_Herbs, with a pungent watery juice and cruciform tetradynamous flowers; fruit a silique or silicle._--Sepals 4, deciduous. Petals 4, hypogynous, regular, placed opposite each other in pairs, their spreading limbs forming a cross. Stamens 6, two of them inserted lower down and shorter (rarely only 4 or 2). Pod usually 2-celled by a thin partition stretched between the two marginal placentæ, from which when ripe the valves separate, either much longer than broad (a _silique_), or short (a _silicle_), sometimes indehiscent and nut-like (_nucumentaceous_), or separating across into 1-seeded joints (_lomentaceous_). Seeds campylotropous, without albumen, filled by the large embryo, which is curved or folded in various ways: i.e. the _cotyledons accumbent_, viz., their margins on one side applied to the radicle, so that the cross-section of the seed appears thus o==; or else _incumbent_, viz., the back of one cotyledon applied to the radicle, thus o||. In these cases the cotyledons are plane; but they may be folded upon themselves and round the radicle, as in Mustard, where they are _conduplicate_, thus o>>. In Leavenworthia alone the whole embryo is straight.--Leaves alternate, no stipules. Flowers in terminal racemes or corymbs; pedicels rarely bracted.--A large and very natural family, of pungent or acrid, but not poisonous plants. (The characters of the genera are taken almost wholly from the pods and seeds; the flowers being nearly alike in all.) SERIES I. Pod 2-celled, regularly dehiscent by 2 valves. [*] Pod compressed parallel to the broad partition. Seeds flat or flattish, orbicular or oval; cotyledons accumbent or nearly so. [+] Pod large, oblong-elliptical, valves nerveless. Seeds in 2 rows. Flowers yellow. 1. Selenia. Leaves pinnatisect. Raceme leafy-bracteate. Seeds winged. [+][+] Pod linear; valves nerveless. Seeds in one row. Flowers yellow only in n. 3. 2. Leavenworthia. Seed winged; embryo straight or nearly so. Annual; stem often scapose, 1--few-flowered. 3. Dentaria. Stem naked below, 2--3-leaved. Pod coriaceous, with thick placentas, long-styled. Seeds wingless; cotyledons thick, very unequal. 4. Cardamine. Stem leafy. Pod coriaceous, with thick placentas. Seeds wingless; cotyledons flattened, equal. [+][+][+] Pod linear, or oblong, or orbicular; valves 1-nerved or nerveless. Seeds in 2 rows (except in species of n. 4). 5. Arabis. Pod long-linear, the flat or flattish valves more or less 1-nerved. Seeds winged or wingless. Flowers white to purple. Stems leafy, at least below. 6. Draba. Pod oval to narrowly oblong or lanceolate; valves flat or flattish, faintly nerved or veined. Seeds wingless, numerous. 7. Alyssum. Pod orbicular; valves veinless, somewhat convex with flattened margin. Seeds wingless, 2--4. [*][*] Pod terete or turgid, or 4-angled by the prominent midnerves. Seeds wingless, more or less turgid. [+] Pods short. (See also n. 10.)