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..
Perennial herbs, vines or shrubs, mostly with milky juice, with opposite alternate or verticillate exstipulate leaves, and mostly umbellate perfect regular flowers. Calyx inferior, its tube very short, or none, its segments imbricated or separate in the bud. Corolla campanulate, urceolate, rotate or funnelform, 5-lobed or 5-cleft, the segments commonly reflexed, mostly valvate in the bud. A 5-lobed or 5-parted crown (corona) between the corolla and the stamens and adnate to one or the other. Stamens 5, inserted on the corolla, usually near its base; filaments short, stout, mostly monadelphous, or distinct; anthers attached by their bases to the filaments, introrsely 2-celled, connivent around the stigma, or more or less united with each other; anther-sacs tipped with an inflexed or erect scarious membrane, or unappendaged at the top, sometimes appendaged at the base; pollen coherent into waxy or granular masses, one or rarely two such masses in each sac, connected with the stigma in pairs or fours, by 5 glandular corpuscles alternate with the anthers. Disk none. Ovary of 2 carpels; styles 2, short, connected at the summit by the peltate discoid stigma; ovules numerous in each carpel, mostly anatropous, pendulous. Fruit of 2 several-many-seeded follicles. Seeds compressed, usually appendaged by a long coma; endosperm cartilaginous, mostly thin; embryo nearly as long as the seed; cotyledons flat.
* Text revised for our first edition by Miss Anna Murray Vail, here somewhat modified.
About 220 genera and 2000 species, of very wide geographic distribution, most abundant in tropical or warm-temperate regions.
Erect or decumbent herbs.
Corona-hoods each with an incurved horn within; eaves mostly opposite.
Corona-hoods prominently crested within: leaves alternate.
Corona-hoods unappendaged or with a thickened crest-like keel.
Corolla-lobes erect; corona-lobes 1-2-awned. Corolla rotate.
Anthers tipped with a scarious membrane; pollen-masses pendulous.
Anthers merely tipped; pollen-masses horizontal.