1918] The Grasses of Illinois 269 



KEY TO THE GENERA OF ILLINOIS GRASSES 



Inflorescence in a single terminal symmetrical (not one-sided) spike, the spike- 

 lets 2- to several-flowered, sessile on the main axis, single or 2 or 3 at each 

 joint (as in wheat, barley, or rye). 

 b. Axis disjointing with spikelets attached; spikelets 3 together at each joint, 

 the lateral ones sterile and reduced to awns (barley). 60. Hordeum 



bb. Axis not disjointing; spikelets 1 to 3 at each joint, all alike. 



c. Spikelets 2 or 3 at each joint of the axis; glumes in front of the spike- 

 let, awl-shaped, not keeled. 61. Elymus 

 ce. Spikelets 1 at each joint of the axis; glumes at the sides of the 

 spikelets. 

 d. Spikelets placed edgewise on the axis, the inner glume wanting 

 except on the terminal spikelet. 58. Loliitm 

 dd. Spikelets placed flat against the axis. 



6. Glumes obsolete or rudimentary; spikelets horizontally spread- 

 ing at maturity. 62. Rystrix 

 ee. Glumes well developed; spikelets appressed to the axis. 



f. Glumes broadly ovate, abruptly awned or pointed, some- 

 times toothed; plants annual (wheat). Triticum 

 ff. Glumes lanceolate, or narrower, awned or awnless, not 

 toothed, 

 g. Lemmas prickly -hisjiid on the keel; glumes 1-ncrved, 

 narrow; plants annual (rye). Secale 

 gg. Lemmas smooth on the keel; glumes several-nerved; 

 plants perennial. 59. Agropyron 

 Inflorescence a panicle, raceme, or aggregation of spikes; panicle may be 

 open, as in oats, or contracted, as in timothy ; racemes usually appear like 

 spikes, but some of the spikelets are pediceled, as Paspalum (Fig. 37), 

 Andropogon (Fig. 23), and crab grass (Fig. 29); the spikes are one-sided 

 and aggregate, as in Bermuda grass (Fig. 193). 

 b. Fruit a bur with barbed spines. 12. Cenchrus 

 bb. Fruit not a bur. 



c. Spikelets unisexual, the staminate and pistillate in different parts of 

 the same inflorescence or in different inflorescences, 

 d. Pistillate sjjikelets in ears in the axils of the leaves; staminate' 

 spikelets in a terminal tassel (com). Zea 



dd. Pistillate and staminate spikelets in different parts of the same 

 inflorescence, 

 e. Inflorescence consisting of 1 to 4 stout spikes, the lower part 

 thick and hard, readily disjointing, the pistillate spikelets 

 embedded in the joints, the staminate spikelets in pairs 

 along the narrow axis of the upper part. 1. Tripsacum 



ee. Inflorescence a large terminal panicle, the pistillate spikelets 

 erect on the ascending upper branches of the panicle, the 

 staminate pendulous on the spreading lower branches. 



13. Zisania 

 cCi Spikelets perfect; or perfect and staminate, or sterile, arranged in 

 pairs (unisexual in Eragrostis hypnoides, but the si^ikelets alike in 

 appearance), 

 d. Spikelets in pairs, one perfect and sessile, the other pediceled 

 and staminate (rarely perfect) or sterile (apparently two pedi- 

 celed spikelets in Holcus and Sorghastrum), on a jointed axis, 

 readily disjointing with the spikelets attached; glumes hard- 

 ened ; spikelets dorsally compressed, the sessile spikelets awned, 

 e. Inflorescence of slender racemes, these single or 2 or 3 to-, 

 gether, not panicled. 3. Andropogon 



ee. Inflorescence an open or contracted panicle. 



f. Panicle densely wooly, of slender, many-flowered racemes; 

 spikelets all perfect. 2. Erianthus 



