11(9). Culms 1.5-9 mm. thick; spikelets of 40 to 350 flowers; achenes bicon- 

 vex (12) 



11. Culms 0.1-1.4 mm. thick; spikelets of 5 to 80 flowers; achenes either bicon- 



vex or trigonous (16) 



12(11). Culms with complete septa (as revealed by dissection) (13) 



12. Culms not septate or irregularly and incompletely septate (15) 



13(12). Culms 1.5-3.5 mm. thick; septa 2-5 mm. apart; tubercle depressed, 

 0.1-0.2 mm. high, in outline confluent with the body; body of 

 achene 0.9-1.1 mm. long 11. E. montatia. 



13. Culms 4-9 mm. thick; septa mostly farther apart; tubercle conic, 1-1.2 mm. 



long; body of achene 1.8-2.2 mm. long (14) 



14(13). Septa very crowded just below the spikelet 12. E. interstincta. 



14. Septa not very crowded just below the spikelet 13. E. equisetoides. 



15(12). Body of achene about 2 mm. long, the surface cellular, appearing as if 

 embedded in plastic; scales obtuse; spikelet 19-36 mm. long, cylin- 

 dric 14. E. cellulosa. 



15. Body of achene 1.2-1.8 mm. long, surface smooth or micropunctate; scales 



usually acute; spikelets 8-25 mm. long 15. E. macrostachya. 



16(11). Achenes biconvex, lustrous, black when mature, the bodies 0.5-1 mm. 

 long; tubercles 0.05-0.2 mm. long; style branches 2(17) 



16. Achenes trigonous or if obscurely so then not black when mature (19) 



17(16). Perennial usually with slender rhizomes; flowers 15 to 25 per spikelet; 

 scales obviously keeled 16. E. flavescens. 



17. Annuals, densely tufted; flowers 28 to 80 per spikelet; scales not or incon- 



spicuously keeled (18) 



18(17). Body of achene 0.7-1 mm. long; tubercle 0.1-0.2 mm. long; culms 

 0.4-1 mm. thick 17. E. caribaea. 



18. Body of achene 0.5-0.6 mm. long; tubercle 0.05 mm. long; culms 0.2-0.3 



mm. thick 18. E. atropurpurea. 



19(16). Body of achene rather sharply trigonous, broadest near the middle, 

 apically confluent in outline with the tubercle which is pyramidal 

 and 0.1-0.2 mm. long; rhizomatous mat-formers; culms 2-12 cm. 

 long, 0.1-0.4 mm. thick 19. E. parvula. 



19. Body of achene either not sharply trigonous or (if so) not confluent with the 



tubercle, or else plants otherwise habitally (20) 



20(19). Tubercle columnar or slightly tapered, blunt, 0.6-1 mm. long, con- 

 fluent with the body of the achene (21) 



20. Tubercle shorter or if as much as 0.6-1 mm. long then constricted basally, 



not confluent (22) 



21(20). Culms usually less than 1 mm. wide, not flattened; spikelets 4-7 mm. 

 long, mostly of 2 to 7 flowers; achene reticulate 20. E. pauciflora. 



21. Culms usually over 1 mm. wide, flattened; spikelets 8-17 mm. long, mostly of 



12 to 30 flowers; achene smooth 21. E. rostellata. 



22(20). Tubercle 0.6-1 mm. long, high-pyramidal, basally truncate; body of 

 achene 1.2-1.7 mm. long, surficially cellular (the cells with promi- 

 nent margins) and olivaceous brown to olivaceous gray when ma- 

 ture 22. E. tortilis. 



22. Tubercle 0.05-0.7 mm. long, mostly pyramidal, low to depressed-pyramidal 



or globose; body of achene 0.5-1.8 mm. long, surficially smooth to 

 warty or punctate but not cancellate, variously colored (23) 



367 



