13. Racemes appressed; fertile lemma nearly as long as spikelet 



13. P. obtusum. 



14(12). Fertile lemma not transversely rugose 12. P. hemitomum. 



14. Fertile lemma transversely rugose (15) 



15(14). Spikelets 2.2-2.4 mm. long 9. P. geminatum. 



15. Spikelets 2.5-3 mm. long 10. P. paludivagum. 



16(11). Sterile palea enlarged and indurate at maturity, expanding the spikelet; 

 blades scarcely broader than their sheaths; spikelets about 2.3 mm. 



long, borne toward the ends of the few slender branches 



14. P. hians. 



16. Sterile palea usually absent or (if present) minute (17) 



17(16). Plants with conspicuous creeping scaly rhizomes (18) 



17. Plants without creeping scaly rhizomes (20) 



18(17). Spikelets short-pedicelled, more or less secund along the nearly simple 

 panicle branches 15. P. anceps. 



18. Spikelets long-pedicelled, not secund, arranged in an open or contracted 



panicle (19) 



19(18). Panicle compact, strongly contracted, elongate and nodding; plants of 

 coastal sands 18. P. amarulum. 



19. Panicle diffuse or only slightly contracted 19. P. virgatum. 



20(17). Panicles narrow and few-flowered; culms erect and wiry; blades drying 

 involute 17. P. tenerum. 



20. Panicles open or contracted, many-flowered 16. P. rigidulum. 



1. Panicum dichotomum L. Fig. 152. 



Perennial; vernal phase (Apr.-Aug.) culms tufted, erect or ascending from 

 a knotted or loose crown, 3-5 (-10) dm. tall, glabrous but the nodes very often 

 with a grayish retrorse beard about 1 (-2) mm. long and often the lower nodes 

 geniculate; sheaths essentially glabrous; ligules minute; blades usually spreading, 

 the upper often reflexed, 3-12 cm. long, 4-15 mm. broad, glabrous or sparsely 

 papillose-ciliate at base, green (often bright, rarely olivaceous) and thin, quite 

 flat; panicle usually elongate-ovoid, usually many-spikeletted, 5-12 cm. long, 

 with the slightly spreading very slender and often flexuous branches usually 

 copiously branched; spikelets 1.4-2.2 mm. long, elliptic, glabrous or pubescent, 

 5- to 7-nerved; second glume usually shorter than fertile lemma; autumnal phase 

 (June-Dec.) much-reduced, much-branched at some nodes, the lower part usually 

 ascending (or reclining from the heavy weight of the top) and bladeless like a 

 slender tree-trunk, the upper part copiously bushy-branched with numerous small 

 blades 2-4 cm. long and 1-3 mm. broad (thin, green, flat or often involute). 

 P. nitidum Lam., P. barbulatum Michx., P. microcarpon Muhl., P. lucidum Ashe, 

 P. yadkinense Ashe. 



In swampy and marshy grounds, bogs, wet peaty meadows and margins of 

 streams, also in moist sandy woodlands, in Okla. (LeFlore Co.) and in e. and s.e. 

 Tex., rare w. to n.-cen. Tex., spring-fall; s.e. Can., e. U.S., Bah. I., Cuba. 



2. Panicum leucothrix Nash. Fig. 152. 



Perennial; vernal phase light olive-green to dark-green; culms 1-5 (-7) dm. 

 tall, ascending (often decumbent at base and somewhat geniculate) weak, slender, 

 glabrous or appressed papillose-pilose, the nodes pubescent or glabrous; sheaths 

 papillose-pilose to puberulent or glabrous; ligule minute but blade at base with 

 a ligulelike tuft or hairs 2-3 mm. long; blades 3-8 mm. broad, about 2-5 cm. 

 long, glabrous or sparsely villous above, puberulent or glabrous beneath, or even 

 velvety-puberulent beneath; panicle 3-8 cm. long, rather densely flowered; spike- 



307 



