Leaves decussate or occasionally in whorls of 3(4), often seemingly in much larger numbers at nodes (‘pseudo-verticillate’; due to the presence of leafy, much-contracted short shoots); blades variable in size and shape, ± ovate to linear, ± acute to obtuse at apex, membranaceous to coriaceous, with 2–4 usually ± indistinct lateral veins on either side of midvein, glabrous or hairy on margins, veins or surfaces, margins ± flat to slightly revolute; petioles short to subobsolete; stipular sheaths ± cup-shaped, with 3–5(7) ± linear setae or fimbriae on either side, the median often (much) longer than the others.
Ovary bicarpellate and biovulate (but one carpel sometimes smaller and ± reduced or in one species completely sterile), mostly shortly hairy; style glabrous, filiform; stigma bifid, lobes filiform.
Stems erect to prostrate, ± terete to obscurely 4-angled, covered with yellowish to whitish, ± spreading or curled multicellular hairs above, often becoming glabrescent below.
Fruit crowned by persistent calyx lobes, dehiscing into two mericarps, each convex on dorsal side and plane or slighdy concave on ventral side.
Seed ± oblong to ± ovoid, granulate, dorsal side convex, ventral side flat, concave or ± hollowed out, with a median longitudinal ridge.
Stamens exserted, filaments inserted at or near throat, thin, ± erect and stiff, glabrous or hairy near base; anthers oblong.
Bud with limb abruptly expanded into a subcylindrical to ovoid head, or limb and tube not distinct from each other.
Corolla glabrous or throat and tube hairy inside, lobes lanceolate to oblong, ± acute at apex.
Calyx: lobes unequal, 1 (2; very rarely 3) enlarged, foliaceous, other lobes minute, setiform.
Flowers paired, on very short, persistent pedicels, hermaphrodite, protandrous, 4–5-merous.
Inflorescence head-or spike-like, usually elongated when in fruit.
Annual or perennial herbs, subshrubs or sometimes dwarf shrubs.