278 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



Sterile or pedicellate spikelets much smaller than the fertile ones, often rudi- 

 mentary (about as large in A. pertusns and A. annulatus with several 

 racemes). 

 Racemes solitary at the ends of the culms or branches, from bractlike 

 sheaths ; rachis joints clavate, the apex with a cuplike hollow. 

 ( Subgenus Schizachyki um. ) 

 Plants annual. 



Culms weak, decumbent ; blades obtuse, 2 to 4 cm. long ; peduncles 



capillary, spreading 2. A. brevif olius. 



Culms erect ; blades acute, usually 5 to 10 cm. long ; peduncles slen- 

 der, but not capillary 3. A. malacostachyus. 



Plants perennial. 



Rachis conspicuously flexuous, very slender, the spikelets spreading. 

 Blades flat ; racemes very numerous in a corymbose panicle ; 



plants robust 4. A. condensatus. 



Blades involute ; racemes few ; plants slender. 



Racemes densely white-villous with long hairs ; blades firm. 



6. A. gracilis. 

 Racemes sparsely villous ; blades rather soft. 



7. A. cubensis. 

 Rachis straight, the spikelets appressed or narrowly ascending. 



Spikelets awnless. 



Racemes 1 to 2 cm. long, partly inclosed in the subtending 

 spathe, numerous, aggregated in a narrow panicle. 



8. A. virgatus. 

 Racemes 4 to 8 cm. long, few, exserted__9. A, salzmanni. 



Spikelets awned. 



First glume of the sessile spikelet villous. 



12. A. hirtiflorus. 

 First glume of the sessile spikelet glabrous or nearly so. 

 Blades terete-filiform ; sessile spikelet 1.5 mm. broad. 



5. A. multinervosus. 

 Blades flat or folded or sometimes involute toward the 

 apex. 

 Sessile spikelet about 4 mm. long, 1 mm. widej 

 blades not over 1.5 mm. wide. 



13. A. tener. 

 Sessile spikelet about 5 mm. long ; blades 2 to 5 



mm. wide 14. A. semiberbis. 



Racemes 2 or more together at the ends of the culms or branches. 

 Plants annual ; racemes sevei-al on capillary flexuous peduncles. 



15. A. piptatherus. 

 Plants perennial ; peduncles not capillary nor flexuous. 



Racemes numerous or several arranged along an axis, forming a 

 panicle. 

 Pedicellate spikelet much smaller than the fertile one, the 



latter not pitted 18. A. saccharoides. 



Pedicellate spikelet about as large as the fertile one. 

 First glume with a pinhole-like pit on the back. 



16. A. pertusus. 



First glume not pitted 17. A. annulatus. 



Racemes 2 to 4 (sometimes more in A. seUoamis), fascicled, sub- 

 tended by a bladeless sheath (spathe), exsertod or partly in- 

 cluded in it. 



