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



Racemes flexuous, the hairs not obscuring the rachis and spikelets; blades more 

 than 10 cm. long, usually elongate. 

 Racemes numerous in a dense flabellate but delicate inflorescence; sessile 



spikelet 5 mm. long 7. A. sericatus. 



Racemes relatively few in a narrow elongate inflorescence; sessile spikelet 

 6 to 10 mm. long. 

 Culms strictly erect; sessile spikelet 6 to 8 mm. long; hairs on the rachis 



and sterile pedicel inconspicuous 8. A. scoparius. 



Culms decumbent at the base, usually very glaucous; sessile spikelet about 

 1 cm. long; hairs on the rachis and sterile pedicel rather prominent. 



9. A. LITTORALIS. 



Sheaths and blades villous; pedicellate spikelet prominent.... 10. A. divergens. 

 Culms solitary or few together; creeping rhizomes developed. 

 Sessile spikelet 8 to 10 mm. long; sterile spikelet mostly not much reduced. 



11. A. MARITIMUS. 



Sessile spikelet 5 to 7 mm. long; sterile spikelet much reduced. 



Rachis tortuous, the joints as long as the sessile spikelets; blades 1 to 3 mm. 



wide, at least some of them involute 12. A. rhizomatus. 



Rachis somewmat flexuous, but not conspicuously tortuous; blades mostly 3 to 



5 mm. wide, flat 13. A. stolonifer. 



Section 2. Arthrolophis 

 la. Pedicellate spikelet staminate, similar to the sessile spikelet, but awnless. 

 Rhizomes short or wanting; rachis joint and sterile pedicel ciliate, the joints short-hispid 



at base; awn of sessile spikelet 1 to 2 cm. long 14. A. gerardi. 



Rhizomes well developed; rachis joint and sterile pedicel densely long-villous; awn of 



sessile spikelet rarely more than 5 mm. long, often obsolete 15. A. hallii. 



lb. Pedicellate spikelet reduced to 1 or 2 glumes, or obsolete, the pedicel only developed; 

 racemes silky-villous. 

 2a. Inflorescence very decompound, the profuse pairs of racemes aggregate in an elongate 

 or corymbose mass; spathes rarely more than 2 mm. wide; pedicellate spikelet obso- 

 lete (see also A. virginicus var. hirsutior) 27. A. glomerattjs. 



2b. Inflorescence not conspicuously decompound nor dense (rather dense in A. virginicus 

 var. hirsutior). 

 3a. Peduncle not more than 1 cm. long, the dilated spathes exceeding the 2 (occasionally 

 3 or 4) racemes. 

 Upper sheaths inflated spathelike, aggregate, the late inflorescence a flabellate tuft. 



28. A. ELLIOTTII. 

 Upper sheaths not inflated and aggregate. 



Blades of the innovations subfiliform; ligule acute, protruding from the folded 



blade; foliage usually glabrous 23. A. perangustatus. 



Blades 2 to 5 mm. wide; ligule minute, concealed within the folded blade; foliage 

 from obscurely to conspicuously pubescent. 



Hairs of the racemes copious... 22. A. longiberbis. 



Hairs of the racemes comparatively sparse. 



Rachis joints shorter than the spikelets; branches glabrous below the spathes. 



25. A. CAPILLIPES. 

 Rachis joints usually as long as the spikelets; branches, at least some of them, 



bearded below the spathes 26. A. virginicus. 



3b. Peduncles 2 cm. long or more. 



4a. Peduncles not more than 5 cm. long, enclosed in the spathe or only slightly ex- 

 serted (see also A. perangustatus). 

 Racemes usually not more than 15 mm. long; ultimate branchlets capillary, spread- 

 ing or recurved, long-villous at summit 24. A. brachystachys. 



Racemes 2 to 5 cm. long. 



Racemes 4 to 6 to a peduncle, tawny; sheaths villous 16. A. mohrii. 



Racemes 2 to a peduncle, silvery or creamy white; sheaths glabrous or nearly so. 

 Pairs of racemes numerous; spathes inconspicuous, at least some of the pe- 

 duncles as much as 5 cm. long 20. A. floridanus. 



Pairs of racemes not more than 10 to a culm; spathes dilated; peduncles 1 to 



3 cm. long 21. A. tracyi. 



4b. Peduncles or most of them 5 to 15 cm. long, long-exserted (short-exserted pe- 

 duncles intermixed with long in A. elliottii and A. subtenuis). 

 Rachis joints longer than the spikelets; racemes 5 to 10 cm. long, conspicuously 



slender and flexuous..- 30. A. campyloracheus. 



Rachis joints not longer than the spikelets; racemes not more than 7 cm. long, 

 usually not more than 5 cm. 



