This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual or perennial branching herbs, with opposite, usually toothed leaves and rather small, long-peduncled discoid and radiate heads, terminal, or in the upper axils, or rays wanting in some species. Involucre campanulate, its bracts in about 2 series, herbaceous, loosely appressed. Receptacle convex or elongated, chaffy, its chaff embracing the disk-achenes and at length falling away with them. Ray-flowers yellow, or white, pistillate, sometimes wanting. Disk-flowers yellow, perfect, their corollas tubular with an expanded 4-5-cleft limb. Anthers truncate at the base. Style-branches of the disk-flowers long, sometimes penicillate at the summit. Ray-achenes 3-sided, or compressed, those of the disk-flowers compressed, margined. Pappus of 1-3 awns, or more. [Greek, spot- or stain-flower, not significant.]
About 30 species, natives of warm and tropical regions. Type species: Spilanthes urens Jacq.
Perennial, usually rooting at the lower nodes; stem slender, simple or branched, spreading or ascending, 8'-2° long, pubescent, or nearly glabrous. Leaves ovate to lanceolate, petioled, acute or acuminate at the apex, or the lower obtuse, coarsely toothed, or nearly entire, 1'-3' long; heads long-peduncled, solitary at the end of the stem and branches, 6"-10" broad; bracts of the involucre oblong to oblong-lanceolate, obtuse or acute; rays 8-12, yellow; receptacle narrowly conic; achenes oblong, most of them roughened when mature and hispidulous; pappus of 1 or 2 very short awns, or none.
In moist or wet soil, Missouri to Texas, east to South Carolina and Florida. June-Sept.