Herbs or shrubs. Leaves with abundant cystoliths at maturity, margins entire, petiolate. Flowers in 1-sided spikes or racemes, each flower subtended by 2 small bracts and 2 bracteoles of about equal size; calyx 5-lobed, the lobes linear to lanceolate; corolla zygomorphic, white, blue, purple, rose, or yellow, funnelform, 5-lobed; stamens 4, in 2 pairs; ovary pubescent, ovules 4; stigma 2-lobed or capitate. Capsules ellipsoid to clavate. Seeds 2-4, orbicular, compressed.

A genus of about 40 species of tropical Africa, Asia, and Malesia. Name derivation obscure, but perhaps derived from the Greek asystatos, meaning loose or having no cohesion.