This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
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.