Senecio vulgaris L., common groundsel. Annual, taprooted, not rosetted, 1—several–stemmed at base, ascending to erect or nodding (somewhat suntracking), 6—52 cm tall; shoots initially ± cobwebby–tomentose especially at leaf bases and in leaf axils, aging sparsely hairy to glabrate, the hairs loose, colorless, weak, composed of ± elongated beadlike cells
Stems ridged, to 3.5 mm diameter, with 3 ridges descending from each leaf, lower stem often purple, aging glabrescent (glabrate).
Leaves helically alternate (the first 2 leaves opposite with pair fused across node), unlobed (lower leaves) and pinnately lobed, ± petiolate (lower cauline leaves) and sessile and commonly clasping (upper cauline leaves), without stipules; petiole weakly defined, to 15 mm long, ± flattened on upper side, winged and grading into blade; blade ovate to oblanceolate or oblong to obovate, (4—)20—100 × (1—)5—20 mm, slightly fleshy, long–tapered at base (lower leaves), rounded to cordate at base with clasping lobes to 5.5 mm long (upper leaves), unevenly dentate and somewhat crisped on margins, lobes deep or shallow, ± opposite to alternate, pinnately veined with midrib conspicuously sunken on upper surface and raised on lower surface, sparsely short–cobwebby (glabrous).
Inflorescence heads in cymelike array (solitary), array terminal, ascendingly branched with several—13(—20) heads or unbranched with 1 head (diminutive individual), head discoid, (2—)3—6 mm across, of (13—)40—60+ flowers, bracteate; bract subtending array = dentate or lobed cauline leaf with clasping base; axes green (pinkish), loosely dense–tomentose (glabrous), finely ridged, peduncle 2—25+ mm long; bracts along peduncle to 15, 1 near midpoint, others helically alternate, bracts subtending involucre (= calyculus) narrowly lanceolate to awl–shaped, 1—2.5 mm long, dark purple–tipped to black–tipped; involucre cylindric, (6—)7—9 × (2—)2.5—4 mm, becoming conic or urn–shaped as fruits develop, phyllaries (8) ca. 21 ± in 1 series, linear, (4—)5.5—7.5 mm long, green–striped with membranous margins, at anthesis some dark purple–tipped or black–tipped or all without dark tips, glabrous, with tips arching over and concealing flowers in bud; receptacle flat, bractlets (paleae) lacking, with shallow pits, hollow beneath receptacle
Disc flower bisexual, radial, 0.8 mm across, 5.5—7(—10) mm long, extending slightly beyond phyllaries; calyx (pappus) of numerous capillary bristles, ≤ corolla length, thin, silky white, with minute ascending barbs (visible at 30×); corolla (4—)5–lobed, 4—5 mm long, glabrous; tube narrowly cylindric, 2.5—3.5 × 0.25 mm, pale green; throat cylindric and slightly wider than tube, 1—1.5 × 0.35—0.4 mm, yellow; lobes ± erect, acute, typically 0.25—0.3 mm long, yellow with green veins; stamens 5; filament fused to base of corolla throat, ca. 1 mm long; anthers fused into cylinder surrounding style, with tips scarcely exserted, basifixed, dithecal, ± 1.5 mm long, yellow with purplish connective, longitudinally dehiscent; pollen yellow; pistil 1; ovary inferior, cylindric to narrowly club–shaped, ± 1.5 mm long, nearly glabrous, 1–chambered with 1 ovule; style eventually exserted to 0.8 mm (after pollination), 2–branched, the branches spreading and stigmatic, yellow, flattened top to bottom, long–papillate on truncate tips
Fruits cypselae, monomorphic, straight–cylindric, 2.3—2.6 × 0.3—0.4 mm, red–brown, shallowly 8—10–ribbed, minutely scabrous on ribs; intact pappus mass spheric, colored like dandelion (Taraxacum), bristles 4—6.5 mm long.
A. C. Gibson & B. A. Prigge