Marrubium vulgare L., horehound. Subshrub, ± evergreen, thick–taprooted, several—many–stemmed at base, with spreading, decumbent, ascending, and sometime erect branches, above midplant often = flowering shoots, canopy fountain–shaped to hemispheric, in range 35—70 cm tall; shoots with only cauline leaves, densely white–tomentose with matted long hairs (especially on summer foliage) and scattered, sessile glandular hairs having colorless, spheric heads, young shoots only faintly aromatic.
Stems young stems 4–sided (± square in ×–section), to 12 mm diameter, tough, white woolly–tomentose.
Leaves opposite decussate, simple, petiolate to subsessile, without stipules; petiole (2—)8—35 mm long, channeled and crescent–shaped in ×–section; blade ovate to round or rhombic, 12—60 × 12—47 mm, truncate to cordate at base, crenate on margins, rounded or obtuse at tip, with several primary veins arising near base but otherwise pinnate with veins strongly raised on lower surface, upper surface conspicuously cobblestonelike with deeply sunken veins, sometimes hidden beneath dense hairs.
Inflorescence paired cymes in dense axillary, headlike clusters (= verticils), verticils 10—50 mm apart, each hemispheric cyme 5—36–flowered and subtended by bract, flowers sessile; bract subtending cyme leaflike, sessile; bract subtending branchlet of cyme or bractlet subtending flower linear awl–shaped, 2.5—5 mm long, green, hooked, persistent.
Flower bisexual, bilateral, 4—5.5 mm across; calyx 10–lobed, narrowly bell–shaped, 4—5 mm long increasing to 2× in fruit, green, 10–ribbed, stellate–tomentose and glandular with sessile spheric hairs; tube ca. 3 mm long, internally glabrous except with ring of hirsute hairs at top; lobes narrowly acuminate, 1—1.5 mm long, for each lobe green and tomentose below midpoint + spreading with a sharp recurved hook and glabrous at tip; corolla 2–lipped, 5–lobed, 6—8 mm long, white; tube ca. 2 mm long, with white hairs forming ring inside tube at level of filament attachment; throat ± 1.5 mm long, with papillate hairs between stamens on lower side; upper lip 2–lobed, ascending to erect, 2—3 mm long, ca. 1 mm wide at base, pubescent on both surfaces; lower lip 3–lobed, 2.5—4 mm long, wider than upper lip, pubescent on outer (lower) surface, the central lobe transversely elliptic to heart–shaped, 2.5—3.5 mm wide, notched, the lateral lobes ascending, triangular, 0.6—1 mm long; stamens 4, fused to top of corolla tube, dimorphic, 2 short and 2 slightly longer, included; filaments 1—1.2 mm long (lower stamens) and 0.5—0.7 mm long (upper stamens), white, bulbous at base; anthers dehiscent before anthesis, dorsifixed, dithecal, 0.6—0.7 mm long, pale yellow to yellowish green, with brownish or olive green connective, longitudinally dehiscent; pollen pale yellow; nectary disc beneath ovary, ringlike; pistil 1; ovary superior, strongly 4–lobed, ± 0.7 mm long, 2–chambered, each lobe with 1 ovule; style attached to ovary base at center of ovary lobes (gynobasic), sigmoidal at base, white; stigma extending beyond anthers, unequally 2–branched, 1 branch ca. 0.5 mm long and other 1/2 as long.
Fruits nutlets, 1—4, 3–sided broadly ellipsoid to oblong–obovoid, 2.2—2.5 × 1.2—1.4 mm, brown, truncate at tip, convex on outer face, flat or concave on inner faces, minutely warty, warts sometimes black; nutlets hidden from view by cone of hairs at top of calyx.
A. C. Gibson & B. A. Prigge