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



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



Spikelets 3.3 to 3.5 mm long, pointed 18. P. fusiforme. 



Spikelets less than 3 mm long, not pointed, or obscurely so. 

 Plants glabrous or nearly so. Autumnal culms erect. 

 Spikelets subsecund along the suberect panicle branches. 



21. P. NEURANTHUM. 



Spikelets not subsecund, the panicle loose and open__ 20. P. ovintjm. 

 Plants pubescent, at least on the lower half. 



Spikelets about 2.4 mm, rarely only 2.1 mm long; vernal blades 7 to 12 

 cm long; autumnal blades not falcate 19. P. arenicoloides. 



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

 tumnal blades much crowded, falcate 14. P. aciculare. 



4' \Bicknelliana 



Spikelets 2.5 to 2.8 mm long; blades not more than 9 mm wide. 



22. P. BICKNELLII. 

 Spikelets 3 mm long; blades as much as 12 mm wide 23. P. calliphyllum. 



5. Nudicaulia 



A single species 24. P. nudicaule. 



6. Dichotoma 



la. Nodes, at least the lower, bearded. 



Spikelets 1.5 to 1.6 mm long, glabrous (occasional individuals with pubescent 



spikelets) 25. P. microcarpon. 



Spikelets 2 mm long or more. 



Spikelets glabrous, 2 mm long; autumnal phase topheavy-reclining. 



32. P. BARBULATUM. 



Spikelets pubescent. 



Blades all velvety; autumnal phase usually sparingly branching. 



27. P. ANNULUM. 



Blades glabrous, or only the lower pubescent or velvety; autumnal phase 

 freely branching. 

 Spikelets 2 mm long; autumnal phase profusely branching. 



26. P. NITIDUM. 



Spikelets 2.2 mm long or more; autumnal phase less profusely branching. 

 Sheaths and upper nodes glabrous 29. P. clutei. 



Lower sheaths and all nodes pubescent. 



28. P. MATTAMUSKEETENSE. 



lb. Nodes not bearded. 

 2a. Spikelets pubescent. 



Culms erect, never becoming vinelike. 



Primary blades spreading; panicles purplish; fruit exposed at summit. 



29. P. CLUTEI. 

 Primary blades erect; panicles green; fruit covered (wood forms with 

 spreading blades may be distinguished from P. dichotomum by pubes- 

 cent spikelets, 2.2 mm long) 30. P. boreale. 



Culms soon prostrate, vinelike, the branches divaricate. 



Plants bright green, culms lax; spikelets not more than 2.1 mm long. 



36. P. LUCIDUM. 

 Plants grayish green, culms stiff; spikelets 2.5 mm long. 



37. P. SPHAGNICOLA. 



2b. Spikelets glabrous. 

 Culms soon prostrate. 



Plants bright green, culms lax; spikelets not more than 2.1 mm long. 



36. P. LUCIDUM. 

 Plants grayish green, culms stiff; spikelets 2.5 mm long. 



37. P. SPHAGNICOLA. 



Culms erect, or the autumnal phase topheavy, never prostrate. 



Spikelets not more than 1.6 mm long; panicles narrow. Plants glaucous 



bluish green 35. P. caerulescens. 



Spikelets 2 mm long or more; panicles open. 



Blades erect, firm; spikelets turgid, strongly nerved. Plants grayish 

 olive green _ 34. P. roanokense. 



