16(15). Horizontal rows of cells counted in vertical series near the middle of 

 one face of the achenial body numbering (14 to) 16 to 30 and 

 all the cells nearly isodiametric, the horizontal cell walls only 

 slightly if at all raised more than the vertical walls; achenes turgid, 

 1 mm. or more thick from the middle of one face to the middle 

 of the other (17) 



16. Horizontal rows of cells usually 11 to 15; (except in R. mixta) those cells 



near the middle of the face greatly vertically elongated and their 

 horizontal walls very prominent and forming horizontal wrinkles on 

 the achene, but those cells near the base and top of the body 

 nearly isodiametric; achenes less turgid, less than 1 mm. thick from 

 face to face (18) 



17(16). Achenial body 2-2.4 mm. long, 1.5-1.8 mm. thick from face to face; 

 horizontal rows of cells 25 to 30 or more; spikelets 4-5.5 mm. 

 long 15. R. Grayi. 



17. Achenial body 1.5-1.8 mm. long, 1-1.5 mm. thick from face to face; hori- 



zontal rows of cells (14 to) 16 to 20; spikelets 2.5-3 mm. long 



16. R. Harveyi. 



18(16). Achenial body twice as long as broad 18. R. inexpansa. 



18. Achenial body less than twice as long as broad (19) 



19(18). Bristles (most of them) surpassing the tubercle (20) 



19. Bristles not surpassing the tubercle (22) 



20(19). Achenial body 1.2-1.3 mm. long, the 2 faces nearly flat; plants never 

 rhizomatous; tubercle 0.3-0.4 mm. long 20. R. Elliottii. 



20. Achenial body 1.3-1.7 mm. long, the 2 faces distinctly convex at least in the 



upper part; plants rhizomatous (this often obscure in exsiccatae); 

 tubercle 0.4-0.9 mm. long (21) 



21(20). Achenial body only 0.8-0.9 mm. broad, with numerous indistinct trans- 

 verse lines 19. R. mixtra. 



21. Achenial body 1.2-1.6 mm. broad, with strong transverse wrinkles 



23. R. caduca. 



22(19). Primary branches of the terminal part of the inflorescence straight, 

 stiffly ascending; faces of the achenial body definitely convex at 

 least in the upper half 24. R. globularis. 



22. Primary branches of the terminal part of the inflorescence arcuate, often 



slightly droopy; faces of the achenial body nearly flat or only very 

 slightly convex (23) 



23(22). Bristles half as long as to nearly as long as the achenial body; Wades 

 usually flat basally 21. R. microcarpa. 



23. Bristles less than half as long as the achenial body; blades usually nearly all 



strongly involute 22. R. perplexa. 



1. Rhynchospora macrostachya Torr. Horned-rush. Fig. 243. 



Tufted perennial; culms 5-10 dm. long, erect, 3-7 mm. thick basally, trique- 

 trous, leafy; basal sheaths becoming markedly fibrous; inflorescence (at maturity) 

 clavate, 3-6 cm. thick, the branches numerous but rather short, erect; spikelets 

 subulate, rich-brown, 15-23 mm. long, the distal third consisting merely of the 

 protruding distal part of the tubercle; bristles; several (4 to 7), about 1.5 to 2.5 

 times as long as the achenial body, stiff, brownish, antrorsely barbed; achenial 

 body 4-5 mm. long, obovate, nearly flat on the 2 faces, dark-brown; tubercle 

 subulate, consisting of the indurated entire portion of the style, 13-18 mm. long, 

 pale-brownish to buffy. R. corniculata var. macrostachya (Torr.) Britt. 



471 



