23(22). Spikelets of 5 to 20 flowers; culms 0.1-0.3 mm. thick; achenes rather 

 sharply trigonous, mostly whitish or maturing to shades of olive, 

 surficially smooth; sheaths long-oblique apically (24) 



23. Spikelets of 20 to 110 flowers; culms 0.2-1.4 mm. thick; achenes mostly not 



so sharply trigonous (except in E. tenuis, E. cylindrica and E. 

 austrotexana), maturing (except in E. tenuis and E. elongata) through 

 shades of yellow to golden-brown or brown; sheaths truncate or 

 only very slightly oblique apically (26) 



24. Spikelets ovoid, 2-5 mm. long; scales rarely appearing distichous (except in 



lance-elliptic, 3-4.5 mm. long, buffy to ferruginous-buflf 



23. E. Baldwinii. 



24. Spikelets ovoid, 2-5 mm. long; scales rarely appearing distichous (except in 



E. minimal), ovate, shorter, usually whitish to purplish (25) 



25(24). Culms about 0.1 mm. thick, strongly recurved, 3-7 (-10) cm. long; 

 spikelets of 5 to 10 flowers; body of achene 0.7-0.8 mm. long, 

 maturing through whitish or olive to olive-gray; tubercle pyramidal, 

 0.15-0.3 mm. high 24. E. minima. 



25. Culms 0.1-0.3 mm. thick, mostly erect, 4-28 cm. long; spikelets of 8 to 15 



flowers; body of achene 0.5-0.6 mm. long, pearly white; tubercle 

 depressed-pyramidal, 0.05-0.15 mm. high 25. E. microcarpa. 



26(23). Scales broadly ovate, subcartilaginous medially, firm-membranous mar- 

 ginally, stramineous in color, somewhat lustrous; body of achene 

 ripening through shades of olive-whitish to brownish-olive and 

 finally to a rich dark-chocolate-brown; bristles conspicuous, reddish- 

 brown at maturity 26. E. albida. 



26. Scales mostly thinner, membranous and usually with some dark pigmenta- 



tion (27) 



27(26). Body of achene light-green, with about 12 rows of coarse transversely 

 linear ceHs; bristles 6 or 7, equaling the achene, greenish; culms 

 often floating on the surface of water 27. E. elongata. 



11. Body of achene (except in E. tenuis) ripening through shades of yellow to 

 golden-brown; bristles various but not as above (28) 



28(27). Body of achene minutely but pronouncedly warty or pitted in vertical 

 lines, 0.6-0.8 mm. long, ripening through shades of ivory to 

 greenish-olive; tubercle pronouncedly depressed, 0.1-0.2 mm. high, 



not much-constricted basally; culms 0.2-0.3 mm. thick 



28. E. tenuis. 



28. Body of achene minutely punctate-reticulate to smooth but not warty, 0.6-1.8 



mm. long, ripening through shades of yellow to brown; tubercle 

 usually conic-globular, usually constricted basally; culms 0.4-1.4 

 mm. thick (29) 



29(28). Scales of spikelet apically rounded, hyaline only in a very narrow 

 rounded border at the apex (30) 



29. Scales of spikelet deltoid or ovate-acute to lance-acuminate, with a more or 



less acute more or less extensive hyaline apex (32) 



30(29). Spikelet lincar-cylindric, acute; body of achene distinctly trigonous, 

 smooth and satiny 29. E. cylindrica. 



30. Spikelet linear-lanceolate or narrowly ovoid to oblong and apically blunt: 



body of achene obscurely trigonous (31) 



31(30). Spikelets linear to narrowly lanceolate, acuminate; achenes smooth or 

 finely pitted; scales acute or acutish 30. E. Parishii. 



31. Spikelets ovoid to oblong, usually apically blunt; achenes punctulate-reticulate 



to nearly smooth; scales mostly obtuse 31. E. montevidensis. 



368 



