FLOWERING PLANTS AND FERNS OF ARIZONA 79 



1. Spikelets not arranged in groups (4). 



4. Spikelets dorsally compressed, with one terminal fertile floret and a sterile 

 or staminate floret below, the latter usually without a palea, the articu- 

 lation below the spikelets, either in the pedicel, ,the rachis, or at base of 

 a cluster of spikelets: Subfamily Panicoideae (5). 

 5. Fertile lemma and palea firmer than the glumes, usually indurate: Tribe 

 Paniceae (6). 

 6. Spikelets subtended by one or more bristles, these distinct or united at 

 base, forming an involucre or spiny bur (7). 

 7. Bristles distinct, persistent; spikelets deciduous 70. Setaria. 



7. Bristles united at base, deciduous, falling with the enclosed spikelets. 



71. Cenchrus. 

 6. Spikelets not subtended by bristles (8) . 



8. Spikelets in open panicles (9). 



9. Fruit dark brown; lemma with more or less prominent white mar- 

 gins, these not inrolled i 65. Leptoloma. 



9. Fruit pale; margins of the indurate lemma inrolled_ 68. Panicxjm. 

 8. Spikelets short-pedicelled on one side of the panicle branches (10). 



10. Second glume mucronate, the sterile lemma mucronate or awned. 



69. ECHINOCHLOA. 



10. Second glume and sterile lemma awnless (11). 



11. Spikelets covered with long silky hairs 63. Trichachne. 



11. Spikelets glabrous or appressed-pubescent (12). 



12. Racemes digitate; first glume present; weedy, decumbent, 



spreading annuals 64. Digitaria. 



12. Racemes panicled (13). 



13. First glume and the rachilla joint forming a swollen ringlike 

 callus below the spikelet; back of the fruit turned away 



from the rachis 66. Eriochloa. 



13. First glume wanting (occasionally present in Paspalum 

 distichum) ; back of the fruit turned toward the rachis. 



67. Paspalum. 

 5. Fertile lemma and palea thin, hyaline, the glumes indurate; sterile lemma 

 like the fertile one (14). 

 14. Spikelets unisexual, the pistillate ones below, the staminate spikelets 



above in the same inflorescence 81. Tripsacum. 



14. Spikelets in pairs, one sessile, the other pedicellate, both usually 



fertile, or the sessile spikelet perfect and the pedicellate one sterile 



or staminate, sometimes much reduced: Tribe Andropogoneae (15). 



15. Spikelets all alike, surrounded by a conspicuous tuft of soft hairs (16) . 



16. Rachis continuous ; fertile lemma awnless 72. Imperata. 



16. Rachis breaking into joints at maturity; fertile lemma awned. 



73. Erianthus. 

 15. Spikelets unlike, the sessile one perfect, the pedicellate spikelet 

 usually sterile, but the sessile spikelet staminate and the pedi- 

 cellate one perfect in genus Trachypoqon (17). 



17. Racemes reduced to one or few joints, these peduncled in a 



subsimple or compound panicle (18). 

 18. Pedicellate spikelets staminate 75. Sorghum. 



18. Pedicellate spikelet wanting, only the pedicel present. 



76. SORGHASTRUM. 



17. Racemes of several to many joints, solitary, digitate, or aggregate 

 in panicles (19). 



19. Spikelets awnless (20). 



20. Rachis joint and pedicel distinct; perfect spikelet lanceolate; 

 tufted perennial; rachis joints and pedicels woolly. 



79. Elyonurus. 



20. Rachis joint and pedicel adnate; perfect spikelet globose; 



freely branching annual with short racemes partly en- 

 closed in the sheaths 80. Hackelochloa, 



19. Spikelets, or at least some of them, awned (21). 



21. Awns delicate, glabrous, not more than 2.5 cm. long. 



74. Andropogon. 



286744°— 42- 



