Limnosciadium pinnatum (DC.) Mathias & Const., arkansas dogshade. Annual, with clustered fibrous roots at base of shoot, 1–stemmed at base, not rosetted, with ascending lateral branches from cauline nodes, initially ± erect but later reclined, in range to 50 cm tall; shoots with 1—2 basal leaves (often short–lived) and finely divided cauline leave to 180 mm long, glabrous, not strongly aromatic.
Stems low–ridged with a broad furrow along each internode, to 4 mm diameter.
Leaves helically alternate, odd–1–pinnately compound with 1—4 pairs of lateral leaflets, petiolate with base sheathing almost 360°, without stipules; petiole to 50 mm long, ridged on lower side, basal sheath to 15 mm long, not inflated, upper portion channeled; blades of leaflets linear, to 40 × 1—2 mm (lateral leaflets) and to 75 × 4 mm (terminal leaflet), entire, acuminate at tip, parallel–veined with principal veins raised on lower surface.
Inflorescence compound umbel, terminal and axillary, with slender axes, flowering canopy several–forked below compound umbels, compound umbel flat–topped, < 20 mm across, umbellets < 7, umbellet with 4+ bisexual and 2—3 staminate flowers, bracteate, glabrous; bract subtending peduncle with sheath to 10 mm long decreasing upward, linear; peduncle to 40 mm long; involucre of bases subtending primary rays, fused at base, narrowly ovate to triangular, 1—2.5 × 0.5 mm, 1–veined, reflexed and persistent; primary rays unequal, slender, < 5—20 mm long; bractlets of involucel subtending outer pedicels awl–shaped, narrowly triangular, < 1—1.5 mm long, green; pedicels unequal, at anthesis to 5 mm long but of staminate flowers to 3 mm long.
Staminate flower radial, ca. 1.8 mm across; sepals 5, unequal, triangular, 0.25—0.5 mm on each flower, fleshy, green with narrow white margins, persistent; petals 5, widely spreading, equal, broadly elliptic, 1 × 0.7—0.8 mm, white, with midridge on upper surface but lacking tail–like tip; stamens 5, free, white; filaments ± 0.8 mm long; anthers dorsifixed, dithecal, inverted heart–shaped, 0.4—0.5 mm long, longitudinally dehiscent; pollen white; pistil absent.
Bisexual flower radial, 2.2—2.5 mm across; sepals 5, unequal, triangular, 0.25—0.5 mm on each flower, fleshy, green with narrow white margins, persistent; petals 5, widely spreading, equal, broadly elliptic, 1 × 0.7—0.8 mm, white, with midridge on upper surface but lacking tail–like tip; stamens 5, free, white; filaments ± 0.8 mm long; anthers dorsifixed, dithecal, inverted heart–shaped, 0.4—0.5 mm long, longitudinally dehiscent; pollen white; pistil 1; ovary inferior, 2–lobed, 0.35 × 0.8 × 0.4 mm, green, glabrous and lacking projections or ribs, 2–chambered, each chamber with 1 ovule; styles 2, ascending, with a nectar–producing platform (stylopodium) on top, D–shaped halves of stylopodium together 0.6 × 0.8 mm, white, styles 0.2—0.25 mm long; stigmas terminal, minute.
Fruit schizocarp, of 2 dry, 1–seeded halves (mericarps), before splitting ± ellipsoid, in range ca. 2.2 × (1—)1.4—1.6 mm + conic stylar structure at top, before drying mericarp wall glossy striped brown and straw–colored (ribs) and smooth, 2 lateral ribs much broader; sepals spreading to ascending, triangular, to 0.5 mm long; stylopodium on top wavy and stigmatic lobes persistent.
A. C. Gibson