GENERA OF GRASSES OF THE UNITED STATES. 13 



Key to the genera of Avcncae. 



la. Spikelets awnless or the upper lemma mucronate (rarely short-awned 



in Splienopliolis) 2 



2a. Articulation bolow the glumes ; glumes distinctly different in 



shape, the second widened above 4G. SIMIKMOPIIOLIS. 



2b. Articulation above the glumes; glumes similar in shape, 44. KOKLKKIA. 



Ib. Spikelets awned (awnless in Trisetum wolfii) 3 



3u. Florets 2, one perfect, the other staminate 4 



4a. Lower floret staminate, the awn twisted, geniculate, ex- 

 sorted 48. ARRHENATHERUM. 



4b. Lower floret perfect, awnless; awn of upper floret hooked. 



51. NOTHOLCUS. 



3b. Florets 2 or more, all alike except the reduced upper ones 5 



5a. Awn arising from between the teeth of a bifid apex, flat- 

 tened, twisted ; inflorescence a simple panicle or reduced 



to a raceme or even to a single spikelet 52. DANTHONIA. 



5b. Awn dorsal, not flattened ; lemma often bifid at apex G 



6a. Spikelets large, the glumes over 1 cm. long 47. AVENA.*-*' 



6b. Spikelets less than 1 cm. long 7 



7a. Lemmas keeled, bidentate; awn arising from above 



the middle 45. TRISETUM. 



7b. Lemmas convex ; awn from below the middle 8 



8a. Rachilla prolonged behind the upper floret; lem- 

 mas truncate and erose-dentate at summit 49. AIRA. 



8b. Rachilla not prolonged ; lemmas tapering into 2 



slender teeth 50. ASPRIS. , 



TRIBE 5, AGROSTIDEAE. 



Spikelets 1-flowered, usually perfect, arranged in open, contracted, 

 or spikelike panicles, but not in true spikes nor in one-sided racemes. 



A large and important tribe, inhabiting more especially the tem- 

 perate and cool regions. The articulation of the rachilla is usualty 

 above the glumes, the mature floret falling from the persistent 

 glumes, but in a few genera the articulation is below the glumes, the 

 mature spikelet falling entire (Alopecurus, Cinna, Polypogon, 

 Lycurus, and Limnodea). The palea is small or wanting in some 

 species of Agrostis. In a few genera the rachilla is prolonged 

 behind the palea as a minute bristle, or sometimes as a more pro- 

 nounced villous stipe (Brachyelytrum, Limnodea, Cinna, three species 

 of Agrostis, Gastridium, Calamagrostis, Ammophila, and Lagurus). 

 In some genera the rachilla joint between the glumes and the lemma 

 is slightly elongated, forming a hard stipe which remains attached 

 to the mature fruit as a pointed callus. The callus is well marked 

 in Stipa (especially in S. spartea and its allies) and in Aristida, the 

 mature lemma being terete, indurate, and convolute, the palea wholly 

 inclosed. In many genera the lemma is awned either from the tip 

 or from the back, the awn being trifid in Aristida. 



