1. Spermacoce glabra Michx. (buttonweed)

Spermacoceodes glabrum (Michx.) Kuntze

Pl. 552 a, b; Map 2561

Plants annual herbs, glabrous at maturity or sometimes somewhat roughened. Stems 10–60 cm long, erect to loosely ascending or trailing, 4-angled or often longitudinally 6-ridged or slightly channeled on opposing sides, glabrous at maturity or sometimes somewhat roughened along the angles. Leaves opposite, sessile or nearly so. Stipules interpetiolar, papery to scalelike, generally truncate to broadly rounded at the tips, usually persistent with the leaves (but sometimes becoming shredded with age), fused to the leaf bases on either side, the sheath 1.5–3.5 mm long, membranous, bearing along its margin 5–7 bristles, these 1.5–6.0 mm long, usually somewhat unequal with the middle ones longer. Leaf blades 2–8 cm long, 3–24 mm wide, narrowly elliptic, angled at the base, angled to a sharply pointed tip, the margins flat, entire, sometimes with minute spinules angled toward the leaf tip, the surfaces glabrous, not glandular, the venation with the midvein and 2 or 3 pairs of secondary veins visible. Inflorescences axillary at mostly the upper stem nodes, dense sessile clusters of 3–20 flowers. Calyces deeply 4-lobed, the lobes 1.5–2.0 mm long, narrowly triangular, glabrous. Corollas 2.5–4.0 mm long, funnelform, 4-lobed to about the midpoint, white, the outer surface glabrous, densely bearded in the throat with the pubescence often extending onto the upper surface of the lobes, these not overlapping in bud. Stamens 4, attached in the corolla throat, partially exserted. Stigmas 2, linear, not exserted. Ovary fully inferior, 2-locular, the ovules 1 per locule. Fruits achenelike, 3–4 mm long, 2–3 mm wide, obconic and somewhat flattened laterally, indehiscent, the surface smooth, leathery to stiffly papery. 2n=28. June–October.

Scattered in the southern half of the state, north locally to Boone and Pike Counties (southeastern U.S. west to Kansas and Texas). Banks of streams and rivers, margins of ponds, lakes, sinkhole ponds, and oxbows, bottomland forests, bottomland prairies, sloughs, and moist depressions of sand prairies; also ditches and wet roadsides.

This species is superficially similar to members of Lycopus L. (bugleweed) in the Lamiaceae, which also has dense axillary clusters of small white flowers. Spermacoce glabra differs in its ovary position (inferior vs. superior); fruit type (unlobed and achenelike vs. nutlets), deeply lobed, unnerved calyces (vs. shallowly lobed or to about the midpoint and strongly nerved), and 4 (vs. 2) stamens, among other characters.