11(10). Pedicels mostly deflexed or recurved; perianth 3-4 mm. long, the lobes 5 

 10. P. Douglasii. 



11. Pedicels mostly erect; perianth less than 3 mm. long, the lobes usually 6 



1 1. P. ramosissimum. 



12(1). Basal leaves well-developed; stem simple; rootstock often thickened and 

 bulblike (13) 



12. Basal leaves (if any) soon withering; stem often branched; rootstock not 



bulblike (14) 



13(12). Inflorescences usually less than 1 cm. wide, the lower flowers bearing 

 bulblets .-...12. P. viviparum. 



13. Inflorescences usually more than 1 cm. wide, with no bulblets 



13. P. bistortoides. 



14(12). Ocreae (sheath around the stem) with marginal cilia less than 1 mm. 

 long (15) 



14. Ocreae with marginal cilia 1.5 mm. long or more (21) 



15(14). Peduncles with numerous stalked glands (16) 



15. Peduncles without stalked glands (sessile glands may occur) (18) 



16(15). Styles and stamens of the same length (or nearly so), not exserted from 

 perianth; achene lenticular, flat or nearly so on both surfaces; 

 flowers white to pink; annual 14. P. pensylvanica. 



16. Styles or stamens exserted; achene lenticular and with at least one side con- 



vex or ridged (17) 



17(16). Inflorescences often solitary, all terminal or nearly so; flowers bright 

 pink; plants aquatic or semiaquatic; perianth in fruit more than 

 4 mm. long 15. P. amphibium. 



17. Inflorescences numerous, lateral and terminal; flowers white or light-pink; 



perianth in fruit usually less than 4 mm. long; stems usually cherry- 

 red at the nodes 16. P. bicorne. 



18(15). Inflorescences usually nodding; perianth with prominent anchor-shaped 



veins near apex; achenes lenticular, flat, about 2 mm. long 



17. P. lapathifolium. 



18. Inflorescences erect; perianth without anchor-shaped veins; achene biconvex, 



oval in cross section (19) 



19(18). Leaf blades less than 2 times as long as wide, long-petioled (usually 25 

 mm. long or more); mature ocreae short, about as wide as high; 

 petioles, stems and peduncles copiously pubescent with strigose 



hairs; perianth 3.3 mm. long or more, pink to pinkish-red 



18. P. orientate. 



19. Leaf blades more than 2.5 times as long as wide, short-petioled (less than 20 



mm. long); mature ocreae at least 1.5 times longer than broad; 

 petioles, stems and peduncles glabrous or pubescent; perianth 2-4.5 

 mm. long, white or pinkish-red (20) 



20(19). Peduncles and ocreolae strigose or stipitate-glandular; stamens or styles 

 strongly exserted from the flowers; perianth in fruit 4 mm. long or 

 more, pinkish-red; inflorescences 1 or 2, terminal.. 15. P. amphibium. 



20. Peduncles and ocreolae not strigose nor stipitate-glandular (glands, if present, 



sessile); stamens and styles mostly included; perianth in fruit less 

 than 3.5 mm. long, white or pinkish-white; inflorescences numerous, 

 terminal and lateral 19. P. densiflorum. 



21(14). Leaf blades less than twice as long as wide; petioles usually 25 mm. or 

 more long; mature ocreae short, about as wide as high; petioles, 

 stems and peduncles copiously pubescent with strigose hairs; 

 flowers pinkish-red 18. P. orientale. 



809 



