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, or some tropical genera shrubs or small trees, with opposite simple exstipulate leaves, and irregular or nearly regular perfect flowers. Calyx inferior, persistent, 4-5-parted or 4-5-cleft, the sepals or segments imbricated, equal or unequal. Corolla gamopetalous, nearly regularly 5-lobed with the lobes convolute in the bud, or conspicuously 2-lipped. Anther-bearing stamens 4, didynamous, or 2 only; anthers mostly 2-celled, the sacs longitudinally dehiscent. Disk annular or cup-like. Ovary 2-celled; ovules 2-10 in each cavity, anatropous or amphitro-pous; style filiform, simple; stigmas 1 or 2. Capsule dry, 2-celled, loculicidally elastically 2-valved. Seeds globose or orbicular, not winged, borne on curved projections (retinacula) from the placentae, the testa close, mostly roughened, often developing spiral threads and mucilage when wetted. Endosperm in the following genera none; cotyledons flat, commonly cordate.
About 175 genera and 2000 species, natives of temperate and tropical regions of the Old World and the New.
Corolla convolute in the bud, nearly regular; stamens 4. Ovules 2 in each cavity; capsule 2-4-seeded.
Ovules 3-10 in each cavity; capsule 6-20-seeded.
Corolla imbricated in the bud, strongly 2-lipped; stamens 2.
Lower lip of the corolla 3-cleft; flowers bracted, not involucrate.
Lower lip of the corolla entire or 3-toothed; flowers involucrate.