2. A. trífida, L. (Great Ragweed.) Stem stout (3–12° high), rough-hairy, as are the large deeply 3-lobed leaves, the lobes oval lanceolate and serrate; petioles margined; fruit obovate, 5–6-ribbed and tubercled.—Var. integrifòlia, Torr. & Gray, is only a smaller form, with the upper leaves, or all of them, undivided, ovate or oval.—Moist river-banks; common.
[*][*] Leaves many of them alternate, all once or twice pinnatifid.
3. A. artemisiæfòlia, L. (Roman Wormwood. Hog-weed. Bitter-weed.) Much branched (1–3° high), hairy or roughish-pubescent; leaves thin, twice-pinnatifid, smoothish above, paler or hoary beneath; fruit obovoid or globular, armed with about 6 short acute teeth or spines.—Waste places everywhere.—Extremely variable, with finely cut leaves, on the flowering branches often undivided; rarely the spikes bear all fertile heads.
4. A. psilostàchya, DC. Paniculate-branched (2–5° high), rough and somewhat hoary with short hispid hairs; leaves once pinnatifid, thickish, the lobes acute, those of the lower leaves often incised; fruit obovoid, without tubercles or with very small ones, pubescent.—Prairies and plains, Ill., Wisc., Minn., and southwestward. Perennial, with slender running rootstocks.
44. XÁNTHIUM, Tourn. Cocklebur. Clotbur.
Sterile and fertile flowers occupying different heads, the latter clustered below, the former in short spikes or racemes above. Sterile involucres and flowers as in Ambrosia, but the scales separate and receptacle cylindrical. Fertile involucre closed, coriaceous, ovoid or oblong, clothed with hooked prickles so as to form a rough bur, 2-celled, 2-flowered; the flower consisting of a pistil and slender thread-form corolla. Achenes oblong, flat, destitute of pappus.—Coarse and vile weeds, with annual roots, low and branching stout stems, and alternate toothed or lobed petioled leaves; flowering in summer and autumn. (The Greek name of some plant that was used to dye the hair yellow; from ξανθός, yellow.)
[*] Leaves attenuate to both ends, with triple spines at the base.
X. spinòsum, L. (Spiny Clotbur.) Hoary-pubescent; stems slender, with slender yellow 3-parted spines at the axils; leaves lanceolate or ovate-lanceolate, tapering to a short petiole, white-downy beneath, often 2–3-lobed or cut; fruit ({1/3}´ long) pointed with a single short beak.—Waste places on the sea-board and along rivers, Mass. and southward. (Nat. from Trop. Amer.)
[*][*] Leaves cordate or ovate, 3-nerved, dentate and often lobed, long-petiolate; axils unarmed; fruit 2-beaked.
X. strumàrium, L. Low (1–2° high); fruit 6–8´´ long, glabrous or puberulent, with usually straight beaks and rather slender spines.—A weed of barnyards, etc., sparingly nat. from Eu. (?) or Ind. (?).