1. Agalinis Raf. (gerardia)

Plants annual herbs (perennial elsewhere), hemiparasitic, often lacking a well-developed taproot, yellowish green to green or dark green, sometimes purplish-tinged, sometimes blackening upon drying. Stems erect or ascending, branched, usually appearing 4-angled (with pairs of ridges decurrent from the leaf bases), glabrous or roughened with minute hairs, these often broadened basally, sometimes pustular-based. Leaves opposite, sessile. Leaf blades linear and entire-margined or (in A. auriculata) lanceolate to narrowly ovate with the margins entire or basally 1- or 2-lobed. Inflorescences open, terminal racemes with leafy bracts (these usually reduced progressively toward the axis tip), the flowers paired at the nodes, sometimes appearing as axillary flowers toward the stem tip or as solitary terminal flowers; variously very short-stalked to long-stalked, the stalks often somewhat thickened toward the tips, lacking bractlets. Cleistogamous flowers absent. Calyces 5-lobed, only slightly zygomorphic, the tube relatively slender at flowering, often 5-angled, the lobes sometimes small and toothlike, persistent, often becoming somewhat enlarged and prominently veiny at fruiting. Corollas 8–33 mm long, 5-lobed, more or less bell-shaped, pink to pinkish purple, rarely white, the tube glabrous or hairy on the inner surface (at least near the tip), the hairs not blocking the throat, the lobes usually slightly shorter than the tube, the lower 3 lobes spreading, the upper 2 lobes either spreading to bent or angled backward or straight to slightly incurved, glabrous or more commonly hairy, the throat usually pale internally with darker pinkish red spots or markings, usually also with a pair of light yellow, longitudinal lines. Stamens with the filaments of 2 lengths, hairy (at least toward the base), the anthers with 2 sacs, these more or less parallel, variously blunt to tapered at 1 end, light yellow, hairy. Style somewhat curved downward and often slightly exserted, the stigmatic portion elongate and somewhat flattened. Fruits 3–15 mm long, globose to subglobose or less commonly elliptic, glabrous. Seeds oblong-ellipsoid to more or less trapezoid, the surface with a network of ridges and pits, variously yellow to light brown, dark brown, or black. About 40 species, North America to South America, Caribbean Islands.

Flowers of Agalinis species are mostly bee-pollinated (C. R. Robertson, 1891). Members of the genus have an extremely broad host range, including a diverse array of herbaceous monocots and dicots, as well as many pines and woody flowering plants (Musselman and Mann, 1978; Musselman et al., 1978; Cunningham and Parr, 1990).

Steyermark (1963) and many other authors chose to treat this genus under the name Gerardia L.. Thieret (1958) reviewed the nomenclatural history and pointed out that the type species of Gerardia corresponds with a different taxon more properly included in Stenandrium Nees, a mostly neotropical genus in the Acanthaceae, and the name has been officially rejected against the conserved Stenandrium. In order to stabilize the nomenclature of the genus, Thieret proposed that the name Agalinis, which had been used for the group in some older floras, be conserved against the older but obscure generic epithet Chytra C.F. Gaertn., which might instead have replaced the misapplied name Gerardia. The name Agalinis has since been officially conserved against Chytra.

Thieret (1958) also noted that the group has been treated variously in a broad sense or with some anomalous species groups segregated into the genera Aureolaria Raf., Tomanthera Raf., and Virgularia Ruiz & Pav. Further research has supported the separation of plants with large, yellow corollas as Aureolaria (Neel and Cummings, 2004; Bennett and Mathews, 2006), with a closer relationship to some of the other yellow-flowered genera in the group than to Agalinis. However, Virgularia (based on a South American species) is now considered part of Agalinis and the generic name Agalinis has been officially conserved against it (D’Arcy, 1978, 1979; Canne-Hilliker, 1988). Likewise, the small segregate Tomanthera is now thought to represent merely a broader-leaved condition within the range of variation of Agalinis (Pennell, 1928; Bentz and Cooperrider, 1978; Canne, 1981; Canne-Hilliker and Kampny, 1991; Neel and Cummings, 2004; Pettengill and Neel, 2008).

The present treatment follows closely the excellent summary of Agalinis in the Ozark Region by John Hays (1998). His insights into morphological variation in the Missouri species have been very helpful.