28(27). Spikelets placed edgewise to the rachis; first glume wanting except in the 

 terminal spikelet 19. Lolium 



28. Spikelets placed flatwise to the rachis 16. Agropyron 



29(26). Spikelets 3 at each node of the rachis, 1 -flowered, the lateral pair 

 pediceled, usually reduced to awns 18. Hordeum 



29. Spikelets 2 or more (occasionally solitary) at each node of the rachis, alike, 



2- to 6-flowered 17. Elymus 



30(25). Spikelets wtih more than 1 perfect floret 35. Leptochloa 



30. Spikelets with only 1 perfect floret, often with additional imperfect florets 



above or below (31) 



31(30). Spikelets with 1 or more modified florets above the perfect one 



39. Trichloris 



31. Spikelets without additional modified florets, the rachilla sometimes pro- 



longed (32) 



32(31). Rachilla articulate above the glumes 36. Cynodon 



32. Rachilla articulate below the glume, the spikelets falling entire (33) 



33(32). Glumes unequal, narrow 38. Spartina 



33. Glumes equal, broad, boat-shaped 37. Beckmannia 



34(24). Spikelets 1-flowered (occasionally some of the spikelets 2-flowered in 

 a few species of Muhlenbergia) (35) 



34. Spikelets 2- to many-flowered (44) 



35(34). Articulation below the glumes, the spikelets falling entire (36) 



35. Articulation above the glumes (38) 



36(35). Glumes long-awned 30. Polypogon 



36. Glumes awnless (37) 



37(36). Rachilla prolonged behind the palea; panicle narrow or open, not dense; 

 glumes not united, not ciliate on the keel 28. Cinna 



37. Rachilla not prolonged behind the palea; panicle dense 29. Alopecurus 



38(35). Glumes longer than the lemma (39) 



38. Glumes not longer than the lemma, usually shorter (the awn tips longer in 



Muhlenbergia racemosa) (42) 



39(38). Glumes compressed-carinate, stiff-ciliate on the keel; panicle dense, 

 cylindric or ellipsoid 31. Phlewn 



39. Glumes not compressed-carinate, not ciliate (40) 



40(39). Glumes saccate at base; lemma long-awned; panicle contracted, shining 

 32. Gastridium 



40. Glumes not saccate at base; lemma awned or awnless; panicle open or con- 



tracted (41) 



41(40). Floret bearing a tuft of hairs at the base from the short callus; palea 

 well-developed, the rachilla prolonged behind the palei as a hairy 

 bristle 26. Calamagrostis 



41. Floret without hairs at the base or with short hairs; palea usually small or 



obsolete 27. Agrostis 



42(38). Lemma awned from the tip or mucronate, 3- to 5-nerved or obscure in 

 some species 33. Muhlenbergia 



42. Lemma awnless or awned from the back (43) 



43(42). Caryopsis at maturity falling from the lemma and palea; seed loose in 



the pericarp, this usually opening when ripe; lemma 1 -nerved 



34. Sporobolus 



111 



