628 MISC. PUBLICATION 200, U. S. DEPT. OF AGRICULTURE 



Blades cordate, 1 to 3 cm. wide (5 to 12 mm. in P. ashei). Spikelets 

 pubescent. 

 Spikelets 2.5 to 3 mm. long. Sheaths glabrous or minutely puberulent. 



16. COMMUTATA. 



Spikelets 3 to 5 mm. long (sometimes but 2.7 mm. long in the hispid- 

 sheathed P. clandestinum) 17. Latifolia. 



Blades not cordate, less than 1 cm. wide. 



Sheaths crisp- or appressed-pubescent. Blades firm; spikelets pubescent. 



9. Columbiana. 



Sheaths glabrous or ciliate only in autumnal phase (sparsely pilose in 



P. curtifolium, the lower velvety in P. mattamuskeetense, rarely 



pilose in P. roanokense and P. caerulescens). 



Vernal culms delicate (sometimes scarcely so in P. albomarginatum 



and P. tenue); spikelets 1.5 mm. or less long (1.6 to 1.7 mm. in 



P. tenue) 11. Ensifolia. 



Vernal culms slender but not delicate, rarely less than 40 cm. tall; 



spikelets 2 to 2.9 mm. long (1.5 mm. in P. microcarpon and P. 



caerulescens). 



Lower internodes short, upper elongate, producing a nearly naked 



culm, leafy at base; spikelets narrowly ovate, 2.7 to 2.9 mm. 



long 5. Nudicatjlia. 



Lower internodes not shorter, the vernal culms about evenly leafy 

 throughout; spikelets elliptic or obovate, not more than 2.5 

 mm. long 6. Dichotoma. 



1. Depauperata 



Spikelets about 3.5 mm. long, beaked 5. P. depaupekattjm. 



Spikelets 3 mm. long or less (sometimes 3.2 mm. long in P. perJongum), not beaked. 



Culms single or few in a tuft; spikelets turgid, blunt, 2.7 to 3.2 mm. long, prairie plants. 



6. P. PERLONGUM. 



Culms in large tufts; spikelets not turgid, 2.2 to 2.7 mm. long; plants of woods. 



Sheaths pilose; spikelets 2.2 to 2.7 mm. long, pilose 7. P. linearifolium. 



Sheaths glabrous; spikelets 2.2 to 2.3 mm. long, glabrous or sparingly pilose. 



8. P. WERNERI. 



2. Laxiflora 



Sheaths retrorsely pilose; spikelets papillose-pilose. 



Blades ciliate and more or less pilose on the surface; spikelets 2 mm. long. 



10. P. XALAPENSE. 



Blades glabrous or nearly so on the surface and margin; spikelets 2.2 mm. long. 



9. P. LAXIFLORUM. 



Sheaths not retrorsely pilose; spikelets pubescent or glabrous. 



Spikelets pubescent, about 2 mm. long 11. P. ciliattjm. 



Spikelets glabrous. 



Blades glabrous on the surface 12. P. polycaulon. 



Blades pilose on the surface 13. P. strigosum. 



8. Angustifolia 



Nodes bearded; plants grayish- villous ; autumnal blades flat. 



Spikelets 2 mm. long 16. P. chrysopsidifolium. 



Spikelets 2.5 to 2.8 mm. long 17. P. consanguineum. 



Nodes not bearded; plants villous only at base, or nearly glabrous; autumnal blades involute 

 or flat. 

 Autumnal blades flat; lower panicle branches spreading or reflexed, or loosely ascending. 



Spikelets 2 mm. long; panicle branches loosely ascending 15. P. bennettense. 



Spikelets 2.5 to 2.8 mm. long; panicle branches widely spreading at anthesis. 



18. P. ANGUSTIFOLITJM. 



Autumnal blades involute; lower panicle branches more or less ascending. 

 Spikelets pointed beyond the fruit, fusiform. 



Spikelets 3.3 to 3.5 mm. long 20. P. fusiforme. 



Spikelets mostly 2.3 to 2.5 mm. long (or to 3 mm. before maturity). 



19. P. PINETORUM. 



Spikelets not pointed beyond the fruit; obovate. 

 Plants glabrous or nearly so. Autumnal culms erect. 



Spikelets subsecund along the suberect panicle branches.... 23. P. neuranthtjm, 



