Descurainia pinnata (Walter) Britt. subsp. pinnata, pinnate tansymustard. Annual, feathery, taprooted, rosetted, 1–stemmed at base, branched or not in canopy, erect to suberect, 30—90 cm tall; shoots with finely dissected basal leaves and cauline leaves, densely pubescent with branched nonglandular hairs (dendritic) and stalked colorless glandular hairs having well–defined heads, strongly scented.
Stems angular becoming ± cylindric, to 6.5 mm diameter, with ridges and lines descending from each leaf, densely glandular–hairy and dendritic–hairy.
Leaves helically alternate, odd–2(—3)–pinnately compound, ± sessile with primary leaflet (pinna) at base, without stipules; blade oblanceolate to oblong in outline, 30—115 × 12—50 mm, gradually reduced upward; leaf axes angular, with scattered glandular hairs; primary and secondary units (pinnules) ovate to obovate; ultimate segments ± obovate, of varying lengths, deeply lobed, the lobes obovate to elliptic, < 2 mm long, acute to rounded at tip, grayish green due to abundant dendritic hairs.
Inflorescence raceme, terminal, many–flowered, flat–topped with open flowers somewhat overtopping buds, lacking bracts; rachis and peduncle glandular–pubescent and with widely scattered unbranched or branched nonglandular hairs; pedicel initially ascending, slender, at anthesis 3—4.5 mm long increasing 2× and ± spreading in fruit, with widely scattered glandular hairs and several ± unbranched nonglandular hairs.
Flower bisexual, radial, 2 mm across; sepals 4, spreading, cupped–elliptic, 1.7—1.9 mm long, green with yellow margin and while open aging completely yellowish (in range lacking rose pigment), with unbranched hairs and lacking glandular hairs, deciduous; petals 4, distinctly clawed, erect, obovate, 1.5—1.8 mm long, bright yellow; stamens 6, free, ± equal with outer 2 short and 4 inner slightly longer; filaments 1.6—1.9 mm long, yellow–green; anthers basifixed, dithecal, 0.4 mm long, yellow, longitudinally dehiscent; pollen yellow; nectaries 6, = minute lobes between filaments, greenish; pistil 1; ovary superior, ± fusiform, ± 2 mm long, greenish yellow, glabrous, 2–chambered, each chamber with ca. 16 ovules attached to septum in 2 rows; style ca. 0.2 mm long, persistent; stigma capitate, unlobed.
Fruit siliqua (silique), dehiscent by 2 valves, ± 20–seeded, fusiform–elliptic and 4–angled or somewhat club–shaped, in range 4—7 × ca. 1 mm, glabrous, with conspicuous midvein on each valve, with seeds often bulging; beak absent; seeds attached to elliptic, translucent septum.
Seed ellipsoid, ca. 1 mm long, reddish brown.
A. C. Gibson