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..
Bitter mostly quite glabrous herbs, with opposite (rarely verticillate) exstipu-late entire leaves, reduced to scales in Bartonia, and regular perfect flowers in terminal or axillary clusters, or solitary at the ends of the stem or branches. Calyx inferior, persistent, 4-12-lobed, -toothed or -divided (of 2 sepals in Obo-laria), the lobes imbricated or not meeting in the bud. Corolla gamopetalous, funnelform, campanulate, club-shaped or rotate, often marcescent, 4-12-lobed or -parted, the lobes convolute or imbricated in the bud. Stamens as many as the lobes of the corolla, alternate with them, inserted on the tube or throat; anthers 2-celled, longitudinally dehiscent; filaments filiform, or dilated at the base. Disk none, or inconspicuous. Ovary superior in our genera, 1- celled or partly 2-celled; ovules numerous, anatropous or amphitropous; style simple, or none; stigma entire, or 2-lobed, or 2- cleft. Capsule mostly dehiscent by 2 valves. Seeds globose, angular or compressed; endosperm fleshy, copious; embryo small, terete or conic.
About 70 genera and 700 species, widely distributed, most abundant in temperate regions. Leaves normal; corolla-lobes convolute in the bud.
Style filiform; anthers usually twisting or recurving when old. Corolla salverform.
Style short, stout or none; anthers remaining straight. Corolla without nectariferous pits, glands or scales. Corolla funnelform, campanulate or clavate.
Corolla without plaits in the sinuses; calyx without an interior membrane.
Corolla with plaits in the sinuses; calyx'with an interior membrane.
5. Dasystephana Corolla rotate.
6. Pleurogyna, 1-2 nectariferous pits, glands or scales at the base of each corolla-lobe. Corolla rotate, a fringed gland at each lobe.
Corolla campanulate, spurred at the base.
Leaves, at least those of the stem, reduced to scales; corolla-lobes imbricated in the bud. Calyx of 2 foliaceous spatulate sepals; upper leaves normal.
Calyx of 4 lanceolate sepals; leaves all reduced to scales.