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



A large tribe, confined mostly to warm regions. The rachis is 

 usually jointed, disarticulating at maturity, with the spikelets at- 

 tached to the joints. In a few genera it is thickened. Sometimes the 

 racemes are shortened to 1 or 2 joints and borne on branches, the 

 whole forming a panicle (as in Sorghum and Sorghastrum) instead of a 

 series of racemes. In a few genera the spikelets of the pair are alike. 

 In Trachypogon the fertile spikelet is pedicellate and the sterile one 

 nearly sessile. The most important economic plants in this tribe are 

 sugarcane and sorghum. 



Key to the genera of Andropogoneae 



la. Spikelets all perfect, surrounded by a copious tuft of soft hairs. 



Rachis continuous, the spikelets falliDg; the spikelets of the pair unequally 

 pedicellate. 



Racemes in a narrow spikelike panicle; spikelets awnless 140. Imperata. 



Racemes in a broad fan-shaped panicle; spikelets awned. 



141. Miscanthus. 

 Rachis breaking up into joints at maturity with the spikelets attached; one 

 spikelet sessile, the other pedicellate. 



Spikelets awnless 142. Saccharum. 



Spikelets awned 143. Erianthus. 



lb. Spikelets unlike, the sessile perfect, the pedicellate sterile (sessile spikelet 

 staminate, pedicellate spikelet perfect in Trachypogon). 

 2a. Pedicel thickened, appressed to the thickened rachis joint (at least parallel 

 to it) or adnate to it; spikelets awnless, appressed to the joint. 

 Rachis joint and pedicel adnate. Annuals. 



Perfect spikelet globose; sterile spikelet conspicuous. 



155. Hackelochloa. 



Perfect spikelet oblong; sterile spikelet minute 153. 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 summit, the spikelets sunken in the hollow below; sterile 



spikelet rudimentary 154. M anisuris. 



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



summit; sterile spikelet staminate or neuter 152. Elyonurus. 



2b. Pedicel not thickened (if slightly so the spikelets awned), neither ap- 

 pressed nor adnate to the rachis joint, this usually slender; spikelets 

 usually awned. 

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

 porting rachis joint or pedicel; awns strong. 

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



panicle 149. Rhaphis. 



Racemes of several to many joints, single. 



Primary spikelet subsessile, sterile, persistent on the continuous axis 

 after the fall of the fertile pedicellate spikelet. 151. Trachypogon. 

 Primary spikelet sessile, fertile; pedicellate spikelet sterile. Lower 

 few to several pairs of spikelets all staminate or neuter. 



150. Heteropogon. 

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

 rachis disarticulating immediately below the spikelet; awns slender. 



Blades ovate. Annual 144. Arthraxon. 



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. 



145. Andropogon. 

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



flexuous peduncles 146. Hyparrhenia. 



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

 subsimple or compound panicle. 



Pedicellate spikelets staminate 147. Sorghum. 



Pedicellate spikelets wanting, the pedicel only present. 



148. Sorghastrum. 



