56 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



17. PANICUM L. Panic grass. 



Annuals or perennials; spikelets 1-flowered, or rarely with a staminate flower below 

 the terminal perfect one, paniculate; glumes very unequal, the first often minute, 

 the second subequal to the sterile lemma; fertile lemma and palea chartaceous- 

 indurated, the nerves obsolete, the margins of the lemma inrolled; grain free within 

 the rigid lemma and palea. 



KEY TO THE SPECIES. 



Spikelets arranged in pairs in 1-sided racemes; plants spread- 

 ing by long stolons 1. P. obtusum. 



Spikelets panicled; plants without stolons. 



Leaf blades of two sorts, those of the stems broad and short. 

 Spikelets less than 3 mm. long; leaf blades thin; 



sheaths glabrous or sparsely hispid 2. P. helleri. 



Spikelets more than 3 mm. long; leaf blades firm; at 



least some of the sheaths hispid 3. P. scribnerianum. 



Leaf blades all alike. 

 Annuals. 



Inflorescence of several more or less secund, spike- 

 like racemes. 

 Spikelets strongly reticulate- veined, glab- 

 rous 4. P.fasciculatum reti- 



culatum. 

 Spikelets not reticulate-veined, pubescent 



and copiously papillose-hirsute 5. P. arizonicum. 



Inflorescence a more or less diffuse panicle. 



First glume very short, not over one-fourth 

 the length of the second ; sheaths glab- 

 rous 6. P. dichotomiflorum. 



First glume longer, half as long as the second 

 or more; sheaths pubescent. 



Panicles somewhat drooping 7. P. miliaceum. 



Panicles erect. 



Panicles large, more than half the 



length of the entire plant 8. P .barbipulvinatum. 



Panicles small, not over one-third 

 the length of the plants. 

 First glume more than three- 

 fourths the length of the 

 second; spikelets 4 mm. 



long 9. P. pampinosum. 



First glume half to two-thirds 

 the length of the second; 

 spikelets not over 3.3 mm. 



long 10. P. hirlicaule. 



Perennials. 



Stems neither bulbous nor rhizomatous. 



Sterile palea enlarged and indurated at ma- 

 turity; glumes acute 11. P. Mans. 



Sterile palea not enlarged; glumes acumi- 

 nate 12. P. hallii. 



