24. Poteridium Spach, Ann. Sci. Nat., Bot., sér. 3. 5: 43. 1846.
American burnet [Genus Poterium and Latin -idium, diminutive]
Alan S. Weakley
Herbs, annual or biennial, 1–10 dm; taprooted. Stems 1–10+, ascending to erect, ˂freely branched˃, glabrous or sparsely pubescent. Leaves deciduous, basal and cauline (basal usually withering before or at flowering); stipules persistent, adnate to petiole, those of basal leaves simple, lanceolate, entire, of cauline pectinately divided; petiole present; blade oblanceolate to obovate, 2–7 cm, herbaceous, leaflets 3–8 pairs, ˂proximally alternate, subopposite, or opposite, distally subopposite or opposite˃, broadly elliptic to obovate, margins flat, pectinately pinnatisect (segment margins nearly parallel), faces glabrous. Inflorescences terminal, also often axillary to distal leaves, 12–100+-flowered, headlike spikes, ˂cylindric, or subglobose when poorly developed˃, glabrous or nearly so; peduncles present, sometimes absent; bracts absent; bracteoles present. Pedicels absent. Flowers 1–3 mm diam.; hypanthium urceolate, compressed at mouth, 1–3 mm, glabrous; sepals 4, ascending to spreading, ovate, ˂margins scarious˃; petals 0; stamens 2 (and opposite inner sepals) or 4, equal to sepals; carpel 1, glabrous, ˂styles repeatedly branched, brushlike˃. Fruits achenes, 1, globose, 1.5–2 mm diam., glabrous; hypanthium persistent, enclosing achenes; sepals persistent, ascending to spreading. x = 7.
Species 2 (2 in the flora): North America.
In North America, Poteridium has, with few exceptions (notably P. A. Rydberg 1908b), been included in Sanguisorba since its segregation by E. Spach. Molecular analyses clarify the distinction of Poteridium from Sanguisorba and bolster the morphologic distinctions (see 25. Sanguisorba for discussion).
SELECTED REFERENCE Rydberg, P. A. 1908b. Poteridium. In: N. L. Britton et al., eds. 1905+. North American Flora.... 47+ vols. New York. Vol. 22, p. 388.