6(5). Bristles straight or slightly curved or none (7) 

 6. Bristles strongly curved (9) 



7(6). Inflorescence usually 2 or more times compound, the clusters loose; achene 

 lenticular or plano-convex; style bifid 5. S. microcarpus. 



I. Inflorescence usually only once compound, the clusters of spikelets commonly 



dense; achene obtusely trigonous; style trifid (8). 



8(7). Rhizomatous; bracts as long as or exceeding the inflorescence; each spike- 

 let with 20 to 40 florets 6. 5. atrovirens. 



8. Not distinctly rhizomatous; bracts shorter than the inflorescence; each spikelet 



with 70 to 200 florets 7. S. georgianus. 



9(6). Principal leaves 3-8 mm. wide; sessile spikelets usually glomerate; curling 

 bristles mostly much longer than the achene 8. S. Uneatus. 



9. Principal leaves 8-12 mm. wide, sessile spikelets usually solitary; curling 



bristles rarely exceeding tne achene 9. S. fontinalis. 



10(1). Achene 0.8-0.9 mm. long 10. S. molestus. 



10. Achene 1.3-4 mm. long (11) 



11(10). Achene 1.3-1.5 mm. long; culms 0.3-1.8 mm. thick; tufted annuals (12) 



II. Achene 1.5-4 mm. long; culms usually thicker; rhizomatous perennials (13) 



12(11). Achene with vertical rows of minute pits 11. 5. koilolepis. 



12. Achene with horizontal ridges 12. S. supinus. 



13(11). Culms 3-20 dm. long, often sharply triquetrous, 2-8 mm. thick, often 

 arcuate (14) 



13. Culms 10-30 dm. long, either terete or only obscurely trigonous, 8-23 mm. 



thick near the base, 2-4 mm. thick just beneath the inflorescence, 

 usually rigidly erect (15) 



14(13). Achene 2.5-3 mm. long; lower scales or the spikelets often much longer 

 than the rest and with strong venation, bracteolelike; inflorescence 



a solitary spikelet or glomerule of 2 to 4 spikelets 



13. 5. americanus. 



14. Achene 1.8-2.6 mm. long; lower scales of the spikelets not differentiated; 



inflorescence a dense glomerule of 5 to 15 spikelets 



14. S. Olneyi. 



15(13). Achene bristles 2 to 4 (16) 



15. Achene bristles 4 to 6 (17) 



16(15). Sheaths (near base of culm) at margins rather regularly retrorsely 



fimbriate-filiferous; bristles ciliate or plumose, not barbed 



15. S. californicus. 



16. Sheaths smooth or merely lacerate; bristles fragile, barbellate or smooth 



16. 5. heterochaetus. 



17(15). Culm obscurely trigonous or flattened; at least the upper sheath with 



a well-developed blade; achene usually more than 3 mm. long 



17. S. etuherculatiis. 



17. Culm terete; sheaths without blades or a much-reduced blade only; achene 



less than 3 mm. long (18) 



18(17). Scales about 5 mm. long, thin-membranous, pale-brown and with con- 

 spicuous elongate reddish glutinous spots (seen under a lens), the 

 distal margin lacerate; achene 1.8-2.9 mm. long 18. S. aciitus. 



18. Scale 3-4 mm. long, firm-membranous, dark-brown, nearly smooth (occa- 



sionally with a few reddish gummy spots near the midveins), the 

 distal margin nearly smooth to slightly lacerate; achene 1.5-2.2 

 mm. long 19. S. validus. 



346 



