1. Thelesperma filifolium (Hook.) A. Gray (stiff greenthread)

Pl. 292 a, b; Map 1235

Plants annual or occasionally short-lived perennial herbs, with taproots or less commonly a somewhat branched, vertical rootstock. Stems 20–70 cm long. Leaves well distributed along the stems. Leaf blades 3–12 cm long, all but occasionally those of the uppermost leaves 2 or 3 times pinnately dissected, the ultimate segments 5–30 mm long. Heads radiate. Involucre with the outer series of 7–10 bracts 4–8 mm long, linear to narrowly triangular with a long-tapered tip; the inner series of bracts 7–10 mm long, mostly fused in the basal 1/4–1/3, the free portion lanceolate to narrowly ovate. Ray florets usually 8, the corolla 10–22 mm long, yellow, occasionally tinged reddish brown toward the base. Disc florets with the corollas 5–7 mm long, yellow to orangish yellow with reddish brown veins (entirely reddish brown elsewhere). Pappus absent or of 2 short, stout awns 0.5–1.0(–2.0) mm long. Fruits 3–6 mm long. 2n=16, 18, 20, 22. May–August.

Uncommon, known thus far only from Greene County, also introduced in Jackson County and the city of St. Louis (Wyoming to New Mexico east to Missouri and Mississippi; Mexico). Glades and tops of bluffs, on limestone substrates; also railroads and open, disturbed areas.

Steyermark (1963) and some other earlier authors called this taxon T. trifidum (Poir.) Britton, but Shinners (1950) explained that the name is of uncertain application and cannot refer to the plants most recent authors have called T. filifolium, as Poiret’s original description noted alternate leaves. Alexander (1955) similarly excluded the name T. trifidum, but without explanation.

Steyermark’s (1963) report of this species from Lawrence County could not be substantiated during the present research. Some authors (Barkley, 1986) have accepted var. intermedium (Rydb.) Shinners, a weakly separable variant in the western portion of the species range characterized by relatively short stems, relatively long outer involucral bracts, and somewhat lighter yellow ray corollas. Our plants correspond to var. filifolium, but the number of intermediate specimens in the region of overlap to the southwest of Missouri makes it impractical to recognize these varieties.