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, rarely somewhat woody, with watery acrid sap, alternate leaves, and racemose or corymbose flowers. Sepals 4, deciduous, or rarely persistent, the 2 outer narrow, the inner similar, or concave, or saccate at the base. Petals 4, hypogynous, cruciate, nearly equal, generally clawed. Stamens 6, hypogynous, tetradynamous, rarely fewer. Pistil 1, compound, consisting of 2 united carpels, the parietal placentae united by a dissepiment; style generally persistent, sometimes none; stigma discoid or usually more or less 2-lobed. Fruit a silique or silicle, generally 2-celled, rarely 1-celled, in a few genera indehiscent or lomentaceous.
Seeds campylotropous, attached to both sides of the septum; endosperm none; cotyledons incumbent, accumbent or conduplicate, or the embryo rarely straight.
About 200 genera and 1800 species, of wide geographic distribution. Also known as Bras-sicaceae.
Key to Genera.
1. Pod continuous, not transversely 2-jointed. A. Cotyledons accumbent or incumbent; pods dehiscent (except in Neslia, Myagrum). a. Pods globose to linear, not greatly elongated nor very broad and flat.
* Pubescence stellate, or of forked hairs.
Pubescence stellate, or the hairs 2-lobed.
Pods orbicular to linear, more or less flattened parallel to the broad partition (Alysseae).
Seeds many in each cell of the oval to linear flat pod.
Seeds few in each cell of the short pod.
Pods little flattened; petals 2-cleft.
Pods much flattened; petals entire.
Flowers white; pubescence of 2-lobed hairs.
Flowers yellow; pubescence stellate.
Pods globose or didymous, swollen (Physarieae).
Pubescence of forked hairs; pods little longer than wide (Camelineae).
Pods dehiscent, several-many-seeded.
Pods obovoid, swollen; flowers yellow.
Pods not swollen, flattened at right angles to the partition; flowers white.
Pods cuneate to triangular-obcordate.
Pods elliptic, the valves with a strong midvein.
Pods indehiscent, reticulated, mostly 1-seeded.
** Pubescence of simple hairs, or wanting.
Pods globose to oblong, scarcely or not at all flattened.
Aquatic with subulate leaves and minute white flowers; cotyledons incumbent (Subularieae).
Aquatic or terrestrial; leaves not subulate; cotyledons accumbent (Cochlearieae).
Flowers yellow, small.
Flowers white, large.
Leaves pinnately divided; pods linear.
Leaves crenate, lobed or pinnatifid; pods globose or oblong.
Terrestrial; leaves undivided or the lower pinnatifid; style very short.
Tall herbs; perennials.
Aquatic: submersed leaves finely dissected, style slender; pod 1-celled.
Pods short, strongly flattened at right angles to the narrow partition; flowers white or purplish.
Pods dehiscent (Lepidieae).
Seeds solitary in each cell of the pod.
Pods smooth, orbicular or ovate.
Pods rugose-reticulate or tuberculate.
Seeds 2-several in each cell of the winged pod.
Pods indehiscent (Isatideae).
b. Pods elongated-linear, or large, broad and very flat in Lunaria and Selenia.
Pods stipitate (Stanleyae).
Pods long-stipitate; sepals reflexed.
Pods short-stipitate; sepals not reflexed.
Pods sessile or very nearly so.
Cotyledons incumbent (Sisymbrieae).
Stigma simple, not 2-lobed.
Leaves broad, cordate; flowers white.
Leaves finely dissected; flowers yellow.
Stigma 2-lobed, the lobes over the placentae.
Flowers yellow or orange.
Leaves various, not cordate-clasping.
Pods 4-angled; hairs of the stem 2-forked with appressed branches.
Pods terete; pubescence, if present, of simple hairs.
Pods narrowly conic, ribbed.
Leaves cordate-clasping, sessile, entire.
Flowers very large, deep purple.
Flowers small, white or pale purple.
Cotyledons accumbent (Arabideae).
Pods narrowly linear, rarely oblong.
Stems leafy, at least above.
Flowers pure yellow; pods terete or nearly so.
Flowers white, pink, violet or purple.
Pods terete, not flattened parallel with the partition.
Pods more or less flattened parallel with the partition.
Seeds winged or wing-margined; pods not elastically dehiscent; pubescence,
or some of it, when present, of branched hairs.
Seeds wingless; pods mostly elastically dehiscent the base; pubescence, if
present, of simple hairs.
Stem leafy below or throughout.
Stem leafless below, 2 - 4-leaved above.
Stem scapose, 1-few-flowered.
Pods very broad and flat.
Flowers yellow; pods finely veined.
Flowers purple; pods reticulate-veined.
B. Cotyledons conduplicate; pod (silique) elongated, beaked; weeds of cultivation (Brassiceae).
Beak of the silique stout, flat or angled.
Valves with 1 strong nerve.
Beak of the silique conic, often short.
Silique terete; seeds in 1 row in each cell.
Silique flattened; seeds in 2 rows in each cell.
2. Pod transversely 2-jointed; fleshy, seaside herbs (Cakileae).