5.03 HYGRORYZA Nees
J.K. Wipff

Plants perennial; aquatic, producing long, floating culms; synoecious. Culms 50–150 cm, spongy, developing adventitious roots at the nodes, branched; branches erect, leafy. Leaves cauline, glabrous, veins tessellate; sheaths open, inflated, serving as floats; ligules absent or hyaline; pseudopetioles present; blades elliptic, ovate, ovate-lanceolate, or oblong. Inflorescences terminal panicles, aerial, lowermost branches whorled; disarticulation beneath the spikelet calluses. Spikelets bisexual, laterally compressed, with 1 floret. Glumes absent or an annular rim; calluses (1)2–10 mm, stipelike, glabrous, junction with the pedicels marked by a tan constriction; lemmas 5-veined, margins clasping the paleas, apices acuminate, awned, awns terminal, antrorsely scabridulous; paleas similar to the lemmas, 3-veined, 1-keeled, acute-acuminate, unawned; lodicules 2, glabrous; anthers 6; styles 2, bases not fused, stigmas laterally exserted, plumose. Caryopses terete, fusiform; embryos small; hila linear, almost as long as the embryo. x = 12. Name from the Greek hygros, ‘wet’ or ‘moist’, and oryza, ‘rice’, referring to its aquatic habit and similarity to rice.

Hygroryza is a monospecific Asian genus that grows in India, Ceylon, and throughout southeast Asia. It forms floating masses, often of considerable extent, in lakes and slow-moving streams, and is sometimes a weed in rice.

SELECTED REFERENCES Bor, N.L. 1960. The Grasses of Burma, Ceylon, India and Pakistan (Excluding Bambuseae). International Series of Monographs on Pure and Applied Biology, Division: Botany, vol. 1. Pergamon Press, New York, Oxford, London, and Paris. 767 pp.; Koyama, T. 1987. Grasses of Japan and Its Neighboring Regions: An Identification Manual. Kodansha, Ltd., Tokyo, Japan. 370 pp.; Watson, L. and M.J. Dallwitz. 1992. The Grass Genera of the World. C.A.B. International, Wallingford, England. 1038 pp.

1. Hygroryza aristata (Retz.) Nees
Asian Watergrass, Water Stargrass

Culms 50–150 cm, floating, branched, flexuous; nodes rooting, roots feathery, whorled. Sheaths glabrous, open, inflated; ligules absent or 0.5–0.8 mm, hyaline, truncate; pseudopetioles shorter than 1 mm; blades 2–8 cm long, 5–20 mm wide, flat, bases rounded to cordate, apices blunt to rounded. Panicles 3–8 cm, pyramidal, with 4–9 branches; lower branches 2–4 cm, whorled, spreading or deflexed, glabrous; pedicels 0.2–2 mm, sometimes absent. Florets (6)7–18 mm (including the stipelike callus), narrowly lanceolate; calluses (1)2–10 mm, stipelike; lemmas 5–8 mm, chartaceous, keeled, veins and margins with hairs, glabrous or pubescent between the veins, awns 5–14 mm; paleas as long as the lemmas, chartaceous, glabrous, keels ciliate or scabrous; anthers 6, about 3.5 mm. Caryopses about 3.5 mm. 2n = 24.

Hygroryza aristata is native to tropical Asia, where it has occasionally been used as forage for cattle (Bor 1960). It is sold for ponds and aquaria, where its long, feathery, adventitious roots have a decorative effect, but it has the potential to become a significant weed problem in the southern United States.