16(15). Leaves small or nearly absent, often on some branchlets transformed 

 into green thorns; plants forming colonies from extensively creep- 

 ing rhizomes 1- A. spinosus. 



16. Leaves well-developed at least in the lower part of the plants; rhizomes (if 



present) not extensively creeping 3. A. subulatus. 



17(15). Lobes of the disk corollas comprising 45 to 75 per cent of the limb; 

 leaves usually pubescent beneath (at least along the midrib) (18) 



17. Lobes of the disk corollas comprising 17 to 45 per cent of the limb; leaves 



typically glabrous beneath (19) 



18(17). Plants with well-developed creeping rhizomes; leaves usually pubescent 

 over the lower surface 13. A. ontarionis. 



18. Plants without well-developed creeping rhizomes; leaves glabrous beneath 



except usually along the midrib 14. A. lateriflorus. 



19(17). Heads very small, with involucre 2.5-3.5 (-4) mm. high and rays 3-6 

 mm. long, numerous, often unilaterally racemiform (20) 



19. Heads larger than above or few, seldom unilaterally racemiform (21) 



20(19). Rameal leaves much-reduced, seldom with any of them as much as 1.5 

 cm. long 15. A. vimineus. 



20. Rameal leaves much larger than above, usually with many of them 1.5 cm. 



long or more 16. A. simplex. 



21(19). Veinlets of the leaf forming a conspicuous reticulum, the areolae mostly 

 nearly isodiametric; lobes of the disk corollas comprising 17 to 



25 per cent of the limb; creeping rhizomes well-developed 



17. A. praealtus. 



21. Veinlets of the leaf forming an obscure reticulm or (if the reticulum is 



evident) the areolae then clearly longer than broad 



18. A. dumosus. 



22(13). Phyllaries with the midrib greenish but this greenish area not expanded 

 toward the apex or at the broadest part not more than twice as wide 

 as at the narrowest part, usually appearing linear, the marginal 

 areas pale and chaffy in texture; leaves (when present) usually 

 subulate, tapered all the way from base to apex, rarely with a few 

 serrations, mostly entire and glabrous; head-bearing branchlets 

 nearly naked (23) 



22. Phyllaries usually with a green splotch toward the apex formed by the 



lateral expansion of the greenish area near the midrib; leaves 

 variously ovate to lanceolate or linear but usually serrate, rarely 

 subulate; head-bearing branchlets usually leafy (25) 



23(22). Leaves small or nearly wanting, often on some branchlets transformed 

 into green thorns; plants forming colonies from extensively creeping 

 rhizomes 1. A. spinosus. 



23. Leaves well-developed at least in the lower part of the plant; rhizomes (if 



present) not extensively creeping (24) 



24(23). Salt marsh and brackish mud perennial 3-6 dm. tall; heads numbering 

 only 3 to 30 per plant or per shoot, broadly turbinate; involucres 

 11-17 mm. broad at top 2. A. temdfolius. 



24. Annual forming colonies in poorly drained areas, (2-) 4-20 dm. tall; heads 



numbering up to several hundred per plant, turbinate; involucres 

 7-11 mm. broad at top 3. A. subulatus. 



25(22). Leaves of midstem linear, 10-25 (-34) mm. long, 1-2 mm. wide; leaves 

 of the head-bearing branchlets numerous, linear-subulate, ascending 

 18. /4. dumosus. 



25. Leaves of midstem usually averaging longer than 20 mm. and always broader 



than 2 mm. (26) 



1617 



