Mikania scandens (L.) Willd., climbing hempweed. Perennial herb, twining, rhizomatous, fibrous–rooted with fleshy clustered roots, not rosetted, arising 1–stemmed from tip of rhizome, arching to trailing over low vegetation; shoots with curving and somewhat twining stem internodes and widely spaced, paired cauline leaves, short–hairy and with sessile glandular hairs having colorless heads.
Stems conspicuously 8–ridged, to 2.5 mm diameter, with medial ridge on each side and 3 ridges descending from each leaf, having ledges across each node with crestlike projections 1—1.5 mm long, internodes 80—200 mm long, purplish red at base of internode.
Leaves opposite decussate, simple, petiolate with a pair of ledges across node, without stipules; petiole shallowly channeled base–to–top, 50+ mm long, with short hairs and sessile glandular hairs, on margin at base of petiole having a thick, pencil–like hair ca. 0.5 mm long resembling a rattlesnake tail; blade ovate to deltate or triangular, in range to 85 × 75 mm, cordate at base, entire or broadly low–dentate and wavy on margins, acuminate at tip, 3–veined from base and pinnately veined above with principal veins raised on lower surface, short–hairy, upper surface with scattered short hairs, lower surface also gland–dotted with sessile glandular hairs in pits.
Inflorescence heads, in flat–topped or domed array of many heads, arrays axillary, in range ca. 40 mm across, of opposite decussate branchlets each with ultimate clusters of 5—10 heads, head ca. 2 mm across, of 4 disc flowers; bract subtending inflorescence leaflike diminutive and spreading; axes of branchlets 6–ridged; bracts subtending cluster 1—2, acuminate–lanceolate, 3—4 mm long, mostly white, short–hairy; peduncle subtending head 4–ridged; involucre ± cylindric 4–sided, 1—1.3 mm diameter, of 4 equal, opposite decussate phyllaries, each phyllary subtending a flower, awl–shaped, ca. 5 × 1 mm, in range outer phyllaries green to above midpoint with white tips and inner phyllaries narrowly elliptic with wide membranous margins and a green midvein, short–hairy; receptacle ± flat, narrow, lacking bractlets (paleae), glabrous; pedicel of flower 0.1—0.15 mm long, white.
Disc flower bisexual, radial, 1—1.2 mm across; calyx (pappus) of ca. 30 capillary bristles in 1 whorl, at anthesis ca. 2.5 mm long, white; corolla 5–lobed, 3.3—3.6 mm long, in bud with minute sessile glandular hairs; tube 1 × 0.35 mm, pale green; throat narrowly funnel–shaped, white, 1 mm wide at orifice, with 5 veins to sinuses; lobes deltate, 0.5 mm, white; stamens 5, fused to top of corolla tube; filaments 1 mm long, white; anthers fused into cylinder surrounding style, basifixed, dithecal, partially exserted, 1 mm long including acuminate appendages 0.25 mm long, white aging reddish, longitudinally dehiscent; pollen white; pistil 1; ovary inferior, narrowly wedge–shaped, 0.7 × 0.4 mm, green, 5–ribbed, 1–chambered with 1 ovule; style exserted from flower and involucre, ca. 5.5 mm long, 2–branched, lower portion 3.5 mm long, the branches ascending to spreading, threadlike, 1.5—2 mm long, white, stigmatic, and papillate above midpoint.
Fruits cypselae (to 4/head), prismatic wedge–shaped and strongly 5–ribbed, in range ca. 2 × 0.5 mm, glossy brown, strongly 5–sided, having a disclike callus at base, densely covered with sessile glandular hairs; pappus of ± 30 capillary bristles 2—3.5 mm long, in range white.
A. C. Gibson