76 



FIRST BOOK OF GRASSES 



branchlets found in the millets, the sand-burs have a 

 complex fascicle of many compound branches. The 

 primary branches disarticulate from the axis, and 

 the whole fascicle, or bur, falls entire with the spike- 

 let permanently inclosed, the grain eventually 

 germinating within it. The simplest form of sand- 

 bur is in Cenchrus myosuroides (Fig. 70) . The struc- 

 ture of this will be comprehended if we conceive of 



Fig. 70. Bur of Cenchrua 



myosuroides. 



Fig. 71. Bur of Cenchrus pauciflorus. 



a dense fascicle of branches with but one of them 

 bearing a single sessile spikelet, the others branching 

 at the base and surrounding the spikelet-bearing one. 

 In the conMnon inland sand-bur, Cenchrus pauci- 

 florus (Fig. 71), the sterile branches are much thick- 

 ened and flattened and are grown together below, 

 their free summits sharp and spine-like. The nu- 

 merous secondary branchlets, in the form of stout 

 spines, spread from the body of the bur formed by 

 the cohesion of the main branches surrounding the 

 one or few sessile spikelets. 



