This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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..
Erect or climbing branching herbs, often armed with hooked stinging or viscid hairs, with alternate or opposite exstipulate leaves, and solitary racemose or cymose, regular and perfect, white yellow or reddish flowers. Calyx-tube adnate to the ovary, its limb 4-5-lobed, persistent. Petals 4 or 5, inserted on the throat of the calyx. Stamens 00, inserted with the petals; filaments filiform, commonly arranged in clusters opposite the petals, the outer sometimes without anthers and petaloid (staminodes); anthers introrse, longitudinally dehiscent. Ovary 1-celled (rarely 2-3-celled), with 2 or 3 parietal placentae; style filiform, entire or 2-3-lobed; ovules anatropous Capsule usually I-celled, crowned with the calyx-limb. Seeds mostly numerous; endosperm scanty.
About 20 genera and 250 species, all but 1 natives of America.
Seeds angled or prismatic, not separated by lamellae; mostly annuals.
Seeds flat, winged, separated by horizontal lamellae; perennials.