OK AMINE AE. 35 



Culms glabrous ; ■ spikelets about 1.25 



mm. long. 19. P. nitidwm. 



Culms at base densely pubescent, with 

 : long, stiff, appressed hairs; spikelets 

 2-2.5 mm. long. 



27. P. Commonsianum. 

 Leaves pubescent on both surfaces, especially be- 

 neath, 

 Hairs on the lower surface short, soft,, and 

 copious. 

 Spikelets about 1.5 mm. long. 



Densely covered with short hairs on 

 the upper surface. 



25. P. lanuginosum. 

 Densely covered with scattered long 



hairs on the upper surface. 



23. P. Tenesseense. 

 Spikelets 2.5 mm. long. 



26. P. scoparium. 

 Hairs on the lower, as well as the upper, sur- 

 face stiff and long, sometimes scat- 

 tered. 



Spikelets 1.5 mm. long. 



Culms slender; hairs on the upper 

 surface of the leaves .extremely 

 long and copious. 



21. P. implicatum. 

 Culms rather stout ; hairs on the 

 upper surface of the leaves shorter, 

 and more scattered, sometimes 

 nearly wanting. 



,22. P. wficiphyllwm. 

 Spikelets a "little more than 2 mm. long. 

 ' ■'"" 24. P. Atlanticum. 

 Spikelets more than 3 mm. long. 



Panicle narrow, its branches appressed. 



29. P. xanthophysum. 

 Panicle open, its 1 branches' spreading. 



28. . P. Scribnerianum.: 

 (2) Middle leaves of the main culm more than 1.5 cm. wide, often 

 3 cm. in width, the base cordate and clasping. 

 Spikelets 2.5 mm. long or less. 



Leaves glabrous on both surfaces. 



Spikelets about 1.5 mm. long; panicle oblong and 



dense; culms simple. 31. P. polyanthe&. 



Spikelets 2.5 mm. long; panicle ovoid, and open; 

 culms branched. 

 Sheaths pubescent. 



Leaf -blades ciliate at the base; spikelets 3 



mm. long or more. 37. P. clandestinum. 

 Leaf -blades naked at the base; spikelets less 

 than 3 mm. long. 38. Pi decoloratum. 

 Sheaths glabrous. 32. P. commutatwm. 



Leaves densely villous. 26. P. scopariwm. 



Spikelets 3 mm. long or more. 



Nodes manifestly and copiously barbed; spikelets 4-5 mm. 

 long. 

 Lower surface of the leaf-blades paler, glabrous. 



35. P. Porterianum. 

 Lower surface of the leaf -blades pubescent, as are also 

 the sheaths and stems. 36. P. pubifolium. 



Nodes naked. 



