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



flowers are produced. The ripe fruits of several species are troublesome to 

 stock on the plains because of the sharp hard points. These fruits are produced 

 sometimes in vast numbers and are carried far and wide by the wind in open 

 country. Aristida adscensionis is one of the annuals that make up the "six- 

 weeks" grasses of the Southwest. 



Lemma articulate with the column of the awns; awns nearly equal. 



Section 1. Arthratherum. 

 Lemma not articulate. 



Lateral awns minute (less than 1 mm. long) or wanting Section 2. Streptachne. 



Lateral awns more than 1 mm. long (rarely obsolete in A. ramosissima), usually well de- 

 veloped Section 3. Chaetaria. 



Section 1. Arthratherum 

 Plants annual. 



Column very short 1. A. desmantha. 



Column 10 to 15 mm. long, twisted 2. A. tuberculosa. 



Plants perennial. 



Culms pubescent -. 3. A. californica. 



Culms glabrous 4. A. glabrata. 



Section 2. Streptachne 



Awn (column) twisted at base. 7. A. orcuttiana. 



Awn not twisted. 



Branches of panicle distant, spreading, mostly more than 5 cm. long, naked at base; awn 



straight or abruptly divergent 5. A. ternipes. 



Branches of panicle short, approximate, 3 to 5 cm. long, floriferous nearly to base; awn 

 curved and flexuous - 6. A. floridana. 



Section 3. Chaetaria 

 la. Central awn spirally coiled at base, the lateral straight. Plants annual. (Group 

 Dichotomae.) 

 Lateral awns half to two-thirds as long as the central, somewhat spreading. 



8. A. BASIRAMEA. 



Lateral awns much shorter than the central, 1 to 3 mm. long, erect. 



Glumes nearly equal. 6 to 8 mm. long; lemma sparsely appressed-pilose, 5 to 6 mm. 



long - 9. A. DICHOTOMA. 



Glumes unequal, the second longer, about 1 cm. long; lemma glabrous except the keel, 



scabrous toward the apex, about 1 cm. long 10. A. curtissii. 



lb. Central awn not spirally coiled (in a few species all the awns loosely contorted in the 

 lower part). 

 2a. Plants annual. (Group Adscensiones.) 



Awns mostly 4 to 7 cm. long, about equal, divergent 11. A. oligantha. 



Awns mostly less than 2 cm. long, often unequal. 



Central awn with a semicircular bend at base, spreading or reflexed. 



Lateral awns much reduced ; lemma about 2 cm. long 12. A. ramosissima. 



Lateral awns one- third to half as long as the central; lemma 4 to 5 mm. long. 



13. A. LONGESPICA. 

 Central awn not sharply curved, the awns about equally divergent. 



Glumes unequal; awns flat at base, 10 to 15 mm. long 14. A. adscensionis. 



Glumes about equal; awns terete, 15 to 20 mm. long 15. A., intermedia. 



2b. Plants perennial. 



3a. Panicle open, the branches spreading (in A. pansa ascending), naked at base. 



(Group DlVARICATAE.) 



Panicle branches stiffly and abruptly spreading or reflexed at base. 



Branchlets divaricate and implicate 16. A. barbata. 



Branchlets appressed. 



Summit of lemma narrowed into a twisted neck 2 to 5 mm. long. 



17. A. DIVARICATA. 



Summit of lemma somewhat narrowed but not twisted 18. A. hamulosa. 



Panicle branches drooping or ascending, not abruptly spreading at base. 



Lateral awns one-fourth to half as long as the central one 19. A. patula. 



Lateral awns about as long as the central, at least more than half as long. 



20. A. PANSA. 

 3b. Panicle narrow, the branches ascending or appressed (branches sometimes some- 

 what spreading in A. parishii and A. purpurea) . 



Column 1 cm. or more long, twisted; glumes awned 21. A. spiciformis. 



Column less than 1 cm. long. 



