Bidens tripartita L.

Swamp Beggar-Ticks

Bidens tripartita plant

Family - Asteraceae

Stem - Erect, to about 1 m, glabrous. Plants taprooted annuals.

Bidens_tripartita_stemStem and node.

Leaf - Leaves all similar, opposite, sessile or petiolate. Blades usually unlobed, lanceolate to elliptic, to 8 cm, sharply toothed, glabrous to moderately pubescent. Deeply 3-lobed leaves occasional present.

Bidens_tripartita_leaf2Leaf adaxial.

Bidens_tripartita_leafLeaf abaxial.

Inflorescence - Solitary terminal heads or in loose, open clusters of heads.

Involucre - Involucral bracts strongly dimorphic. Outer bracts 4-9, to 35 mm, loosely ascending to spreading, oblanceolate to linear, green, leafy, the margins entire or finely toothed, often also with spreading hairs, the outer surface glabrous or sparsely short-hairy. Inner bracts 7-8, to 12 mm long, narrowly ovate to ovate, usually yellow, glabrous.

Bidens_tripartita_involucreInvolucre.

Head - Heads typically discoid. Chaffy bracts present, narrowly lanceolate, usually purplish-tinged at the tip. Ray florets when present 1-5, the corolla inconspicuous, 3-8 mm long, yellow. Disc florets 20-40, the corollas 3-4 mm long, sometimes only 4-lobed, yellow. Pappus absent or more commonly of 3 or 4 awns mostly 2-3 mm long, these barbed, erect or somewhat spreading at fruiting.

Bidens_tripartita_headHead.

Bidens_tripartita_receptacleReceptacle and chaffy bracts.

Fruits - Fruits 3-11 mm long, linear to narrowly wedge-shaped, flattened and somewhat 3- or 4-angled in cross-section, the faces each with a longitudinal nerve, dark brown to purplish black, glabrous or finely pubescent, sometimes also with minute tubercles

Flowering - July - October.

Habitat - Streambanks, pond margins, sloughs, swamps, fens, bottomland forests.

Origin - Native to the U.S.

Other info. - This is one of three Missouri species of Bidens which (usually) has simple rather than compound leaves. This feature in combination with discoid heads is a strong indicator for the species. Ray florets are occasionally present but never showy.

The Bidens tripartita complex contains species native to both the Old World and North America. Morphological differentiation is problematic and more study of this group is needed.

Photographs taken at Catawissa Conservation Area, Franklin County, MO, 9-14-2017; also at Shaw Nature Reserve, Franklin County, MO, 9-29-2013 (SRTurner).



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