GENERA OF GRASSES OF THE UNITED STATES. 
13 
Key to the genera of Aveneae. 
la. Spikelets awnless or the upper lemma mucronate (rarely short-awnetl 
in Sphenopholis) 2 
2a. Articulation below the glumes ; glumes distinctly different in 
shape, the second widened above 4G. Sphenopholis. 
2b. Articulation above the glumes ; glumes similar in shape^ 44. Koeleria. 
lb. Spikelets awned (awnless in Trisetum wolfii) 3 
3a. Florets 2, one perfect, the other staminate 4 
4a. Lower floret staminate, the awn twisted, geniculate, ex- 
serted 48. Arrhenatherum. 
4b. Lower floret perfect, awnless; awn of upper floret hooked. 
51. NOTHOLCTJS. 
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 6 
6a. Spikelets large, the glumes over 1 cm. long 47. A vena. 
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 l-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 usually 
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. 
