304 CONTRIBUTIONS 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, 8 mm. long, 
52. P. virgatum, 
Spikelets glabrous; blade margins sharply serrulate. 
Spikelets eliiptic 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 8 to 10 mm. wide, flat or becoming involute in 
drying, e 
