HITCHCOCK AND CHASE NORTH AMERICAN PANICUM. 147 



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



mm. long 83. P. perlongum. 



Culms in large tufts; spikelets not turgid, not over 2.7 

 mm. long. 



Sheaths pilose 84. P. linearifolium. 



Sheaths glabrous 85. P. werneri. 



Blades usually not elongated; secondary panicles not at 

 the base. 

 Spikelets attenuate at base, mostly prominently pustu- 

 lose; blades narrow, stiff, strongly nerved, tapering 

 from base to apex. 

 Nodes bearded; plants grayish-villous; autumnal 

 blades flat. 



Spikelets 2 mm. long 92. P. chrysopsidifo- 



lium. 



Spikelets 2.5 to 2.8 mm. long 93. P. consanguineum. 



Nodes not bearded; plants villous only at the base, or 

 nearly glabrous. 

 Autumnal blades flat; lower panicle branches 



spreading or deflexed 94. P. angustifolium. 



Autumnal blades involute; lower panicle branches 

 more or less ascending. 

 Plants glabrous or nearly so; autumnal culms 

 erect. 

 Spikelets subsecund along the suberect panicle 



branches 98. P. neuranthum. 



Spikelets not subsecund; panicle loose and 



open 97. P. ovinum. 



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 . 96. P. arenicoloides. 

 Spikelets not over 2 mm. long; vernal blades 



4 to 6 cm. long, autumnal blades falcate. . 91. P. aciculare. 

 Spikelets not attenuate at base. 

 Sheaths retrorsely pilose; blades soft and lax. 



Panicle branches ascending, forming a compact 



panicle; spikelets 1.6 mm. long 87a. P. xalapense stric- 



tirameum. 

 Panicle branches loosely spreading. 

 Blades ciliate and more or less pilose on the sur- 

 face; spikelets 2 mm. long 87. P . xalapense . 



Blades glabrous or nearly so on the surface and 



margin; spikelets 2.2 mm. long 86. P. laxiflorum. 



Sheaths not retrorsely pilose. 

 Ligule manifest, mostly 2 to 5 mm. long. 

 Sheaths, or all but the lowest glabrous; spike- 

 lets not over 1.6 mm. long. 

 Panicle narrow, one-fourth to one-third as wide 



as long 116. P. spretum. 



Panicle open, nearly as wide as long. 



Spikelets 1.5 mm. long 117. P. lindheimeri. 



Spikelets 1.1 mm. long 119. P. longiligulatum. 



Sheaths pubescent. 



