(Source: Payne, 1970.)
1. Staminate and pistillate florets in the same head; ray florets pistillate, disk florets staminate
...... IVA - marsh‑elder.
1. Staminate and pistillate florets borne in separate heads.
2. Pistillate heads 2‑flowered and with many, sharply-hooked spines; staminate heads lacking phyllaries
......... XANTHIUM - cocklebur.
2. Pistillate heads 1 (‑2) ‑flowered with a few vestigial spines or none; staminate heads with involucres of connate phyllaries
......... AMBROSIA - ragweed.
AMBROSIA - ragweed
1. Pistillate involucres with 2 flowers and 2 sharp beaks
........... A. tomentosa (Franseria discolor) - skeleton-leaf bur-sage.
1. Pistillate involucres with 1 flower and 1 beak.
2. Leaves palmately lobed or unlobed; staminate involucres marked with dorsal striations; usually all cauline leaves opposite; plants annual (s = 2x, 4x; x = 12) .
.... A. trifida - giant ragweed.
2. Leaves pinnately or bipinnately lobed or parted; staminate involucres lacking dorsal striations; upper cauline leaves usually alternate.
3. Plants perennial with horizontal runner-like underground roots; involucral spines blunt or absent; leaves usually coarsely lobed (s = 2x, 3x, 4x, 5x, 6x, 7x, 8x, ca 11x, 12x, 16x; x = 9)
.... A. psilostachya - western or perennial ragweed.
3. Plants annual with taproots; involucral spines usually sharply pointed; leaves usually delicately lobed and parted (s = 2x; x = 18, 17) .
.... A. artemisiifolia - common ragweed.
IVA - marsh‑elder
1. Plants perennial; phyllaries basally connate (s = 2x, 3x; x = 18)
..... I. axillaris - poverty-weed.
1. Plants annual; phyllaries free.
2. Leaves ovate, coarsely serrate; heads subtended by prominent bracts; phyllaries 3‑4 (s = 2x; x = 17)
...... I. annua - rough marsh-elder.
2. Leaves subcordate to ovate, usually coarsely lobed and toothed; heads ebracteate; phyllaries 5 (s = 2x; x = 18) .
...... I. (Cyclachaena) xanthifolia - big marsh-elder.
XANTHIUM - cocklebur
(s = 2x; x = 18)
1. Leaves pinnately lobed; stems bearing long, golden, three-rayed, axillary spines ..
........... X. spinosum - spiny cocklebur.
1. Leaves coarsely palmately lobed; stems unarmed
........... X. strumarium - common cocklebur.
Plants of Wisconsin
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