1. I. frutéscens, L. Shrubby at the base, nearly smooth (3–8° high); leaves oval or lanceolate, coarsely and sharply toothed, rather fleshy, the upper reduced to linear bracts, in the axils of which the heads are disposed, in leafy panicled racemes; fertile flowers and scales of the involucre 5.—Salt marshes, coast of Mass. to Va. and southward.
2. I. ciliàta, Willd. Annual (2–6° high), rough and hairy; leaves ovate, pointed, coarsely toothed, downy beneath, on slender ciliate petioles; heads in dense spikes, with conspicuous ovate-lanceolate rough-ciliate bracts; scales of the involucre and fertile flowers 3–5.—Moist ground, from Ill. southward.
§ 2. CYCLACHÆ̀NA. Heads in panicled spikes, scarcely bracteate; corolla of the 5 fertile flowers a mere rudiment or none.
3. I. xanthiifòlia, Nutt. Annual, tall, roughish; leaves nearly all opposite, hoary with minute down, ovate, rhombic, or the lowest heart-shaped, doubly or cut-toothed, or obscurely lobed; heads small, crowded, in axillary and terminal panicles.—N. W. Wisc. to Minn., Kan., and westward.
43. AMBRÒSIA, Tourn. Ragweed.
Sterile and fertile flowers occupying different heads on the same plant; the fertile 1–3 together and sessile in the axil of leaves or bracts, at the base of the racemes or spikes of sterile heads. Sterile involucres flattish or top-shaped, of 7–12 scales united into a cup, containing 5–20 funnel-form staminate flowers, with slender chaff intermixed, or none. Anthers almost separate. Fertile involucre (fruit) oblong or top-shaped, closed, pointed, resembling an achene (usually with 4–8 tubercles or horns near the top in one row), and enclosing a single flower which consists of a pistil only; the elongated style-branches protruding. Achenes ovoid; pappus none.—Coarse homely weeds, with opposite or alternate lobed or dissected leaves, and inconspicuous greenish flowers, in late summer and autumn; ours annuals, except the last. (The Greek and later Latin name of several plants, as well as of the food of the gods.)
§ 1. Sterile heads sessile in a dense spike, the top-shaped involucre extended on one side into a large, lanceolate, hooded, bristly-hairy tooth or appendage; fertile involucre oblong and 4-angled.
1. A. bidentàta, Michx. Hairy (1–3° high), very leafy; leaves alternate, lanceolate, partly clasping, nearly entire, except a short lobe or tooth on each side near the base; fruit with 4 stout spines and a central beak.—Prairies of Ill., Mo., and southward.
§ 2. Sterile heads in single or panicled racemes or spikes, the involucre regular.
[*] Leaves opposite, only once lobed; sterile involucre 3-ribbed on one side.