Pennisetum is closely related to the genus Cenchrus, and the boundary between them is unclear.
Some species now in Pennisetum were once members of Cenchrus, and some have been moved back.
There are three kinds of bristle, therefore some species have all three, while others do not.
A main morphological character used to distinguish them is the degree of fusion of the bristles in the inflorescence, but this is often unreliable.
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