26 MISC. PUBLICATION 200, U. S. DEPT. OF AGRICULTURE 



Rachis joint and pedicel adnate. Annuals. 



Perfect spikelet globose; sterile spikelet conspicuous 164. Hackelochloa. 



Perfect spikelet oblong; sterile spikelet minute 162. Rottboellia. 



Rachis joint and pedicel distinct, the sessile spikelet appressed to them, its first glume 

 lanceolate. 

 Racemes subcylindric; rachis joints and pedicels glabrous, much thicker at the sum- 

 mit, the spikelets sunken in the hollow below; sterile spikelet rudimentary. 



163. Maxisuris. 

 Racemes flat; rachis joints and pedicels woolly, not much thicker at the summit; 



sterile spikelet staminate or neuter 161. Elyoxurus. 



2b. Pedicel not thickened (if slightly so the spikelets awned), neither appressed nor adnate 

 to the rachis joint, this usually slender; spikelets usually awned. 

 3a. Fertile spikelet with a hairy-pointed callus, formed of the attached supporting 

 rachis joint or pedicel; awns strong. 

 Racemes reduced to a single joint, long-peduncled in a simple open panicle. 



158. Chrysopogox. 

 Racemes of several to many joints, single. 



Primary spikelet subsessile, sterile, persistent on the continuous axis after the fall 



of the fertile pedicellate spikelet 160. Trachypogox. 



Primary spikelet sessile, fertile; pedicellate spikelet sterile. Lower few to several 



pairs of spikelets all staminate or neuter 159. Heteropogox. 



3b. Fertile spikelet without a callus (a short callus in Hyparrhenia), the rachis disarticu- 

 lating immediately below the spikelet; awns slender. 



Blades ovate. Annual 153. Arthraxox. 



Blades narrow, elongate. 



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



Lower pair of spikelets like the others of the raceme 154. Axdropogox\ 



Lower pair of spikelets sterile, awnless. Racemes in pairs on slender flexuous 



peduncles - 155. Hyparrhexia. 



Racemes reduced to one or few joints, these mostly peduncled in a subsimple or 

 compound panicle. 



Pedicellate spikelets staminate 156. Sorghum. 



Pedicellate spikelets wanting, the pedicel onty present 157. Sorghastrum. 



TRIBE 14. TRIPSACEAE 



Spikelets unisexual, the staminate in pairs, or sometimes in threes, 2-flow- 

 ered, the pistillate usually single, 2-flowered, the lower floret sterile, em- 

 bedded in hollows of the thickened articulate rachis and falling attached to the 

 joints, or enclosed in a thickened involucre or sheath or, in Zea, crowded in 

 rows on a thickened axis (cob) ; glumes membranaceous or thick and rigid, 

 awnless; lemmas and palea hyaline, awnless. Plants monoecious. 



This small tribe of seven genera is scarcely more than a subtribe of Andro- 

 pogoneae, differing chiefly in the total suppression of the sterile spikelet of a 

 pair, the fertile spikelet being pistillate only and solitary; staminate spikelets 

 paired. It is also known as Maydeae. 



Key to the genera of Tripsaceae 



Staminate and pistillate spikelets in separate inflorescences, the first in a terminal tassel, the 

 second in the axils of the leaves. 

 Pistillate spikes distinct, the spikelets embedded in the hardened rachis, this disarticulat- 

 ing at maturity 167. Euchlaexa. 



Pistillate spikes grown together forming an ear, the grains at maturity much exceeding 



the glumes 168. Zea. 



Staminate and pistillate spikelets in separate portions of the same inflorescence, the pistillate 

 below. 

 Spikes short, the 1- or 2-flowered pistillate portion enclosed in a beadlike sheathing bract. 



165. Coix. 



Spikes many-flowered, the pistillate portion breaking up into several 1-seeded joints; no 



beadlike sheathing bract 166. Tripsacum. 



