Flowers grouped into heads Asteraceae
Involucral bracts of the female heads or of the outer florets enclosing the fruit(s) at maturity and forming a burr (Fig. 42)
Burrs with hooked appendages
Burrs 5–8, whorled, each burr derived from a single ray floret enclosed in a bract Acanthospermum
Burrs solitary or few together in the axils of alternate leaves, each burr containing 2 fruits Xanthium