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Annual (sometimes biennial) or perennial herbs or subshrubs, glabrous or almost so (calyx sometimes hairy), of damp to aquatic habitats. Stems ± quadrangular. Leaves simple, opposite and usually decussate, sometimes whorled, sessile or subsessile, 1-veined; stipules absent. Inflorescences axillary, (1–) 3–many-flowered; flowers in cymes (dichasial), umbels or crowded into bracteate heads, or solitary on long pedicels, usually regular (actinomorphic), (3) 4–6-merous. Hypanthium (calyx tube) campanulate or urceolate to globose, faintly (4–) 8-veined or markedly ribbed, usually glabrous; epicalyx-like appendages obvious or minute or absent; sepals (calyx lobes) small. Petals absent or usually 4, small and obscure, sometimes showy, caducous or (in former Hionanthera, not in Australia) persistent. Stamens 4 (–8), inserted in lower half of calyx tube, episepalous. Ovary (1–) 2–5-locular. Capsule thin-walled, hyaline, dehiscence by irregular splitting (initially circumscissile dehiscence followed by irregular splitting generally in former Nasaea species) or septicidal by valves, with numerous tiny seeds or (in former Hionanthera, not in Australia) 2–5 seeds.
Herbs, annual, tending to become anthocyanic with age. Stems erect, glabrous; young branches often 4-angled or narrowly winged. Leaves opposite, usually decussate, sessile or subsessile, membranous. Inflorescences cymose, with small, white, membranous bracteoles. Flowers 4(-6)-merous, actinomorphic. Floral tube campanulate or urceolate, becoming globose or nearly so in fruit, 4(-6)-lobed, noticeably 4-8-veined; sepals short, less than 1/3 length of floral tube; epicalyx segments between sepals small or absent. Petals absent to 4, caducous. Stamens 2-8. Ovary incompletely [1 or]2-4-loculed, globose; ovules numerous; style much shorter to much longer than ovary, style base persistent in fruit; stigma capitate. Capsule globose, hyaline, smooth walled, not finely striated, irregularly dehiscing from apex. Seeds many, golden-brown, obovoid, concave-convex, ca. 1 mm. 2n = 18, 24, 26, 28, 30, 32, 34, 36, 40, 48, 66.
Annual herbs, simple or sparsely branched, the young branches obscurely tetragonal. Leaves opposite, sessile. Inflorescence of ( 1-to) several-flowered cymes gathered into axillary clusters; flowers (5-, 6-or) 4-merous, bisexual, actinomorphic. Hypanthium urceolate, appendiculate or exappendiculate, the calyx lobes valvate. Petals present or absent. Stamens of the same or twice the number of calyx lobes or less, inserted about a third from the base of the hypan-thium. Ovary generally 4-carpellary, superior, sessile, the placentation generally axile; ovules many; style sometimes obsolete; stigma capitate. Fruit an irregularly dehiscent ovoid capsule, the hypanthium and style persistent; seeds many, minute.
Fls typically 4-merous; hypanthium globose or campanulate, usually with small intersepalar appendages; sep short; pet present or not, equaling or barely surpassing the sep; stamens 4(8); ovary imperfectly 2–4-locular; disk wanting; capsule rupturing irregularly, not striate; glabrous annuals of wet places, with narrow, sessile, opposite lvs and inconspicuous, bibracteolate fls in small axillary cymes of(1–)3–15. 25, mainly tropical.
Capsule globose or ellipsoid, included in the calyx or ± exserted, thinly membranous, dehiscing transversally and irregularly; seeds numerous, very small, angular.
Ovary incompletely 2–4(5)-locular, sometimes unilocular; style 0 or ± long, not continuous with placentas; stigma capitate; ovules numerous, axile or on the septa.
Stamens as many as or twice the number of calyx-lobes, rarely fewer, with the filaments inserted at 1/4–1/2 of the calyx-tube length; anthers didymous.
Dichasia (1)3-many-flowered, sub-sessile or pedunculate; bracteoles small, membranous, whitish.
Annual (sometimes biennial?) glabrous (rarely with the calyx minutely hairy) herbs.
Petals 4–8, very caducous or absent, small, obovate, spathulate or rounded.
Calyx campanulate or urn-shaped, 8-ribbed; appendages absent or very short.
Stems erect or decumbent, 4-gonous or 4-winged, simple or ± branched.
Flowers 4 (5–8)-merous, never heterostylous.
Leaves decussate, sessile, entire, 1-nerved.
