19(18). Sepals about twice as long as the petals; petals 2-3 mm. long; head of 



achenes cylindroid, 11-17 mm. long; stems never rooting 



5. R. pensylvanicus. 



19. Sepals equal to or somewhat shorter than the petals; petals (3-) 5-10 mm. 



long; head of achenes ovoid, 7-12 mm. long 6. R. Macounii. 



20(18). Petals (when fully expanded) usually 8-18 mm. long, large and con- 

 spicuous, 1.5 to 3 times as long as the sepals (21 ) 



20. Petals minute, 2-4 or rarely 6-7 mm. long, shorter than or slightly exceeding 



the sepals (22) 



21(20). Stems rooting at the nodes, some of them usually stoloniferous; recep- 

 tacle usually hispid 1. R. repens. 



21. Stems never rooting; receptacle glabrous 2 R. acris. 



22(20). Receptacle glabrous; nectary scale not forming a pocket, free laterally 

 for its entire length 3. R. uncinatus. 



22. Receptacle hispid; nectary scale forming a pocket at least basally, the distal 



margin sometimes proliferating into a flap with free margins 



4. R. recurvatus. 



23(17). Achenes mostly 40 to 130, rarely fewer; sepals usually 8-10 mm. long, 

 rarely less; petals 8 to 18 9. R. macranthus. 



23. Achenes 10 to 30, rarely more; sepals 6-8 mm. long; petals 5 (24) 



24(23). Stems rooting at the nodes (especially when plants are in wettish soils); 

 plants usually stoloniferous 7. R. carolinianus. 



24. Stems never rooting; roots both tuberous and filiform 8. R. fascicularis. 



25(16). Head of 70 to 150 achenes globose, 10-20 mm. in diameter; nectary 

 scale usually distally ciliate; herbage glabrous; roots large and 

 fleshy, 2-3 mm. thick, the cluster conspicuous and dense; stems 

 prostrate or ascending 19. jR. ,qlaberrimus. 



25. Head of achenes cylindroid to ovoid or globose, if globose 2.5-7 mm. in 



diameter, the horizontal diameter in any case never more than 9 

 mm. (26) 



26(25). Sepals covered dorsally with dense conspicuous long reddish-brown hair 

 13. R. Macauleyi. 



26. Sepals not covered dorsally with reddish-brown hair (27) 



27(26). Petals 1-3.5 mm. long, shorter than or scarcely equal to the sepals; 

 achenes glabrous 18. R. abortivus. 



27. Petals (when fully expanded) 4-18 mm. long or in plants with canescent 



achenes {R. inamoenus) rarely shorter (28) 



28(27). Achenes swollen or broadened at the bases and therefore obovoid-oblong 

 14. R. EschscholtzH. 



28. Achenes obovoid to flattened-obovoid or discoid, not swollen nor flat-stiped 



at the base (29) 



29(28). Petals 2.5-8 mm. long; achenes glabrous; stems mostly 1- to 4-flowered 

 (rarely 6) 15. R. inamoenus. 



29. Petals (6-) 8-18 mm. long or (in plants with canescent achenes or with 



5- to II -flowered stems) sometimes shorter (very rarely none in 

 R. pedatifidus) (30) 



30(29). Achenes glabrous 17. R. pedatifidus. 



30. Achenes canescent (31) 



31(30). Nectary scale ciliate, the adjacent surface of the petal sometimes hairy 

 also 16. R. cardiophyllus. 



31. Nectary scale and the petal glabrous (32) 



931 



