Draba platycarpa Torrey & A. Gray, broadpod draba. Annual, taprooted, rosetted, 1—several–stemmed at base, lateral branches mostly unbranched, forming at successive nodes, ± ascending, to 35 cm tall; shoots with basal leaves and 4—10 cauline leaves with basal leaves present during late fruiting, short–hirsute with treelike branched hairs (dendritic).
Stems cylindric, to 3 mm diameter, lower primary axis with long, unbranched hairs mixed with shorter dendritic hairs.
Leaves helically alternate, simple, ± petiolate (basal leaves) and sessile but not clasping (cauline leaves), without stipules; petiole ± flattened and indistinct from blade; blade obovate to elliptic, < 10—30 × 4—12 mm, tapered at base, with several low teeth to subentire on margins, obtuse to acute at tip, pinnately veined with midrib sunken on upper surface and raised on lower surface, having mixed long hairs and dendritic hairs (basal leaves) to only dendritic hairs (most cauline leaves).
Inflorescence raceme, terminal, many–flowered, with open flowers overtopping buds, lacking bracts, villous–hirsute with mixed long unbranched and shorter branched hairs; axis stemlike, green; pedicel slender, at anthesis ascending and ca. 2 mm long later spreading and increasing fruit.
Chasmogamous flower bisexual, radial, 2—2.5 mm across; sepals 4, erect, equal, elliptic to ovate or oval, 1.5—1.8 mm long, center green and short–villous with unbranched hairs and having narrow white margins and at tip, deciduous; petals 4, erect, obovate, 2.8—3.4 × 1—1.3 mm long, white, rounded at tip, glabrous; stamens 6, free, subequal, with 2 outer short and 4 inner slightly longer; filaments gradually tapered to tip, 1.3—1.7 mm long, of outer stamens 0.3—0.4 mm shorter, whitish; anthers basifixed, dithecal, ± 0.35 mm long, pale light yellow, longitudinally dehiscent; pollen pale yellow; nectaries 4, minute; pistil 1, 1.2—1.4 mm long; ovary superior, ellipsoid compressed side–to–side parallel to septum, green or tinged reddish, initially appearing glabrous soon becoming short–hirsute with ascending, unbranched hairs, 2–chambered, each chamber with many ovules; style ± 0.2 mm long, greenish; stigma capitate.
Cleistogamous flower bisexual, radial, ca. 1 mm long; sepals 4, erect, subequal, ± elliptic and cupped, 1 mm (ovary faces) and 1.4 mm long (opposite edges of ovary) and 0.5 mm wide, tinged red–purple, with short dendritic hairs; petals absent; stamens 6; filaments slender, to 1 mm long, green; anthers somewhat heart–shaped, ca. 0.1 mm long; pistil 1, ca. 1 mm long; ovary superior, ellipsoid scarcely compressed, green with short dendritic hairs, 2–chambered, each chamber with many ovules; style and stigma ca. 0.2 mm long, white, at same height as anthers.
Fruits siliquas (silicles), dehiscent by 2 valves parallel to translucent septum, dimorphic, of chasmogamous flowers flattened elliptic to obovate–elliptic, ± 5.5—7 × 2.6—3 mm, dark tan, ± pinnately veiny, uniformly short–hirsute with hairs to 0.35 mm long and having enlarged bases (pustulate), inner surface ± pinnately veiny; translucent septum persistent on plant; pedicel in fruit widely spreading, to 8 mm long, pubescent with dendritic hairs; of cleistogamous flowers shorter, more plump, with short dendritic hairs.
Seed ellipsoid to ovoid compressed side–to–side, 0.5—0.7 × 0.3—0.45 mm, brown to red–orange or red, not winged, having minute projections on faces, sides furrowed showing radicle; cotyledons acumbent.
A. C. Gibson