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



9b. Sheaths pubescent. 



Pubescence ascending or appressed. 



Spikelets 3 to 3.2 mm. long; first glume conspicuously remote. 



65. P. MALACOX. 

 Spikelets 3.5 to 4 mm. long; first glume not remote.... 96. P. oligosaxthes. 

 Pubescence spreading, sometimes sparse. 



Plants robust, about 1 m. tall; blades usually 2 cm. or more wide. 



113. P. CLAXBESTIXUM. 



Plants rarely more than 50 cm. tall; blades rarely more than 1.5 cm. "wide. 

 Panicle about as wide as long; blades ascending or spreading. 



Spikelets attentuate at base, 3.5 to 4 mm. long.... See 14. Pedicellata. 

 Spikelets not attentuate at base, not more than 3.3 mm. long. 



Spikelets 3.2 to 3.3 mm. long; blades firm; sheaths, or some of them, 



more or less hispid 95. P. scribxeriaxum. 



Spikelets not more than 3 mm. long; blades rather thin; sheaths, or 



some of them, glabrous or sparselj' hispid 94. P. helleri. 



Panicle narrow, the branches erect (sometimes ascending in P. ivilcoxianum), 

 or spreading at anthesis only; blades erect. 

 Spikelets not more than 3 mm. long; blades not more than 6 mm. wide. 



92. P. WILCOXIAXUM. 

 Spikelets 3.7 to 4 mm. long; blades 8 to 20 mm. wide. 



Blades papillose-hispid 98. P. leibergii. 



Blades glabrous on both surfaces 99. P. xaxthophysum. 



6b. Spikelets less than 3 mm. long. 



10a. Blades elongate, not more than 5 mm. wide; secondary panicles at the base only 

 or wanting. 

 Culms single or few in a tuft; spikelets turgid, 2.7 to 3 mm. long. 



6. P. perloxgum. 

 Culms in large tufts; spikelets not turgid, not more than 2.7 mm. long. 



Sheaths pilose 7. P. lixearifolium. 



Sheaths glabrous 8. P. werxeri. 



10b. Blades usually not elongate; secondary panicles not at the base. 



11a. Spikelets attentuate at base, mostly prominently pustulose. Blades narrow, 

 stiff, strongly nerved, tapering from base to apex. 

 Xodes bearded; plants grayish-bilious; autumnal blades flat. 



Spikelets 2 mm. long 16. P. chrysopsidifolfcm. 



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



Xodes not bearded; plants villous only at the base, or nearly glabrous. 

 Autumnal blades flat. 



Spikelets 2 mm. long; panicle branches loosely ascending. 



15. P. BEXXETTEXSE. 



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



18. P. AXGUSTIFOLIUM. 



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



Spikelets pointed beyond the fruit, fusiform 19. P. pixetorlm. 



Spikelets blunt, obovate. 

 Plants glabrous or nearly so. 



Spikelets subsecund along the suberect panicle branches. 



23. P. XEURAXTHUM. 



Spikelets not subsecund; panicle loose and open 22. P. oyexcm. 



Plants pubescent, at least on the lower half. 



Spikelets about 2.4 mm. long; vernal blades 7 to 12 cm. long; autumnal 



blades not falcate 21. P. arexicoloides. 



Spikelets not more than 2 mm. long; vernal blades 4 to 6 cm. long; au- 

 tumnal blades falcate 1 14. P. aciculare. 



lib. Spikelets not attentuate at base. 



12a. Sheaths retrorsely pilose. Blades soft and lax. 



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



10. P. xalapexse. 

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



9. P. LAXIFLORFM. 



12b. Sheaths not retrorsely pilose. 



13a. Ligule manifest, mostly 2 to 5 mm. long, at least 1 mm. long. 

 Sheaths, or all but the lowest, glabrous; spikelets not more than 1.6 mm. long. 

 Panicle narrow, one-fourth to one-third as wide as long.... 40. P. spretoi. 

 Panicle open, nearly as wide as long. 



