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..
Herbs, with alternate opposite verticillate or basal leaves, and perfect regular flowers, in terminal or axillary racemes, spikes, umbels or corymbs, or solitary in the axils or at the summit of a scape. Calyx free from the ovary (adnate to its lower part in Samolits), 4-9-parted or cleft (usually 5-parted), persistent or rarely deciduous. Corolla gamopetalous in our species (wanting in Glaux), 4-9-lobed or cleft (usually 5-cleft), rotate, funnel form, salverform or campanu-late, deciduous. Stamens as many as the corolla-lobes and opposite them, hypogynous or rarely perigynous, inserted on the tube or base of the corolla; filaments distinct, or connate at the base; anthers introrse, attached by their backs to the filaments, 2-celled, the sacs longitudinally dehiscent. Disk obsolete, or none. Ovary superior (partly inferior in Samolus), 1-celled; placenta central, free; ovules anatropous, or mostly amphitropous; style 1; stigma simple, capitate, entire. Capsule 1-celled, 2-6-valved; rarely circumscissile or indehiscent, the valves erect or recurved, entire or 2-cleft. Seeds few or several, the testa adherent to the fleshy or horny copious endosperm; embryo small, straight; cotyledons obtuse.
* Lobes or segments of the corolla erect or spreading, not reflexed.
† Lobes of the corolla imbricated, at least in the bud.
Ovary wholly superior.
Terrestrial scapose plants; leaves not pinnatifid.
Corolla-tube longer than the calyx; style slender.
Corolla-tube shorter than the calyx; style short.
Aquatic leafy-stemmed plant; leaves finely pinnatifid.
Ovary adnate to the calyx.
†† Lobes of the corolla convolute or valvate, at least in the bud (corolla wanting IN NO. 9).
Capsule longitudinally dehiscent.
Corolla rotate, or rarely short-funnelform.
Stem leafy throughout; flowers yellow.
Staminodia none; corolla-lobes convolute: flowers axillary or racemed.
Staminodia 5; each corolla-lobe curved around its stamen.
Staminodia 5, tooth-like; flowers in axillary spike-like racemes.
Leaves whorled at the top of the stem; flowers white.
Corolla none; flowers minute, solitary in the axils.
Capsule circumscissile; flowers axillary.
Corolla longer than the calyx; stamens borne on its base; leaves opposite.
Corolla shorter than the calyx; stamens borne on its tube; leaves alternate.
** Segments of the corolla reflexed; plants scapose.