2. Diodia L. (buttonweed)

Plants annual or perennial herbs, sometimes woody at the base. Stems erect or ascending to prostrate or loosely ascending form a spreading base, sometimes mat-forming, usually 4-angled. Leaves opposite, subsessile, with interpetiolar stipules, these generally truncate to broadly rounded at the tips, persisting with the leaves, fused to the leaf bases on either side, membranaceous, bearing along the margins 3–11 bristles, at least some of which are more than 3 mm long. Inflorescences axillary, of solitary, sessile flowers or sessile, 2–4-flowered clusters. Flowers homostylous, often subtended by small bracts. Calyces deeply 2- or 4-lobed, usually persistent at fruiting. Corollas 4-lobed, funnelform to saucer-shaped, white to light pink. Stamens 4, inserted near the top of the corolla tube, the anthers exserted or not. Ovary fully inferior, 2-locular, the ovules 1 per locule. Style 1, slender, either unbranched and with a 2-lobed, capitate stigma or 2-branched and with long linear stigmas, exserted or not. Fruits schizocarps, dry, hairy, at maturity splitting from the base into 2 mericarps. Five to about 30 species, North America, Central America, South America, Caribbean Islands, and possibly Africa; introduced in Asia, Pacific islands.

In their taxonomic revision of Diodia, Bacigalupo and Cabral (1999) treated the American species traditionally ascribed to the genus as belonging to four different genera. They transferred D. teres and about fifteen other species to the genus Diodella Small, based mainly only differences in fruit morphology. However, the taxonomic relationships among these genera are still not well-understood. The as-yet not fully published doctoral studies of Dessein (2003) include a preliminary molecular analysis of the tribe Spermacoceae Bercht. & J. Presl, which supports the division of Diodia into several groups, but also does not resolve the relationships between these groups. Diodia teres is thus retained in the genus Diodia for the present, with the knowledge that it will likely become recognized as part of a segregate genus (likely Diodella Small) in the future. Diodia is related to Spermacoce. For further discussion, see the treatment of that genus.