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



2a. Rachilla prolonged behind the palea as a minute bristle. 



Lemma pubescent 1. A. avenacea. 



Lemma glabrous. 



Spikelets 2 mm. long 2. A. thurberiana. 



Spikelets 3 mm. long 3. A. aequivalvis. 



2b. Rachilla not prolonged. 



Glumes scabrous on the keel and on the back; panicle contracted, lobed, the short 



branches densely verticillate 4. A. semiverticillata. 



Glumes scabrous on the keel only; panicle open or, if contracted, not lobed nor with 

 densely verticillate branches. 



Plants tufted; dwarf alpine species 10. A. humilis. 



Plants with rhizomes or stolons; taller species of low and medium altitudes. 



Branches of panicle naked at base, the panicle open and delicate ; ligule as much as 

 2 mm. long on culm leaves, less than 1 mm. on the innovations. 



9. A. TENUIS. 

 Branches of panicle or some of them floriferous from base; ligule as much as 6 mm. 

 long. 

 Panicle contracted, the branches appressed; long stolons developed in isolated 



plants. Culms decumbent at base 6. A. palustris. 



Panicle open, the branches ascending, no long stolons developed. 



Culms producing rather stout creeping leafy stolons 7. A. nigra. 



Culms decumbent at base; rhizomes wanting 5. A. stolonifera. 



Culms erect; rhizomes developed 8. A. alba. 



lb. Palea obsolete, or a minute nerveless scale (in A. exarata and A. calif ornica as much as 

 0.5 mm. long or more). 

 3a. Plants annual, lemma with a slender awn, geniculate or flexuous. 



Lemma awnless 11. A. rossae. 



Lemma with a slender geniculate or flexuous awn. 



Awn flexuous, delicate; Southeastern States 12. A. elliottiana. 



Awn geniculate; Pacific coast. 



Spikelets about 1.5 mm. long; lemma awned below the tip 13. A. exigua. 



Spikelets at least 2.5 mm. long; lemma awned from the middle. 



Apex of lemma obscurely toothed or nearly entire; lemma 1.7 to 1.9 mm. long. 



16. A. MICROPHYLLA. 



Apex of lemma bearing 2 or 4 delicate awns. 



Lemma pilose; glumes 3.5 to 4 mm. long 15. A. kennedyana. 



Lemma glabrous except on the callus; glumes 5 to 6 mm. long. 



Lemma relatively firm, scabrous. 3.2 to 3.5 mm. long; palea nearly Y% as 



long as the lemma 17. A. aristigltjmis. 



Lemma thin, glabrous, 3 mm. long; palea obsolete.— 14. A. hendersoni. 

 3b. Plants perennial; lemma awned or awnless, the awn when present not much exserted. 

 4a. Plants spreading by creeping rhizomes (those of A. lepida short). 



Hairs at base of lemma 1 to 2 mm. long 18. A. hallii. 



Hairs at base of lemma minute or wanting. 



Rhizomes short; alpine tufted plants 19. A. lepida. 



Rhizomes long and slender. 



Panicle spikelike 20. A. pallens. 



Panicle open 21. A. diegoensis. 



4b. Plants without rhizomes, stolons sometimes developed. 



5a. Panicle narrow, contracted, at least some of the lower branches spikelet-bearing 



from the base. 



Culms slender, not more than 20 cm. tall, in dense tufts with numerous basal 



leaves; blades not more than 7 cm. long, mostly less, less than 2 mm. wide; 



panicles seldom more than 5 mm. wide. 



Culms spreading; panicles strict, greenish; lemma with a minute awn or the 



midnerve ending below the summit 22. A. blasdalei. 



Culms erect; panicle narrow but loose, purple; lemma awnless, the midnerve 



reaching the summit 23. A. variabilis. 



Culms taller, stouter, not in tufts with dense basal foliage; blades or some of them 

 at least 8 to 10 cm. long and 3 to 5 mm. wide, commonly much larger; glumes 

 scabrous on the keel. 

 Panicle from loose to dense; lemma acute, not toothed; palea minute. 



Panicle loose, the branches verticillate, not densely flowered at base; awn of 



lemma twisted, geniculate 25. A. ampla. 



Panicle dense to loose, the branches crowded and densely flowered at base; 

 lemma awnless or (in vars. pacifica and monolepis) awned. 



24. A. EXARATA. 



