Erect herbs or shrubs with cystoliths, sometimes with spines in leaf axils. Leaves petiolate; blades entire, often spine-tipped. Inflorescences axillary clusters or terminal spikes, each flower in axil of large bract sometimes with apical spine; bracteoles usually 2 at base of calyx. Calyx consisting of two unequal pairs of lobes inserted at right angles to each other, outer pair, larger than inner pair (lateral), each lobe entire or with toothed margins, sometimes spine-tipped; corolla with tube narrow at base, widening into throat, lobes 5, of equal length, lowest lobe fused to adjacent lobes for a shorter length than other 4 lobes, giving a 2-lipped appearance, upper lip 4-lobed, lower lip 1-lobed; stamens 4, inserted at base of throat, one pair fertile and far exserted, other pair fertile or sterile and included, sometimes with a small staminode, anther thecae of exserted stamens 2, parallel, inserted at equal levels, medifixed, slightly diverging at base, without appendages, those of included stamens similar or reduced; disk annular; ovary with 1-2 ovules per locule, style long, glabrous, stigma entire. Capsules ovoid, acuminate, 1-2 seeds borne on conspicuous retinacula at base; seeds large, diskoid, covered with long, appressed trichomes which expand and become mucilaginous when moistened.