234 American Midland Naturalist Monograph No. 7 



14. Bracts scarious. 



15. Leaves mostly basal, spatulate or obovate, the stem-leaves 

 small ; plants perennial, stoloniferous, dioecious or 

 polygamous 22. Antennaria 



15. Leaves all or mostly cauline; plants annual or biennial, 



not stoloniferous or dioecious; all the flowers fertile, 

 the central ones perfect, surrounded by pistillate ones 



23. Gnaphalium 



14. Bracts not scarious. 



16. Bracts with tips hooked; coarse biennial weeds with large 



ovate leaves; heads globose 52. Arctium 



16. Bracts not hooked. 



17. Principal bracts in only one series (often with a few 

 small bractlets at the base of the head ) . 

 18. Pappus scabrous; flowers all perfect; plants 



perennial 49. Cacalia 



18. Pappus smooth; marginal flowers pistillate, the 



disk-flowers perfect; plants annual, with strong 



odor 50. Erechtites 



17. Bracts in more than one series. 



19. Leaves all or mostly opposite or whorled (or some 



of the upper ones alternate) 7. Eupatorium 



19. Leaves alternate. 



20. Bracts striate, imbricated in 3 or more equal 

 series; pappus plumose; achenes striate, 

 nearly terete; leaves minutely resinous-dotted 



9. Kuhnia 



20. Bracts not striate, in 1-3 series; achenes flat- 

 tened; pappus merely scabrous; leaves not 

 resinous-dotted. 

 21. Heads racemose; outer bracts foliaceous; 

 pappus copious, of soft bristles; achenes 



terete species of 19. Aster 



21. Heads paniculate; bracts all narrow, not 

 foliaceous; pappus-bristles short, brittle; 



achenes compressed 



species of 20. Erigeron 



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



22. Stems twining; leaves opposite, triangular-hastate; flowers pink 



8. Mikania 



22. Stems not twining. 



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



or white 7. Eupatorium 



23. Leaves alternate or basal. 

 24. Leaves prickly. 



25. Heads 1-flowered, in capitate clusters 53. Echinops 



25. Heads many-flowered, distinct. 



26. Pappus-bristles plumose 54. Cirsium 



26. Pappus-bristles not plumose. 



27. Receptacle bearing numerous bristles 



55. Carduus 



27. Receptacle without bristles 56. Onopordum 



24. Leaves not prickly. 



