MONOPETALOUS EXOGENOUS PLANTS. 341 



SILPHIUM, L. 



Heads many-flowered ; the rays numerous, pistillate, fer- 

 tile, in a single row; disk-floiuers cylindrical, sterile; the 

 sti/le undivided; scales of the involucre leafy, imbricated 

 in several rows, the innermost smallest, chaff-like; recep- 

 tacle small, with linear acutish chaff; fertile achenia in 3-4 

 rows, round or obovate, flat, broadly winged, 2-toothed or 

 emarginate at the apex; the sterile ones slender; 'pappus 

 none, or represented by the two teeth of the achenia. — Tall 

 resinous herbs, with alternate opposite or whorled leaves, 

 and large heads of yellow flowers in corymbose panicles. 



* stems terete^ marly naked ; leaves alternate, the lowest large, sen^ate or variously 

 lobed, long-peiioltd, the others small and scattered. 



S. LACINIATUM, L. Stem hispid or smooth ; leaves very 

 rough or hispid, on clasping petioles, pinnately parted; 

 the divisions oblong or lanceolate, acute, lobed or toothed; 

 heads large, spicate or racemose ; scales of the involucre 

 ovate, tapering into a long and spreading point, ciliate ; 

 achenia round-obovate, emarginate. — Varies with the more 

 numerous sessile and clasping leaves less deeply parted. — 

 Stem 6°-8° high ; lowest leaves l°-2° long; heads If-S' in 

 diameter. 



S. TEREBINTHINACEUM, L. Stem smooth, naked above ; 

 leaves rough-hairy, undivided, cordate-oval or oblong, 

 coarsely serrate, on slender petioles ; heads small, loosely 

 panicled; scales of the involucre oval or obovate, obtuse, 

 smooth ; achenia obovate, emarginate or 2-toothed. Stem 

 4°-8° high; radical leaves 2° long; heads 1' wide. 



S. COMPOSITUM, Michx. Smooth; leaves cordate-ovate 

 or reniform, angularly toothed or variously lobed, long- 

 petioled ; heads small, corymbosely panicled ; scales of the 

 involucre obovate or oblong, obtuse ; achenia roundish, 

 deeply emarginate; rays 6-10. Stem 3°-6° high; leaves 

 G-12'; heads J' in diameter. 



