Herbs (or more rarely shrubs) with opposite rarely verticillate simple entire or lobed leaves. Flowers usually in bracteolate spikes or racemes. Calyx inferior, 4- or 5-lobed, sometimes very small, and occasionally obsolete. Corolla ringent or bilabiate, the lower lip overlapping the upper in bud, rarely 1-lipped. Stamens usually 2, sometimes 4, and didynamous. Capsule two-celled, two-valved; valves opposite the partition. Seeds exalbuminous, 2 or more in each cell, attached to a woody placenta which splits through the axis and adheres to the valves. There are about 150 genera and 1500 species, nearly all tropical.