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..
Usually slender annual or perennial caulescent shrubby herbs. Leaves thinnish, mostly narrow, entire or shallowly toothed; buds mostly erect. Flowers yellow, diurnal, in terminal spikes or racemes. Calyx-tube slender, dilated at the throat; calyx-segments finally reflexed, the tips united in the bud or nearly so. Petals 4, spreading. Stamens 8, the alternate ones longer; filaments filiform; anthers linear. Ovary usually club-shaped, 4-angled; united styles filiform; stigma 4-cleft; ovules numerous, on slender stalks, in many rows. Capsules more or less club-shaped, nearly sessile or long-stalked, 4-winged or rarely sharply 4-angled. Seeds numerous, not angled, without a tubercle. [In honor of Prof. C. Kneiff, of Strassburg, who wrote on cryptogamic botany.]
Stem-leaves filiform-linear; capsules 4-angled or very slightly 4-winged.
Stem-leaves never approaching filiform; capsules prominently winged.
Pedicels and capsules pubescent.
Stems decumbent, spreading.
Stems erect or nearly so; body of the capsule more or less elongated.
Pedicel longer than the body of the capsule, the wings thick and pubescent.
Pedicels shorter than the body of the capsule, the wings thin and glabrous.
Hirsute; flowers 1 1/2'-2 1/2' wide.
Merely puberulent; flowers 1' wide or less.
Pedicels and capsules glabrous or glabrate.
Capsules oblong or nearly so.
Plants not glaucous, usually pubescent: capsules less than 4" long.
Plants somewhat glaucous, glabrous; capsules more than 5" long.