1. Mazus pumilus (Burm. f.) Steenis (mazus)

M. japonicus (Thunb.) Kuntze

Pl. 478 e–g; Map 2186

Plants annual or possibly short-lived perennial herbs, usually with a slender taproot. Stems 1 to several from the base, 5–20 cm long (including the inflorescence), loosely ascending to ascending, individually usually unbranched or few-branched, bluntly 4-angled, the angles slightly ridged, not winged, sparsely pubescent with short, curved hairs, also glandular toward the tips (and inflorescence axes). Leaves in a basal rosette and opposite, usually becoming alternate toward the stem tip, sessile or the basal ones with an indistinct, winged petiole. Leaf blades 1–4 cm long, those of the basal leaves noticeably larger and spatulate to oblong-spatulate or occasionally obovate-spatulate, tapered at the base, rounded at the tip, those of the stem leaves shorter, oblong to elliptic or somewhat spatulate, variously tapered or truncate at the base, sometimes somewhat clasping the stem, rounded to bluntly pointed at the tip, the margins of both types irregularly wavy to bluntly toothed (sometimes the upper leaf blades nearly entire), the surfaces sparsely to moderately short-hairy. Inflorescences terminal, open racemes of 3–15(–20) alternate flowers, each subtended by an inconspicuous, linear bract 1–3 mm long, the stalks 3–10 mm long at flowering, elongating to 7–16 mm at fruiting. Calyces 3–5 mm long at flowering, becoming enlarged to 5–9 mm at fruiting, not becoming papery, actinomorphic, bell-shaped, the 5 lobes slightly shorter than to slightly longer than the tube, elliptic to ovate-elliptic, short-hairy, and somewhat angled or thickened along the midrib of each sepal, but not winged, somewhat spreading at flowering and fruiting, sharply pointed but not hooked at their tips. Corollas 7–10 mm long, lavender to purple and somewhat glandular on the outer surface, the upper lip somewhat hooded, shallowly notched at the tip, lavender to purple on the inner surface, the lower lip with the lateral lobes much larger than the middle lobe, white or very pale lavender on the inner surface, the base of the lip with a pair of prominent, rounded, longitudinal ridges, these with a series of large, yellow spots. Ovaries appearing 2-carpellate (with a pair of sutures on opposing sides), containing numerous ovules. Fruits capsules, 3.0–4.5 mm long, globose, glabrous. Seeds 0.3–0.4 mm long, oblong-ellipsoid to narrowly ellipsoid, the surface appearing pebbled or with a fine network of ridges, brown. 2n=16–52.

Introduced, uncommon and sporadic in eastern Missouri (native of Asia, introduced widely in the U.S. except for most of the Rocky Mountains and Great Plains regions). Banks of streams; also lawns and open disturbed areas. May–October.

Mazus pumilus is sometimes cultivated as a ground cover or rock garden plant. It was long known as M. japonicus (H.-l. Li, 1954; Steyermark, 1963), but in his studies of the flora of New Guinea, van Steenis (1958) discovered that an older epithet was available. Botanists in China sometimes recognize several varieties, in which case the North American materials appear to correspond to var. pumilus. However, the distinctness of the varieties is not clear.