POACEAE. 29 



flower. Stamens 3. Styles united below. Stigmas plumose. Grain free, en- 

 closed in the scales. [Ancient Greek name for some grass, probably Millet.] 

 About 12 species, in tropical and temperate regions. Type species: Cenchrus 

 echinatus L. 



Involucre 2-spikeleted, armed at the base with commonly shorter often reflexed 

 spines, usually glabrous, or merely puberulent at the base. 

 Involucre, over all, 8-12 mm. in diameter. 



Body of the involucre 5-6 mm. thick, finely pubescent. 1. C- carolinianus. 

 Body of the involucre 8-10 mm. thick, densely woolly. 2. ('. tribuloides. 

 Involucre, over all, 5 mm. in diameter or less. 3. C. microcephalus. 



Involucre more than 2-spikeleted, armed at the base with 

 barbed bristles. 

 Spines on the usually 3-spikeIeted involucre basal and slen- 

 der, rarely any above, erect : bristles very slender. 4. C. viridis. 

 Spines on the usually 4-6-spikeleted involucre not basal but 

 occurring on various parts, spreading or often reflexed ; 

 bristles stouter and almost spine-like. 5. C. echinatus. 



1. Cenchrus carolinianus Walt. Fl. Car. 79. 1788. 



Culms erect or decumbent from an annual root, 2-9 dm. long. Sheaths 

 usually loose, compressed, smooth; leaves 612.5 em. long; spikes 2.5-6.25 

 cm. long, sometimes partially included in the upper sheath ; involucres crowded 

 on the scabrous rachis, 2-spikeleted, globose, pubescent except at the base, the 

 spines s'tout; spikelets about 6 mm. long. 



Sand dunes, cultivated grounds, and waste places from South Bimini and 

 Andros to Mariguana, Turks Islands, Inagua, Anguilla Isles and Salt Cay Bank : 

 Maine to Nebraska. Arizona, Texas and Florida : West Indies and tropical conti- 

 nental America. Referred by Hitchcock, Coker, Dolley, Mrs. Northrop, and Herrick 

 to C. tribuloides L. Small Bur-grass. Devil-grass. 



2. Cenchrus tribuloides L. Sp. PI. 1050. 1753. 



Culms at first erect, later prostrate and forming mats, 46 dm. long, 

 branching; leaf -sheaths compressed; blades 6-12 cm. long, 4-8 mm. wide, 

 smooth or rough, usually flat; spikes 2-6 cm. long; involucres 6-20, 1.5-2 cm. 

 broad, enclosing 2 spikelets, pubescent, the spines 3-4 mm. long; spikelets 67 

 long, usually not exserted beyond the involucre. 



Sandy sea-beaches. New Providence (according to Hitchcock) : New York to 

 Florida and Mississippi ; Brazil. Large Bur-grass. 



3. Cenchrus microcephalus Nash; Hitchc. & Chase, Contr. IT. S. Nat. Herb. 



18:356. 1917. 



Culms at length prostrate and branching, up to 1 m. long, smooth and 

 glabrous ; leaf-sheaths glabrous, or often ciliate on the margins ; blades up to 

 1 dm. long, or those on the innovations longer, 2-3 mm. wide, rough, commonly 

 pubescent on the upper surface, ascending; spike 3-4 em. long, exserted; in- 

 volucres 8-12, 5-6 mm. long, 5 mm. in diameter or less, the larger spines broad 

 and flat, long-ciliate, the basal spines much shorter, not ciliate; spikelets 2, 

 much exserted beyond the body of the involucre. 



Moist soil, Little Harbor Cay and Frozen Cay, Berry Islands. Endemic. 

 Bahama Bur-grass. 



^ 4. Cenchrus viridis Spreng. Syst. 1: 301. 1S25. 



C. echinatus viridis Griseb. Fl. Br. W. I. 556. 1864. 



Culms erect or ascending, usually 6-8 dm. high, branched or simple, the 

 leaf-blades flat, 4 dm. long or less. Spike usually dense and cylindric, S-12 

 cm. long and often much exserted; involucres usually 3-spikeleted, armed below 

 with slender spines, the body about 4 mm. in diameter, the barbed bristles 

 slender. 



Cultivated land. Andros, New Providence, Eleuthera, Long Island and Mari- 

 guana : Florida : West Indies and tropical continental America. Green Bur-grass. 



