304 CONTEIBUTIONS FROM THE NATIONAL HERBARIUM. 



Spikelets 2 to 2.2 mm. long ; plants glabrous. 



17. P. melanospermum. 

 Spikelets 2.4 to 3 mm. long ; plants more or less pubescent. 



18. P. convexum. 

 Plants perennial. 



Spikelets elliptic; sterile lemma not transversely rugose. 



16. P. wrightii. 

 Spikelets obovate to suborbicular ; sterile lemma undulate- 

 rugose just within the raised margin. 

 Blades flat, lax, broader at base than the summit, of the 



sheath; culms decumbent at base 15. P. olivaceum. 



Blades conduplicate at base, stiff; culms erect. 



14. P. plicatulum. 

 Fruit not dark brown and polished (brown but not polished in P. 

 virgatum ) . 

 Racemes numerous, usually 15 or more (5 to 10 in P. secans, 

 1 to 2 meters tall). 

 Spikelets about 1.3 mm. long, subhemispheric, pubescent. 



44. P. paniculatum. 

 Spikelets 1.8 mm. or more long. 

 Spikelets pubescent. 



Fruit pale ; spikelets elliptic, 2 mm. long. 



56. P. coryphaeum. 

 Fruit brown ; spikelets obovate, 3 mm. long. 



52. P. virgatum. 

 Spikelets glabrous ; blade margins sharply serrulate. 



Spikelets elliptic or narrowly obovate; rachis glabrous; 



lower sheaths not nodulose 53. P. secans. 



Spikelets suborbicular or broadly obovate ; rachis pilose, at 

 least sparsely so ; lower sheaths nodulose. 

 Rachis densely pilose ; spikelets 1.8 to 2 mm. long. 



55. P. densum. 

 Rachis sparsely pilose ; spikelets 2 to 2.2 mm. long. 



54. P. millegrana. 

 Racemes 1 to 5, rarely more. 

 Spikelets 2 mm. or more long. 

 Racemes subcylindrical, solitary (rarely 2) ; spikelets soli- 

 tary, mostly rugose. 



Blades pubescent, flat or subinvolute 24. P. nanum. 



Blades glabrous, concavo-convex in cross section. 



Spikelets 2 mm. long; glume and sterile lemma not 



pointed beyond the fruit 22. P. filiforme. 



Spikelets 2.7 to 3 mm. long; glume and sterile lemma 



pointed beyond the fruit 23. P. lindenianum. 



Racemes not cylindrical, one or more ; spikelets not rugose. 

 Blades not over 2.5 mm. wide, strongly involute ; spikelets 

 pubescent. 

 Blades glabrous, usually exceeding the racemes. 



25. P. alterniflorum. 

 Blades pilose, short, clustered toward the base. - 



26. P. rottboellioides. 

 Blades 3 *to 10 mm. wide, flat or becoming involute in 

 drying. 



