This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Herbs, with opposite leaves, or the upper sometimes alternate, and perfect irregular flowers, racemose in our species. Calyx inferior, 4-5-cleft or 4-5-parted or sometimes split to the base on the lower side. Corolla gamopetalous, irregular, the tube oblique, often decurved, the limb slightly 2-lipped, 5-lobed, the lobes nearly equal, the 2 upper ones exterior in the bud. Anther-bearing stamens 4, didynamous, or the posterior pair sterile; anthers 2-celled, the sacs longitudinally dehiscent. Ovary 1 - celled, with 2 parietal placentae expanded into broad surfaces, or 2-4-celled by the intrusion of the placentae or by false partitions; ovules numerous or few in each cavity of the ovary, anatropous; style slender; stigma 2-lobed or 2-lamellate. Fruit various in the different genera. Seeds oblong or orbicular, compressed, wingless or narrowly winged; endosperm none; embryo large; cotyledons fleshy, flat; radicle short and straight.
Three genera and about 12 species, mainly tropical.