244 American Midland Naturalist Monograph No. 2 



19. Heads paniculate; bracts all narrow, not foliaceous; 



pappus-bristles short, brittle; achenes compressed 



species of 15. Erigeron 



9. Flowers pink, purple, blue, or yellow (rarely white). 



20. Stems twining; leaves opposite, triangular-hastate; flowers pink 7. Mil^ama 



20. Stems not twining. 



21. Leaves opposite or whorled, not prickly; flowers purple, blue, or white.... 



6. Eupalorium 



21. Leaves alternate or basal. 

 22. Leaves prickly. 



23. Heads 1 -flowered, in capitate clusters 49. Echinops 



23. Heads many-flowered, distinct. 



24. Pappus bristles plumose 50. Cirsium 



24. Pappus bristles not plumose 51. Carduus 



22. Leaves not prickly. 



25. Bracts of the involucre pectinate, or tipped with a rigid spine 



51 . Centaiirea 



25. Bracts neither pectinate nor with a rigid spine. 



26. Bracts with hooked tips; coarse biennial weeds with large ovate 

 chiefly basal leaves; heads globose; flowers purple (rarely 



white) ; receptacle bristly 48. Arclium 



26. Bracts not hooked; receptacle not bristly. 



27. Flowers yellow; bracts in 1 series 47. Senecio 



27. Flowers not yellow; bracts imbricated in 2-several series. 

 28. Pappus bristles plumose or barbellate; heads in long 



racemes or spikes; leaves narrow, entire, rigid 



9. Lialris 



28. Pappus bristles not plumose. 



29. Pappus (in our species) double, the outer bristles 

 short; heads many-flowered, in corymbose cymes; 



bracts imbricated in several series 4. Vernonia 



29. Pappus bristles approximately the same length, not in 

 two series. 

 30. Heads 2-5-nowered, aggregated into dense clus- 

 ters subtended by foliaceous bracts; flowers all 



perfect and alike; bracts 8 5. Elephanlopus 



30. Heads many-flowered, corymbose; flowers of 2 

 kinds in the same head; bracts imbricated; 

 plants camphor-scented 16. Pluchea 



1. Pappus not of capillary bristles, either of rigid awns, small chaffy scales, or reduced 

 to a mere crown, or entirely lacking. 

 3L Heads radiate. 



32. Rays yellow. 



33. Pappus of 2-6 awns or bristles. 



34. Leaves all or mostly opposite or whorled; bracts not hooked. 



35. Terrestrial or marsh plants with one kind of leaves; achenes flat or 



angled 33. Bidens 



35. Partly submerged aquatics with the submerged leaves finely dis- 

 sected; achenes terete 34. McgaloJonta 



34. Leaves alternate; bracts often gummy, with recurved or hooked ti[)s.-.. 



10. CrinJelia 



33. Pappus none, or of few short teeth or scales. 

 36. Leaves all or mostly opposite or whorled. 



37. Achenes thick, not at all, or scarcely flattened. 



38. Leaves thin, deeply angulate-lobed or lyrate-pinnatifid ; rays 



small or none 20. Polymnia 



38. Leaves thick, entire or serrate; rays conspicuous. 



39. Bracts obtuse; ray-flowers pistillate, fertile, papery and per- 

 sistent on the achene 23. Heliopsis 



