Perennial herbs or shrubs, sometimes with long, thin, tuberous roots. Leaves petiolate or sessile; blades elliptic, ovate-lanceolate, oblong-spathulate, acuminate or obtuse apically, margin entire, slightly undulate or repand-crenate, with numerouscylindrical cystoliths. Inflorescences axillary, branching, long peduncled cymes or, 1-2(-3) more or less sessile, occasionally long pedicellate flowers in upper axils, combined into a terminal raceme or spike; bracts one or absent; bracteoles 2. Flowers usually large, showy, regular, sometimes curved, with cleistogamous flowers often present; calyx usually 5-lobed, lobes narrow, mostly equal; corolla purple, lavender, whitish, occasionally red or rose, infundibuliform, sometimes saccate, tubular at base, upper part straight or oblique, widened and campanulate, limb of 5 equal, obtuse, spreading, nearly regular lobes, in bud convolute; stamens 4, didynamous, inserted at top of corolla tube, filament pairs connected at base by a membrane, anthers parallel, 2-thecous, thecae equal, oblong-sagittate, muticous at base, staminodes 0; disk annular; ovary with 4-13 superposed ovules per locule, style slender, usually slightly recurved at apex, stigma simple, subulate or 2-lobed, lobes flattened, equal or posterior lobe short or obsolete. Capsules oblong or clavate, somewhat flattened, base very short or contracted into a solid, elongated stipe, retinacula present, septa with retinacula remaining attached to inner wall of mature capsule; seeds 4-20, lenticular, suborbicular, glabrous when dry, covered or rimmed with sticky mucilaginous trichomes when moistened.
Guianas present, Pantropical present, pansubtropical present, temperate zones of both hemispheres present
The second largest genus in the family with more than 250 species, pantropical and pansubtropical with a number of species in the temperate zones of both hemispheres; in the Guianas 11 species.
In the Flore de la Guyane Française (), Lemée treated 2 species which not occur in the Guianas:
Arrhostoxylon coccineum (L.) Nees ( Ruellia coccinea (L.) Vahl) from the Greater Antilles; and Dipteracanthus paniculatus (L.) Nees ( Ruellia paniculata L.) occuring in C America, Venezuela, Colombia and Brazil.