Acmella is a genus of flowering plants in the family Asteraceae (order Asterales), formally described in 1807. The genus is native to the Americas and has since been introduced widely across Asia, Africa, the Pacific islands, and Australia.
Members of the genus are annual or perennial herbs with branching stems that typically reach 10 to 20 centimetres in length, growing either prostrate or erect. The leaves are arranged oppositely, with smooth or toothed margins, and are usually covered in rough or soft hairs. Flower heads are generally solitary at the tips of branches; each head bears several to many disc florets with bell-shaped throats and four or five triangular lobes, typically yellow or orange. Some species also produce ray florets — usually 5 to 20 or more — in yellow or orange, and occasionally white or purple. Disc florets are bisexual, while any ray florets are pistillate. Some species lack ray florets entirely.
The best-known member of the genus is Acmella oleracea, which has been cultivated for centuries. It is used as a food plant, a medicinal herb, an insecticide, and an ornamental. Its widespread use as a remedy for toothache and oral infections gave rise to its common name, the toothache plant.
Etymology
The genus name Acmella derives from Greek akme (ακμή), meaning "point" or "edge," a reference to the sharp, pointed scales of the receptacle that are characteristic of the genus.
Distribution
Acmella is native to the Americas, spanning tropical and subtropical regions. Several species have been introduced to Asia, Africa, the Pacific islands, and Australia, often spreading as weeds of disturbed habitats and cultivated ground.
Cultural Uses
Acmella oleracea has been used for centuries in traditional medicine across tropical regions as a local anaesthetic for toothache and oral infections, leading to its common name "toothache plant." The plant contains spilanthol, an alkylamide that produces a tingling, numbing sensation on contact with mucous membranes. It is also eaten as a leafy vegetable in parts of South and Southeast Asia, and used as a traditional insecticide.
