Herbs, shrubs, or often trees in tropical regions, mostly with opposite leaves and solitary or clustered mostly axillary perfect flowers. Stipules usually none. Calyx persistent, free from the ovary, but generally enclosing it, the limb toothed and often with accessory teeth in the sinuses. Petals as many as the primary calyx-teeth or none, inserted on the calyx. Disk annular or none. Stamens various, inserted on the calyx. Anthers versatile, longitudinally dehiscent. Ovary sessile or stipitate, 2-6-celled or sometimes 1-celled; style 1; stigma capitate or 2-lobed; ovules 00, rarely few, anatropous. Capsule 1-several-celled, variously dehiscent or sometimes indehiscent. Seeds without endosperm; cotyledons flat, often auricled at the base.

About 21 genera and 400 species, of wide geographic distribution, most abundant in tropical America.

Calyx-tube campanulate or hemispheric; flowers regular.

Flowers small, axillary, solitary or few; low herbs.

Petals 4 in our species; capsule bursting irregularly.

1.

Ammannia.

Petals none; capsule indehiscent.

2.

Didiplis.

Petals 4; capsule septicidally dehiscent.

3.

Rotala.

Flowers large, in axillary cymes; large aquatic shrub.

4.

Decodon.

Calyx-tube cylindric; flowers regular.

5.

Lythrum.

Calyx-tube tubular, oblique; flowers irregular.

6.

Parsonsia.