68 



CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



to be the large-burred coastal plant which had been distinguished as C. macro- 

 cephalus. The name C. carolinianus Walt, was then applied to this species, 

 but Walter's diagnosis does not agree with its characters and it has not been 

 found in Walter's region. 1 



DESCRIPTION. 



Plants annual, sometimes forming large mats ; culms 20 to 90 cm. long, com- 

 pressed, rather stout, scabrous or rarely pubescent at the summit, spreading, 

 ascending or rarely suberect, from a decumbent base, usually freely branch- 

 ing; sheaths pubescent along the margin, rarely throughout, sometimes with a 

 tuft of white hairs at the summit, loose, those below the spikes commonly in- 

 flated ; ligule ciliate, nearly 1 mm. long ; blades usually flat but sometimes sub- 

 involute or folded, spreading, 3 to 15 cm. long, 2 to 7 mm. wide, tapering from 

 base to apex, scabrous on the upper surface and sometimes on the lower, often 

 pilose near the base above ; spikes numerous, short-exserted or partly included, 

 1 to 10 cm. long (commonly 3 to 8 cm. long), the burs rather crowded, the 

 slender axis flexuous, scabrous, sometimes pilose; burs (excluding the spines) 



Fig. 17. — Cenchrus pauciflorus. From the type specimen. 



3 to 7 mm. wide (commonly 4 to 6 mm.), pubescent, often densely so, rarely 

 nearly glabrous ; spines numerous, spreading or reflexed, flat, broadened at base, 

 the lowermost shorter and relatively slender, some of the upper ones com- 

 monly 4 to 5 mm. long, usually villous at the base ; body of the bur often with 

 one deep cleft on the outer face, the lobes commonly about eight, erect or 

 spreading or one or two inflexed, usually villous at the base, rigid and spine- 

 like ; spikelets commonly two, 5 to 7 mm. long, about 2 mm. wide ; first glume 

 usually not over one-third the length of the spikelet ; second glume and sterile 

 lemma subequal or the lemma nearly as long as the turgid acuminate-pointed 

 fruit. 



This species reaches its most characteristic development in the interior of the 

 United States and on the Mexican plateau, where it is a coarse weed 2 in sandy 

 ground, forming mats as much as 50 cm. in diameter. Eastward the species 

 appears to be introduced, though it seems to be native in Florida. On the 

 Atlantic coastal plain it is often more slender, with the blades sometimes 

 folded, approaching C. gracillimus in habit. In the Colorado Desert it is some- 

 times dwarfed, forming mats only 3 to 5 cm. in diameter, the spikes reduced 

 to one or two burs. In western Mexico and Central America specimens with 

 smaller burs (about 3 mm. wide, excluding the spines) are found, besides the 



1 See discussion, p. 76. 



2 A study of the barbs on the spines of this species and a speculation as to 

 the cause of the irritation produced by them when left in the flesh was pub- 

 lished by Gayle (Bot. Gaz. 17: 126, 127. 1892). 



