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..
Annual or perennial herbs, often swollen at the nodes, with opposite entire exstipulate leaves, and perfect, polygamous, or rarely dioecious regular flowers, the sap watery. Sepals 4 or 5, persistent, united into a tube or cup. Petals equal in number to the sepals, or rarely none, often with a scale at the base of the blade. Stamens twice as many as the sepals, clawed, perigynous; anthers longitudinally dehiscent. Ovary 1, stipitate, mainly i-celled (rarely 3-5-celled); styles 2-5; ovules and seeds several or many (in all our species), attached to a central column. Fruit generally membranous, a capsule, dehiscent by valves or teeth. Seeds mainly amphitropous; embryo nearly straight, and peripheral to the endosperm; cotyledons mainly incumbent.
About 20 genera and perhaps 600 species, widely distributed, most abundant in the northern hemisphere.
Calyx-ribs at least twice as many as the teeth, running both into the teeth and into the sinuses.
Styles 5, alternate with the foliaceous calyx-teeth.
Styles 3-5, when 5, opposite the short calyx-teeth.
Styles 5, capsule several-celled at the base.
Styles 3, rarely 4.
Styles 5, capsule I-celled to the base.
Calyx s-ribbed, 5-nerved, or nerveless, or striate-nerved.
Calyx conspicuously scarious between its green nerves.
Calyx not bracteolate at the base.
Calyx bracteolate at the base.
Calyx not at all scarious.
Petals appendaged at the base of the blade.
Petals not appendaged at the base of the blade.
Calyx strongly 5-angled, not bracteolate.
Calyx terete or nearly so, subtended by bractlets.