578 MISC. PUBLICATION 200, IT. S. DEPT. OF AGRICULTURE 



Spikelets 1.8 to 1.9 mm long, slightly pointed; blades not more 

 than 8 mm wide 20. P. propinquum. 



Foliage conspicuously pubescent (or sparsely so in exceptional speci- 

 mens of P. pubescens). 

 Culms erect or nearly so. 



Blades from sparsely to rather densely pilose, rather thin. 



18. P. PUBESCENS. 



Blades puberulent on both surfaces with long hairs intermixed 



or the lower surface nearly or quite glabrous except for a 



few long hairs along midrib and margin, usually rather firm. 



17. P. STRAMINEUM. 



Culms widejy spreading or prostrate. 



Foliage coarsely hirsute; plants commonly relatively stout. 



15. P. SUPINUM. 

 Foliage finely puberulent; plants usually grayish olivaceous. 



16. P. PSAMMOPHILUM. 



4b. Racemes terminal on the primary culm or leafy branches, no truly 

 axillary racemes. 

 6a. Spikelets conspicuously silky-ciliate around the margin, the hairs as 

 long as the spikelet or longer. 

 Racemes commonly 3 to 5; culms geniculate at base. 



29. P. DILATATUM. 



Racemes commonly 12 to 18; culms erect 30. P. urvillei. 



6b. Spikelets not ciliate. 



7a. Fruit dark brown and shining. 



Plants perennial; sterile lemma wrinkled 40. P. plicatulum. 



Plants annual; sterile lemma not wrinkled 41. P. boscianum. 



7b. Fruit pale to stramineous (brown but not shining in P. virgatum). 

 8a. Plants robust, 1 to 2 m tall. 



Spikelets pubescent at least toward the summit; fruit brown at 



maturity 39. P. virgatum. 



Spikelets glabrous; fruit pale. 



Culms ascending; leaves crowded toward the base. 



36. P. DIFFORME. 



Culms erect or suberect, leafy throughout. 



Glume and sterile lemma slightly inflated and wrinkled, 



green 37. P. floridanum. 



Glume and sterile lemma not inflated and wrinkled, rusty- 

 tinged 38. P. GIGANTEUM. 



8b. Plants not robust, if more than 1 m tall, culms relatively 

 slender. 

 9a. Spikelets suborbicular or broadly obovate or broadly oval. 

 Spikelets turgidly plano-convex, 3.5 to 4 mm long. 



36. P. DIFFORME. 

 Spikelets depressed plano-convex or lenticular, 2.2 to 3.4 mm 

 long. 

 Spikelets solitary; glume and sterile lemma firm. 



Spikelets orbicular, 3 to 3.2 mm long, scarcely one-third 

 as thick; blades usually equaling the base of the 



panicle or overtopping it 33. P. circulare. 



Spikelets longer than broad, more than one-third as thick; 

 panicle usually much exceeding the blades. 

 Sheaths and blades pilose, mostly conspicuously so. 



32. P. LONGIPILUM. 



Sheaths and blades from glabrous to sparsely pilose. 



31. P. LAEVE. 



Spikelets paired and solitary in the same raceme (rarely all 

 solitary or all paired). 

 Spikelets 2.2 to 2.5 mm (rarely to 2.8 mm) long; foliage 



not conspicuously villous 34. P. praecox. 



Spikelets 2.7 to 3.4 mm long; lower sheaths and blades 

 mostly conspicuously villous at least at base. 



35. P. LENTIFERUM. 



9b. Spikelets elliptic to oval or obovate. 



Culms decumbent at base, rooting at the lower nodes (occa- 

 sional plants in dry situations erect), branching. 

 Spikelets turgidly plano-convex, 3 to 3.2 mm long; culms 

 rather stout 6. P. pubiflorum. 



