No. 7. 

 CENCHRUS TRIBULOIDES L. 



Plant annual. Roots slender. 



Culms glabrous, a few inches to 3 feet high, usually branching from the base 

 and procumbent, rooting at the lower nodes, or sometimes the shorter plants erect. 



Leaves of the stem 3 to 10; sheaths glabrous, rarely ciliate on the margins 

 above, usually loose, commonly contiguous; blade 6 inches long or less, 1 to 2 lines 

 broad, flat or sometimes involute; ligule about -^ line long, deeply densely fimbriate. 



Inflorescence a short-pedunculate or partly sheathed spike of clusters, 4 inches 

 long or reduced to a single cluster, the rachis nearly smooth. Clusters composed 

 of 2 to 3 spikelets surrounded by an involucre. Involucre thick, coriaceous, cleft 

 to the base on the side next the rachis or on both sides, inclosing the spikelets: 

 the outer surface provided with numerous bristles and spines flattened below and 

 retrorsely barbed, those toward the base of the involucre smaller, the larger 2$ lines 

 long. 



Spikelets 2 to 3 in each involucre, 1 at least fully developed and with the 

 following structure (the others rudimentary in various degrees). 



Glumes 4; 3 lower membranaceous; first short, ovate, acute, 1- to 3-nerved, 

 empty; second broadly lanceolate, 5-nerved, acute, nearly as long as the involucre, 

 empty; third like the second, but subtending a flower; fourth (flowering) a little 

 larger and similar in form to the second and 'third, but thin-coriaceous. 



Flowers 2. Lower staminate, with a thin hyaline 2-nerved palet; stamens 3. 

 Upper with a thin-coriaceous palet, hermaphrodite; ovary flattened, circular in 

 outline; stamens 3. anthers ^ line long, dehiscing much earlier than those of the 

 staminate flower. 



Grain inclosed in the spikelet, spikelet inclosed in the involucre, the whole 

 disarticulating from the spike together. 



PLATE VII; a, external view of the involucre; b, the same cut open to show 

 the spikelets; c, a single fully developed spikelet opened to show the parts. On 

 the left are the second glume, the fourth glume, and the hermaphrodite flower; on 

 the right are the first glume, the third glume, and the staminate flower. The first 

 glume should be inserted below the second, and is represented twice too long, as 

 are the anthers of the hermaphrodite flower. 



This species is too common in sandy grounds, where its spiny burs are an an- 

 noyance to men and beasts and an injury to the wool of sheep that graze near it. 



