212 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



the first usually minute, rarely obsolete; sterile lemma few to several-nerved, inclosing 

 a palea and often a staminate flower or empty; fruit subindurate, smooth, the lemn a 

 acuminate, mostly nerved toward the summit, its margins thin an<l usually flat; 

 the palea of similar texture or a little thinner, its tip sometimes acuminate and free 



from the clasping lemma; stamens 3; stigmas plumose; grain usually oblong, dorsally 

 compressed, with a punctiform hilum, free within the lemma and palea. 



Annuals or perennials, mostly tall and robust, confined to the tropics and subtropics. 

 There are probably 80 species or more, over half of them confined to Africa. 



The spikelet of Pennisetum glaucum, presumably developed under cultivation, 

 has assumed a form somewhat analogous to that of Indian corn. '1 he fascicles are 

 persistent on the axis, the glumes are much reduced, and the enlarged grain at 

 maturity protrudes from the lemma and palea. 



The simplest form of Pennisetum is found in the African species of the section 

 Beckeropsis, in which there is a single bristle below the spikelet. In the South 

 American P. mutilatum (Kuntze) Hack, the single bristle is usually shorter than the 

 spikelet, or is sometimes obsolete. Pennisetum exaltatum (Anderss.) Leeke, of the 

 Galapagos, connects these one-bristled forms with such few-bristled species as the 

 North American Pennisetuvx durum. In 1 1 ijmenachne montana Griseb., of Argentina, 

 is found what suggests an approach to Pennisetum. In that the lower spikelets of the 

 panicle are sometimes subtended by a bristle. The uppermost spikelet also is often 

 thus subtended; that is, the spikelet-bearing branchlet is prolonged into a scabrous 

 bristle. r lhe subindurate fertile lemma and palea and the lanceolate spikelets of 

 Pennisetum also suggest relationship to llijmenachne. At the other extreme, such 

 species of Pennisetum as P. karwinslyi and P. ciliure, with bristles united at the very 

 base and with several spikelets in a fascicle, approach such species of Cenchrua as 

 (.'. myosuroides. 



KEY TO THE SPECIES. 



Bristies about 4 cm. long; panicles oval, feathery; spikelets 10 to 12 mm. long. 



1. P. villosum. 

 Bristles rarely over 2 cm. long, most of them much shorter; panicles cylindric 

 or nearly so; spikelets not over 7 mm. long. 

 Grain at maturity subglobose, bursting through the lemma and palea; panicle 



solidly dense, 2 cm. or more thick; plants annual 4. P. glaucum. 



Grain permanently inclosed in the lemma and palea; panicles less than 2 cm. thick, 

 not solid; plants perennial. 

 Fascicles, or most of them, with 2 or more spikelets. 

 Bristles free throughout, some of them plumose. Fascicles pedunculate 



spikelets pedicellate 2. P. orientale triflorum. 



Bristles united at the base into a minute disk. 

 Inner bristles conspicuously plumose, much exceeding the spikelets. 



3. P. ciliare. 

 Inner bristles sparsely (rarely not at all) ciliate, only the innermost much ex- 

 ceeding the spikelets 5. P. karwinskyi. 



Fascicles with but one spikelet. 



Bristles conspicuously plumose 6. P. setosum. 



Bristles not plumose. 



Panicles terminal on the primary culm and leafy branches only. 



Blades involute, not. over 5 cm. long 10. P. doiningense. 



Blades flat or involute-pointed only, 10 cm. or more long. 

 Spikelets 2 to 2.5 mm. long; axis about 0.5 mm. thick. 7. P. antillarum. 

 Spikelets 4.5 mm. or more long; axis 1 to 3 mm. thick. 



Bristles scant, most of them scarcely exceeding the spikelets or shorter; 

 panicle stiff, densely flowered; 8. P. crinitum. 



wered.' 



