Cirsium horridulum Michx. var. horridulum, horrid thistle, yellow thistle. Annual or biennial, horrifically spinescent, taprooted, flat–rosetted, 1–stemmed at base, with several lateral branches each with terminal head, in range 35—90 cm tall; shoots with 20+ basal leaves, photosynthetic cauline leaves decreasing upward, and a set of leaves hiding developing head with comblike series of spines, cobwebby–woolly.
Stems 20+–ridged, to 15 mm diameter, tough, with a principal ridge + several minor ridges descending from each cauline leaf, ridges rounded, initially grayish and cobwebby–woolly aging without most hair; pith continuously hollow and to 10+ mm wide at base (white pith mostly destroyed).
Leaves helically alternate, deeply pinnately lobed with lateral lobes never in pairs, petiolate, without stipules; basal leaves to 500 × 90 mm, petiole not clearly defined, lobes irregular in shape, to 55 mm long, principal lobes and sublobes wavy and with spine tips = endings of principal veins, coarsely toothed on lobe margins, the principal spines < 10 mm long and straw–colored, spines on each lobe variable in length, at sinuses midrib having blade wings, fleshy midrib + green wings ca. 15 mm wide, pinnately veined with principal veins raised on lower surface, midrib triangular and pale green and white); cauline leaves decreasing upward (to 40 × 11 mm) and becoming flatter and more spinescent, at top of shoot (hiding involucre) cauline leaves reduced to a midrib lined with 40+ sharp–pointed lateral spines, the spines simple and some branched at base, < 2—20 mm long, white, highly variable in length and diameter along each side of infolded midrib, midrib with fine cobwebby–woolly hairs.
Inflorescence head, terminal and axillary, appearing spinescent, head at anthesis ca. 35 mm across but appearing much wider, of 250+ bisexual disc flowers, flowers helically alternate, bracteate; peduncle short above the uppermost cauline leaf; involucre fully hidden by a dense set of spinescent cauline leaves, bell–shaped, at anthesis ca. 40—45 × 30—35 mm, of 200+ helically alternate, stiff and spine–tipped phyllaries, in numerous series and unequal, phyllaries narrowly triangular, < 15 × 2 mm (the outermost phyllaries) to long–tapered linear with flexible tip, 50 × 2 mm (the innermost phyllaries), margins cobwebby–ciliate with tawny fine hairs, outer surface cobwebby–hairy approaching point at tip, inner series of phyllaries not ciliate, outer surface glabrous at base and above mostly scabrous with short upward–pointing hairs and cobwebby–tomentose approaching tip; in fruit involucre aging cup–shaped to 80 mm across; receptacle flat to slightly domed aging domed, lacking bractlets (paleae) subtending flowers, with white hairs between ovaries 3.5—5 mm long.
Disc flower bisexual, radial, to 1.5 mm across, 21—25 mm long; calyx (pappus) fine, soft, feathery capillary bristles (plumose) 100+ in several series, before anthesis 17—20 mm long increasing 2× and ascending in fruit, silky–white, fused to ring at base, some lateral hairs 5+ mm long; corolla 5–lobed, ca. 24 mm long; tube 5–ribbed with deep furrows, ca. 4.5 × 0.6—0.7 mm, white, lacking hairs; throat ± cylindric, 7—8 mm long, 0.8 mm diameter at base to 1 mm diameter at top, cream–colored, lacking hairs; lobes erect and only scarcely gapping by anther appendages and stigma passing through, strap–shaped, 6—7 × 0.3 mm or splitting longer, white, with blunt, glossy, glutinous callus at tip, papillate on outer lower) surface; stamens 5, fused to top of corolla tube; filaments never straight, 6—7 mm long, mostly red with white next to anther tails; anthers fused into cylinder surrounding style, basifixed, dithecal, ca. 11 mm long including white, acuminate appendages 1 mm long and flat, linear tails ± 2 mm long, sacs cream–colored, appendages minutely toothed at tip, longitudinally dehiscent; pollen white; pistil 1; ovary inferior, ± wedge–shaped, 1 mm long, whitish, glabrous, 1–chambered with 1 ovule; nectary disc surrounding base of style, ca. 0.3 mm long, white (corolla not bulging); style to top of anther appendages, cylindric, 20—23 mm long, white, lacking hairs, 2–branched, the branches stigmatic, ca. 2 mm long but appressed and coherent.
Fruits cypselae (achenes), knife sheathlike compressed side–to–side, 5.5—6 × 1.5—2 mm, straw–colored, blunt to nearly truncate at base, with rim at top and within rim having a projecting, persistent base of style to 0.8 mm long, faces with several fine groove and wrinklelike texture; pappus of 100+ silky capillary hairs in several series fused by ring at base, ca. 35 mm long, ring easily dislodged from fruit.
A. C. Gibson