Description: Perennial aquatic herbs; clump-forming; rhizomatous.
Leaves all basal; sheath spongy, containing prominent air spaces.
Inflorescences well-branched, branches short and upright to elongate and arching; rachis internodes conspicuously zigzagged; bracts deciduous, both bracts and prophylls falling with flower if fruit not set, leaving proximal portion of rachis bare, each bract subtending 1 flower pair, herbaceous to leathery. Flowers pale to dark purple (corolla and staminodes); sepals very short, persistent in fruit, membranous, floral tube indistinct; corolla lobes subequal to strongly unequal; outer staminode 1, petal-like, showy; callose staminode mainly fleshy, narrow apical rim petal-like; cucullate staminode with 2 appendages, subterminal, finger-like; stylar movement helical when tripped; styles with 1 appendage, elongate, strap-like.
Fruit a capsule, 1-seeded, indehiscent. Seeds smooth; aril reduced.
Distribution and occurrence: Species 6 (1 in the flora): warm temperate to tropical regions, southeastern North America, Mexico, West Indies, Central America, South America (to northern Argentina), west Africa.
| One species in NSW: Thalia dealbata |
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