composite. 177 



Heads of flowers large, few, arranged in loose panicles, the rays much longer than 

 the involucre, bright yellow; disk dull yellow. 



23. SILPHIUM, Linn. Rosin-plant. 



&t. silphion, the ancient name of a medicinal plant of Africa, transfered to the ge- 

 nus by Linnajus. 



Heads many-flowered. Ray-flowers numerous, ligu- 

 late, pistillate and fertile, their broad flat ovaries imbricated 

 in 2 to 3 rows; disk-flowers perfect but sterile, tube short. 

 Receptacle somewhat convex, chaffy. Involucre bell- 

 shaped, with imbricated scales in several rows, the outer with 

 loose leaf-like summits. Achenia broad and flattened, sur- 

 rounded by a ring which is notched at the top, destitute of 

 pappus, or with 2 teeth confluent with the winged margin ; 

 those of the disk abortive, with an obsolete crown-like pap- 

 pus. — Coarse and tall rough perennial herbs, with a copious 

 resinous juice, and large heads of yellow flowers in corymbose pan-* 

 icles. 

 *S!em round or slightly Wangled, leafy: leaves undivided, 



1. S. trifoliatum, L. Three-leaved Rosin Plant'" 



Stem tall and rather slender, smooth, often glaucous ; stem leaves lanceolate* 

 pointed, entire or scarcely serrate, rough, on very short petioles, in whorls of 3 or 

 •4; upper lea ves opposite; heads loosely panicled ; involucre smooth; scales broadly 

 ovate, rather obtuse; achenia broadly oval, sharply 2-toothed at the top. 



Dry woods and plains. Aug. Stem 5 to ,6 feet high, slightly angled, purplish- 

 Leaves thick, 3 to 5 inches long. Heads loosely cymose on rather long peduncles- 

 Bays 12 to 16, expanding about 2% inches, bright yellow. 



* * Stem square; leaves opposite, connate. 



2. S. perfoliatum, L. Cup Plant. 



Stem stout, square; leaves ovate, coarsely toothed, connate-perfoliate, narrowed 

 towards the base; heads in a trichotomous cyme, the central on a long peduncle. 



Rich soil along streams. Aug. Stem 4 to 6 feet high, often branched above* 

 Leaves 6 to 16 inches long, the upper united by their bases and forming a cup" 

 shaped disk, the lower narrowed into winged petioles, which are connate by their 

 bases. Heads large, with 15 to 25 yellow rays. Achenia winged and variously 

 notched. 



24. AMBROSIA, Toum. Rag-weed. 



Gr. ambrosia, the food of the gods, a term strangely applied. 



Heads monoecious ; the fertile at the base and the sterile 

 at the top of the spike. Sterile involucre flattish or 

 top-shaped, composed of 7 to 12 scales united into a cup, 

 containing 5 to 20 funnel-form staminate flowers. Fertile 

 involucre oblong or top-shaped, closed, pointed, and usu- 

 ally with 4 to 8 horns near the top in one row inclosing a 

 single pistillate flower. Achenia ovoid. — Herbaceous chief- 



