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, rarely shrubs, with alternate or opposite leaves, no stipules or mere glands in their places, and axillary spicate or racemose generally perfect regular or sometimes irregular flowers. Calyx-tube adnate to the ovary, often prolonged beyond it, the limb 2-6-lobed (usually 4-lobed). Petals 2-9 (usually 4), convolute in the bud, rarely none. Stamens usually as many or twice as many as the petals, inserted with them on the summit of the calyx-tube, or on the epigynous or perigynous disk. Ovary 1-6-celled (usually 4-celled); styles united; stigma capitate, discoid or 4-lobed; ovules 00 in each cavity, generally anatropous. Fruit a capsule or small nut. Seeds mostly small; endosperm very little or none; embryo straight.
About forty genera and 400 species of wide geographic distribution, most abundant in America. 1. Floral whorls of 4 parts or more. A. Fruit a many-seeded capsule, opening by valves or by a pore. * Calyx-tube not prolonged beyond the ovary. Seeds naked; calyx persistent on the fruit. Stamens 4, in 1 row.
Flowers sessile; petals none, or very small; leaves petioled; capsules short, the top
Flowers long-stalked; petals conspicuous; leaves sessile; capsules elongated, curved, with
a prominent 4-lobed stylopodium.
Leaves alternate; stems erect or ascending.
Stamens 8-12, in 2 rows.
Seeds furnished with a tuft of silky hairs; calyx deciduous.
** Calyx-tube prolonged beyond the ovary; calyx deciduous.
Seeds furnished with a tuft of silky hairs.
Seeds naked or sometimes tuberculate.
Stamens equal in length.
Stigma deeply 4-cleft, its segments linear.
Ovules and seeds horizontal, in 2 or rarely more rows, prismatic-angled.
Ovules and seeds ascending, not angled.
Buds erect; petals yellow; ovules and seeds in 2 rows.
Stigmas entire or slightly 4-toothed.
Calyx-tube longer than the ovary; stigma disk-like, entire.
Calyx-tube shorter than the ovary; stigma disk-like, slightly 4-toothed.
Stamens unequal in length, the alternate longer.
Ovules and seeds many, on slender funiculi; capsules club-shaped.
Flowers white, pink or reddish.
Ovules or seeds few, sessile, in 1 or 2 rows.
Plants normally acaulescent.
Capsules obtusely or retusely 4-angled; seeds furrowed along the raphe.
Capsules sharply 4-angled or winged; seeds with a tubercle at one end.
Stems wiry, diffuse; capsules sharply 4-angled.
Stems stout, not diffuse; capsules 4-winged.
* Text revised with the assistance of Dr. John K. Small.
B. Fruit indehiscent, nut-like.
Calyx-tube obconic; filaments with scales at the base; ovary 4-celled.
Calyx-tube filiform; filaments unappendaged; ovary 1-celled.
2. Floral whorls of 2 parts.