Streptanthus petiolaris A. Gray, brazos rockcress. Annual, taprooted, not rosetted, 1–stemmed at base, unbranched or sometimes with inflorescence branches in canopy, erect, in range 34—109 cm tall; shoot with several basal leaves and mostly cauline leaves, all leaves with well–developed petioles, to midplant pilose and above midplant glabrous.
Stems ± cylindric, to 6 mm diameter, green and somewhat glaucous or purple at base, internodes to 65 mm long.
Leaves helically alternate, deeply pinnately lobed (basal and lower cauline leaves) and unlobed (upper cauline leaves), petiolate, without stipules; petiole broadly and shallowly channeled, 12—70 mm long decreasing upward, green or purplish and glaucous, pilose (lower leaves) or glabrous (upper leaves); blade of lower leaves to 70 × 45 mm, with 1—4 pairs of subopposite lateral lobes, having sinuses often to the midrib, lobes ovate or oblong to obovate, to 22 mm long, ± obtuse at lobe tips, terminal lobe deltate and dentate, pinnately veined with principal veins raised on lower surface, pilose; blade of unlobed leaves narrowly lanceolate to lanceolate–linear, to 70 × 13 mm decreasing upward, the lower ones with short basal lobes or not, low–dentate and wavy on margins, acute at tip, pinnately veined, glabrous and often red–purple on margins.
Inflorescence raceme terminal or later with axillary racemes developing downward in canopy, 20+–flowered, flat–topped and ca. 12 mm across; peduncle cylindric, < 20 mm long; pedicel ascending, cylindric, 6—7 mm long increasing 2× in fruit, glabrous.
Flower bisexual, radial, in range 4.5—5 mm across; sepals 4, oblong–lanceolate, 4—4.5 × 1.1—1.3 mm, white to below midpoint and green above midpoint having a purplish line near margins, glabrous; petals 4, oblanceolate (claw not distinct from limb), in range 5.5—6.3 × 2—2.2 mm, “claw” white to above midpoint, limb lavender to light purple, rounded at tip, pinnately veined; stamens 6, free, outer 2 short and inner 4 longer; filaments erect, ca. 2.5 mm long (short stamens) and 3.5—3.8 mm long (long stamens), white, glabrous; anthers dorsifixed, dithecal, 0.9—1.1 mm long, pale yellow in bud with a tiny purple spot at tip, longitudinally dehiscent; pollen pale yellow; nectaries = irregular disc connected next to bases of stamens, to 0.15 mm long, green, nectar–producing in bud; pistil 1, 3—3.5 mm long, green; ovary superior, cylindric, ca. 3 mm long, slightly glaucous, 2–chambered, each chamber with to 25 ovules; style short, hidden by terminal, domelike, 2–lobed stigma, stigma positioned at midpoint of anthers of long stamens.
Fruit siliqua (silique), short–stalked (gynophore < 0.5 mm long), ascending from ascending pedicel, dehiscent by 2 valves from septum, to 50–seeded, linear, < 65—75+ × (2.4—)3—3.7 mm, green, strongly flattened parallel to septum, scarsely bulging on faces at seeds; beak (style) 2.2—3 mm long.
Seed winged, thin, and strongly flattened, ± heart–shaped and asymmetric, in range 3.8—4.2 × 2.8—3.3 mm, continuous to notch at funiculus, at maturity papery, in range 0.7—1.2 mm wide, seed chamber low–biconvex, glabrous.
A. C. Gibson