COMPOSITE. 249 



sterile flowers in long, naked racemes ; fertile flowers sessile below, each with 

 a 6-ribbed involucre, terminating in 6 tubercles. A tall, rank, herbaceous 

 plant. In low grounds. August. 



2. A. ARTEMISLEFOLIA (Roman Wormwood). Stem erect, slender, 

 branching, more or less hairy ; leaves bipinnatifid, nearly smooth above ; 

 lower ones opposite, upper ones alternate ; sterile racemes naked, terminal, 

 loosely panicled ; fertile flowers sessile in the axils of the upper leaves. 

 Homely weed, 2-4 feet high. August-September. 



21. Iva. Heads discoid ; marginal flowers 1-5, pistillate, with a 

 tubular corolla. Involucre-scales few, mostly in one row. Receptacle 

 hairy. Achenia obovoid, obtuse. Pappus none. y. 



I. FRUTESCENS (Marsh Elder. Highwater Shrub). Nearly or quite 

 smooth ; stem shrubby, with opposite branches ; leaves oval or lanceolate, 

 coarsely serrate, with 3 prominent veins, petiolate, the lower ones opposite, 

 the upper alternate, narrow ; heads greenish-white, on short, recurved pedi- 

 cels, in long, axillary racemes, arranged in a long, leafy, terminal panicle. 

 Along the sea-shore, 3-6 feet high. August. 



22. Centaurea. Heads many-flowered. Flowers all tubular ; 

 marginal ones mostly enlarged and sterile, resembling ray-flowers. 

 Involucre imbricate. Receptacle bristly. 



C. CYANUS (Bachelor's Button). Stem erect, branching, downy ; leaves 

 linear, sessile, downy ; heads solitary, ovoid ; sterile flowers longer than the 

 disk ; involucre-scales fringed. A garden annual. July. 



23. Cirsium. Heads discoid, many-flowered. Flowers all perfect. 

 Involucre-scales imbricated in many rows. Receptacle bristly. Pap- 

 pus of capillary, plumose bristles in a ring. Achenia oblong, com- 

 pressed, smooth. 



1. C. ARVENSE (Canada Thistle). Root creeping; stem erect, branch- 

 ing; leaves pinnatifid, with spiny teeth, smooth or somewhat downy beneath ; 

 heads small, light purple, numerous, in a terminal, loose panicle ; involucre- 

 scales closely appressed, tipped with minute spines. A weed, spreading rap- 

 idly by its creeping roots. July. 



2. C. LANCEOLATUM (Common Thistle). Stem very branching, leafy; 

 leaves deeply pinnatifid, decurrent, hispid above, white and woolly beneath ; 

 heads large, numerous, purple ; involucre-scales spreading, lanceolate, tipped 

 with long and formidable spines. July-September. 



3. C. PUMILUM (Pasture Thistle). Stem very stout, hairy, nearly or 

 quite simple ; leaves pinnatifid, clasping at base, green on both sides ; seg- 

 ments variously lobed and cut, ciliate, spinose ; heads very large, i'-3' in 

 diameter, fragrant, about 1-3 in number, purple, rarely whitish ; involucre- 

 scales spinous. August. 



24. Cichorium. Heads many-flowered. Involucre double ; outer 

 of 5 short, leafy scales ; the inner of 8-10. Receptacle chaffy. Pap- 

 pus short, chaffy. Achenia striate. y 



