Roots usually thickened, branched. Caudex short, woody, usually covered with remnants of leaf bases. Leaves in whorls of 3 or 4(-6), rarely 2, opposite, linear to oblong-lanceolate, entire to pinnatipartite, spinose. Inflorescence of several verticillasters, each subtended by a whorl of leaflike bracts. Flowers sessile or shortly pedicellate, enveloped in an involucel; involucels campanulate, with 8-16 spiny teeth, 2 significantly longer than others. Calyx tube oblique, campanulate; limb 2-lipped, lips 2-or 3-lobed or emarginate. Corolla tube elongate; limb spreading, 2-lipped; upper lip 2-lobed, lower one 3-lobed. Fertile stamens 2, inserted at corolla throat; staminodes 2, at base of corolla tube, cordate. Nectary 1, anterior, at base of corolla tube, 3-lobed. Ovary inferior, 1-loculed, enveloped in an involucel; style usually longer than stamens; stigma disklike; ovule single, pendulous. Achenes rugose, columnar, slightly or markedly obliquely truncate.
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