6. Matricaria L. (wild chamomile)

Contributed by John H. Pruski

Plants annual (short-lived perennial herbs elsewhere), with taproots, usually strongly aromatic. Stems erect or ascending, usually branched, finely ridged or lined, glabrous or less commonly hairy. Leaves alternate, sessile or short-petiolate with winged petioles and somewhat broadened, more or less clasping bases. Leaf blades deeply 1–3 times pinnately lobed, glabrous or less commonly hairy, the ultimate segments mostly linear. Inflorescences of solitary heads or loose clusters at the branch tips, sometimes appearing paniculate, the stalks glabrous or minutely hairy, bractless. Heads radiate or discoid. Involucre bell-shaped to broadly ovoid, the bracts mostly in 2 or 3 loosely overlapping series, subequal, elliptic-lanceolate to narrowly oblong or linear-oblanceolate, rounded or angled to a minute, sharp point at the tip, glabrous or sparsely to moderately and minutely hairy, green when young but soon becoming tan to straw-colored, the midrib not keeled, green to reddish brown and often somewhat glandular. Receptacle conical or strongly convex, tending to elongate and become more conical as the fruits mature, hollow, naked. Ray florets (when present) pistillate, the corolla white, not glandular, the upper surface sometimes with minute papillae. Disc florets perfect (the outer series of florets in discoid species occasionally only pistillate), numerous, the corolla yellow or sometimes greenish yellow, occasionally minutely glandular, the 4 or 5 lobes usually lacking resin canals (these appearing as minute, brown lines or streaks), persistent, the tube often somewhat flattened toward the tip, becoming swollen at fruiting. Pappus absent or a short collar or crown. Fruits wedge-shaped in profile, bluntly angled in cross-section, slightly flattened, curved and slightly asymmetrical at the base, the tip often obliquely truncate, 3–5-nerved, the surface otherwise smooth, glabrous or glandular, light brown to brown with lighter nerves. Seven species, worldwide.

The nomenclature and application of the name Matricaria and of several species within the genus have remained unstable. Linnaeus (1753) originally treated five species in Matricaria, of which only two are still referred to the genus, and he later changed the circumscription and application of the two most widespread of his species (Linnaeus, 1755). Selection among the five species to typify the generic name also has been complex and confusing (Grierson, 1974; Jeffrey, 1979), leading Rauschert (1974) to reject the name Matricaria as a nomen confusum in favor of Chamomilla Gray. However, this interpretation has been rejected by many subsequent authors (see Gandhi and Thomas, 1991; Applequist, 2002).

The genus is closely related to Tripleurospermum, which differs in its solid, more or less dome-shaped receptacles, corolla lobes with resin canals (visible as small, darker lines or streaks), and fruits that are asymmetrically and strongly 3-ribbed (each rib is about as big as the achene body), with the surface between the ridges wrinkled to weakly tuberculate and having 1 or 2 resiniferous or mucilaginous glands. Species of Tripleurospermum are often known as scentless chamomiles because the plants are essentially odorless, whereas the two species of Matricaria that grow in Missouri are aromatic when bruised or crushed (often strongly so).