1. Marrubium vulgare L. (common horehound)
Pl. 436 a, b;
Map 1961
Plants perennial
herbs, with short, stout rhizomes, sometimes taprooted and flowering the first
year. Stems 30–70(–100) cm long, erect to loosely ascending, often from a
spreading base, usually bluntly 4-angled, unbranched or more commonly branched,
densely pubescent with white, woolly hairs, sometimes in patches, the upper
portion often somewhat less densely hairy. Leaves opposite, short- to
long-petiolate, the petioles unwinged or winged toward the tip, with a pungent,
odor usually described as pleasant when bruised or crushed. Leaf blades 1–6 cm
long, broadly elliptic to broadly ovate or nearly circular, unlobed, the
margins scalloped or bluntly to less commonly sharply toothed, the upper
surface with strongly impressed veins, densely woolly when young, often
becoming glabrous (sometimes only in patches) at maturity, the undersurface
with strongly raised veins, usually persistently woolly, some of the hairs
often stellate, also with usually conspicuous sessile glands, these often
partially obscured by the hairs. Inflorescences axillary, dense clusters of numerous
flowers per node, these sessile or nearly so. Bractlets 1.5–6.0 mm long,
shorter than to about as long as the calyces, linear, sharply pointed, somewhat
spinescent, and sometimes hooked at the tips. Calyces 5–8 mm long, not or only
slightly zygomorphic, lacking a lateral projection, symmetric at the base, more
or less cylindric to bell-shaped, the tube with 10 fine or faint nerves, with
dense, relatively long, fine hairs in the mouth, the 10 lobes shorter than to
nearly as long as the tube (frequently 5 longer lobes alternating with 5
somewhat shorter lobes), more or less spreading, triangular to narrowly
triangular, tapered to sharply pointed, usually hooked, more or less spinescent
tips, densely woolly on the outer surface, becoming slightly enlarged and
leathery or hardened at fruiting. Corollas 4–6 mm long, zygomorphic, white to
pale cream-colored, lacking darker purple spots, the outer surface densely
hairy (except on the lowermost lobe), the tube narrowly funnelform, the lobes
somewhat shorter than the tube, the upper lip straight or arching upward,
divided into 2 slender lobes, the lower lip with the central lobe obovate and
broader than the 2 lateral lobes, often shallowly notched at the tip. Stamens
4, not exserted, the anthers small, the connective short, the pollen sacs 2,
spreading, usually yellow. Ovary deeply lobed, the style appearing more or less
basal from a deep apical notch. Style not exserted, with 2, short, unequal
branches at the tip. Fruits dry schizocarps, separating into usually 4 nutlets,
these 1.8–2.5 mm long, obovoid to oblong-ellipsoid, broadly rounded at the tip,
the surface tan to yellowish brown with darker mottling, relatively smooth or
finely pebbled, glabrous. 2n=34, 36. May–September.
Introduced,
widely scattered, known mostly from historical collections (native of Europe,
Asia, Africa, Macaronesia; introduced nearly worldwide, including throughout
the U.S.). Glades and ledges and tops of bluffs; also pastures, fencerows,
barnyards, railroads, roadsides, and open disturbed areas.
Steyermark
(1963) noted that horehound has a long history of medicinal use as a stimulant,
tonic, laxative, cold remedy, and an aid for rheumatism and dyspepsia. It
formerly was a popular ingredient in some cough drops and a flavorant in some
candies. Diggs et al. (1999) noted that the plant is most often observed in
areas frequented by livestock and that the nutlets often become caught in
animal fur and wool. It is thus a troublesome weed for wool producers.
The species is
treated here in the traditional broad sense in which it has been included in
the North American floristic literature. R. E. Brooks (1986) pointed out that
most of the North American material attributed to M. vulgare does not
key well in European floristic treatments, differing in details of pubescence
and calyx morphology. In the absence of a modern monograph of the genus, it is
not possible to assess the introduced American plants in the context of
variation among native populations of M. vulgare or other species. More
studies are needed.