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Key to Cenchrus

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1 Bristles plumose or antrorsely scabrous, free or fused < ½ their lengths.
  2 Primary bristles (immediately subtending each spikelet) scabrous.
    3 Panicles with 9-16 fascicles per cm of length; plants 0.3-1.2 m tall
    3 Panicles with 30-40 fascicles per cm of length; plants 2-8 m tall
  2 Primary bristles conspicuously long-ciliate.
      4 Spikelets 9-12 mm long
      4 Spikelets 2.5-7 mm long.
        5 Fascicles not disarticulating from the rachises; fascicles 33-160 per cm of inflorescence; panicles 4-200 cm long; leaves 7-70 mm wide
        5 Fascicles disarticulating from the rachises at maturity; fascicles 8-37 per cm of inflorescence; panicles 2-32 cm long; leaves 2-13 mm wide.
          6 Spikelets 4.5-7 mm long; leaves 2-3.5 mm wide, folded or conduplicate and superficially appearing even narrower; rachis pubescent
          6 Spikelets 2.5-5.6 mm long; leaves 2-13 mm wide, flat; rachis scabrous.
             7 Inner bristles fused for <¼ of their length; many outer bristles exceeding the spikelets; terminal bristles 10.5-23 mm long, noticeably longer than the other bristles in the fascicle
             7 Inner bristles fused for ⅓-½ of their length; outer bristles not exceeding the spikelets; terminal bristles 2.9-6.5 mm long, usually not noticeably exceeding the other bristles in the fascicle
1 Bristles glabrous, retrorsely scabrous, or strigose, usually at least some bristles fused > ½ their lengths.
               8 Involucre of bristles only, these not fused into a bur; perennial, to 2 m tall
               8 Involucre of spines fused into a coherent bur, sometimes also with bristles; annual or perennials, to 1 m tall.
                 9 Spines in a single whorl, subtended by numerous smaller, narrower, free outer bristles.
                   10 Spines fused at the base only, the lower surfaces with 1-3 grooves
                   10 Spines fused for >1/3 their length, the lower surfaces not grooved.
                     11 Rachis internodes 0.8-1.7 mm long; most of the outer bristles equal to or slightly longer than the flattened inner bristles (spines)
                     11 Rachis internodes 2-4 mm long; most of the outer bristles much shorter than the flattened inner bristles (spines)
                 9 Spines in multiple whorls or irregular in their disposition (if few and in a single whorl, then not subtended by smaller, narrower bristles).
                       12 Plants perennial, long-lived, clump-forming; burs not imbricate, usually glabrous; leaf blades 1-3.5 mm wide
                       12 Plants annual or perennial, short-lived and not clump-forming; burs imbricate, usually pubescent, leaf blades (1-) 3-14.2 mm wide.
                          13 Burs (excluding the spines) 9-16 mm long, 4-6 mm wide, the spines 4-8 mm long; spikelets 1 (-2) per bur, concealed; leaf blades 3-14 mm wide
                          13 Burs (excluding the spines) 5.5-12 mm long, 2.5-6 mm wide, the spines 2-7 mm long; spikelets 2-4 per bur, exserted at the tip; leaf blades 1-5 (-7) mm wide.
                            14 Spines slender, (30-) 45-75, 3.5-7 mm long; spikelets 6-8 mm long; most of the outer spines very slender (bristle-like) and ranging from patent to reflexed; inner spines terete, not or hardly flattened at their base (at most 1 mm wide); plant always annual
                            14 Spines stout, 6-10 (-40), 2-5 mm long; spikelets 3.5-6 mm long; bristle-like outer spines fewer to almost lacking, if present reflexed; inner spines stout and more or less conical, distinctly flattened with a base up to 3 mm wide; plant annual or (more often) perennial