HITCHCOCK AND CHASE — NORTH AMERICAN GRASSES. 



73 



times as much as 3 mm., broad, some of them 5 to 8 cm. long, long-villous on the 

 inner face and margins of the broad base, the hairs of the margin rather stiffly 

 spreading, the ends needle-like and retrorsely barbed ; body of the bur with no 

 deep cleft on the outer face, the tips of the spikelets usually not showing above 

 the base of the clefts, the lobes six to eight, mostly about equal and simulating 

 the larger spines, erect to spreading, villous on the inner face and on the mar- 

 gins at the base like the spines, the outer surface glabrous or nearly so above 

 the base ; spikelets usually two, 7 to 8 mm. long, about 3 mm. wide ; first glume 

 about one-third the length of the spikelet; second glume sometimes minutely 

 puberulent on the lower part of the middle internerves, slightly shorter than 

 the sterile lemma, this slightly shorter than the acuminate-pointed fruit. 



Fig. 19. — Cenchrus tribuloides. From Amer. Gr. Nat. Herb. 621, Virginia. 



Cenchrus tribuloides usually is readily recognizable by its short-jointed, 

 leafy, decumbent culms and large woolly burs. In Chase 4531 from South 

 Carolina and in most of the specimens from the West Indies, however, the 

 burs are not conspicuously villous, the pubescence being scarcely or not at all 

 longer or more copious than in C. pauciflorus. In the specimen f-om Costa 

 Rica the burs are nearly glabrous. Because of the habit of the plants and 

 because of their large burs, with bodies not deeply cleft and with hidden or 

 nearly hidden spikelets, these specimens are referred to C. tribuloides. In 

 Shafer 2737 from Cuba, Millspaugh 1162 from Cayman Brae, and Chase 6561 

 from Porto Rico, the burs are scarcely larger than in extreme specimens of 

 C. pauciflorus, and some of them are slightly cleft, showing the upper part of 

 the spikelets. It is a puzzling fact that in the West Indies, at the eastern edge 

 of the range of C. tribuloides, this species and C. pauciflorus, whose center of 

 distribution is far to the west of that of C. tribuloides, approach each other, 

 while in the Gulf States, where their ranges meet, they do not. 



115803—20 6 



