COMrOSITiE. (C0Mr0SITE FAMILY.) 211 



28. PABTHMIUM, L. Parthenium. 



Heads many-flowered, inconspicuously radiate ; the 5 ray-flowers with very 

 short and broad obcordate ligules not projecting beyond the woolly disk, pistil- 

 late and fertile ; the disk-flowers staminate with imperfect stj les, sterile. Invo- 

 lucie hemispherical, of 2 ranks of short ovate or roundish scales. Receptacle 

 conical, chaffy. Acheuia only in the ray, obcompressed, surrounded by a slen- 

 der callous margin, crowned with the persistent ray-corolla and a pappus of 2 

 small chaffy scales. — Leaves alternate. Heads small, corymbedj the flowers 

 whitish. (An ancient name of some plant, from Trapfftvos, vir,/in.) 



1. P. illtcgrifolium, L. Rough-pubescent (l°-3° high) j leaves ob- 

 long or ovate, crenate-toothed, or the lower (3' -6' long) cut-lobcd below tho 

 middle ; heads many, in a dense flat corymb. y. — Dry soil, Maryland to Wis- 

 consin, and southward. 



29. IV A, L. Marsh Elder. Highwater-shrub. 



Heads several-flowered, not radiate; the pistillate fertile and the staminato 

 sterile flowers in the same heads, the former few (1-5) and marginal, with a 

 small tabular corolla ; the latter with a funnel-form 5-toothed corolla. Scales 

 of the involucre few, roundish. Receptacle small, with narrow chaff among 

 the flowers. Achenia obovoid or lenticular. Pappus none. — Herbaceous or 

 shrubby coarse plants, with thickish leaves, the lower opposite, and small 

 greenish-white heads on short recurved peduncles in the axils of the leaves or 

 of bracts. (Derivation unknown.) 



1. I. frilteSCC'tlS, 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 arc disposed, forming 

 leafy panicled racemes; fertile flowers and scales of the involucre 5. — Salt 

 marshes, coast of Massachusetts to Virginia, and southward. Aug. 



2. I. Cilia tu, Willd. Annual (2° -8° high), rough and hairy ; leaves ovate, 



pointed, coarsely toothed, downy beneath, on slender ciliute petioles ; heads in dense 

 panicled spikes, with conspicuous ovate-lanceolate rough-ciliatc bracts ; scales 

 of the involucre and fertile flowers 3-5. — Moist ground, from Illinois south- 

 ward. Aug. - Oct. 



30. AMBROSIA, 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, 

 composed of 7-12 scales united into a cup, containing 5-20 funnel-form stami- 

 nate flowers ; with slender chaff intermixed, or none. Fertile involucre (fruit) 

 oblong or top-shaped, closed, pointed, and usually with 4-S tubercles or horns 

 near the top in one row, enclosing a single flower which is composed of a pistil 

 only ; the elongated branches of the style protruding. Achenia ovoid : pappus 

 none. — Chiefly annual coarse weeds, with opposite or alternate lobed or dis 



