Robert W. Freckmann Herbarium



Tribe 10a. HELIANTHEAE (s.l.) Subtribe AMBROSIINAE


(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.

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