea present; relatively long (about equalling the lemma); entire (acute); awnless, without apical setae; textured like the lemma; not indurated (papery); several nerved (3); one-keeled. Lodicules present; 2; free; membranous; glabrous; not toothed; heavily vascularized. Stamens 6. Anthers about 1 mm long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small (about 3.5mm long); not noticeably compressed (terete). Hilum long-linear. Embryo small. Endosperm hard.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; costal and intercostal. Intercostal papillae over-arching the stomata (but only very slightly, at the ends); several per cell (each long- and interstomatal cell with one large central papilla, this and the entire cell surface ornamented by numerous small cuticular papillae - only the guard cells and subsidiaries lack ornamentation). Long-cells of similar wall thickness costally and intercostally (walls of medium thickness). Intercostal zones exhibiting many atypical long-cells (most being short to more or less isodiametric). Mid-intercostal long-cells having markedly sinuous walls. Microhairs present; elongated; ostensibly one-celled; these in the form of minute, seemingly 1-celled trichomes - their bases presenting the appearance of those of conventional microhairs at first sight. Stomata common. Subsidiaries non-papillate; dome-shaped and triangular. Guard-cells overlapped by the interstomatals (very slightly), or overlapping to flush with the interstomatals. Intercostal short-cells common; not paired (solitary); silicified. Intercostal silica bodies minutely oryzoid-type. Costal short-cells conspicuously in long rows. Costal silica bodies oryzoid (exclusively, a large version).

Transverse section of leaf blade, physiology. C3. Mesophyll with non-radiate chlorenchyma; with arm cells, or without arm cells (? - these not apparent in the poor material seen); with fusoids. The fusoids external to the PBS. Leaf blade with distinct, prominent adaxial ribs. Midrib with one bundle only. Combined sclerenchyma girders present. Sclerenchyma all associated with vascular bundles.

Special diagnostic feature. Plants aquatic, with inflated leaf sheaths serving as floats.

Cytology. Chromosome base number, x = 12. 2n = 24. 2 ploid.

Classification. Watson & Dallwitz (1994): Bambusoideae; Oryzodae; Oryzeae. Soreng et al. (2015): Oryzoideae; Oryzeae; Zizaniinae. 1 species (H. aristata).

Distribution, phytogeography, ecology. Eastern Asia.

Hydrophytic.

Economic aspects. Significant weed species: H. aristata (in rice).

References, etc. Leaf anatomical: studied by us.

Illustrations. • Hygroryza aristata, with Leersia, Zizania and Elytrophorus: Wu Zhengyi, Flora Yunnanica 9 (2003). • Hygroryza aristata, 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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