y colored parts; staminate and
pistillate flowers in separate heads (or rarely in the same heads);
coarse weeds with inconspicuous flowers (summer and autumn) --2.

1b. Flowers with some petal-like parts, usually brightly colored or
white --10.

2a. Leaves toothed or lobed --3.

2b. Leaves deeply pinnatifid or dissected (4-15 dm. high; flowers
in erect spikes) (Ragweed) --7.

3a. Fruit or pistillate flowers thickly covered with sharp hooked spines
(3-10 dm. high) (Cocklebur) --4.

3b. Fruit not spiny --8.

4a. With spines on the stem at the base of the leaves
=Cocklebur, Xanthium spinosum.=

4b. Without any spines on the stem --5.

5a. Body of the bur smooth or slightly hairy
=Cocklebur, Xanthium canadense.=

5b. Body of the bur and the spines densely pubescent --6.

6a. Body of the bur more than twice as long as thick; a common weed
=Cocklebur, Xanthium commune.=

6b. Body of the bur less than twice as long as thick
=Cocklebur, Xanthium echinatum.=

7a. Leaves twice-pinnatifid =Ragweed, Ambrosia artemisiifolia.=

7b. Leaves once-pinnatifid =Ragweed, Ambrosia psilostachya.=

8a. Leaves deeply 3-lobed (1-5 m. high)
=Giant Ragweed, Ambrosia trifida.=

8b. Leaves serrate or obscurely lobed --9.

9a. Stem simple or sparingly branched; pistillate heads in the axils of
the upper leaves (1-3 m. high)
=Giant Ragweed, Ambrosia trifida var. integrifolia.=

9b. Stem much branched; heads all alike, in panicles
=Marsh Elder, Iva xanthifolia.=

10a. Flowers all strap-shape; juicy milky. (The central flowers
must be examined carefully, since they are frequently much
smaller than the marginal ones) --11.

10b. Flowers all tubular, with regular. 4-5-lobed corollas --45.

10c. Flowers both tubular and strap-shape; heads radiate (in a few
species the rays are small and may be overlooked by mistake)
--108.

11a. Flowers blue (summer and autumn) --12.

11b. Flowers orange, yellow, white, or purplish --15.

12a. Heads 2.5 cm. wide, or larger --13.

12b. Heads 1.5 cm. wide, or smaller --14.

13a. Leaves line