Today the involucre. It may be an unfamiliar word but everyone has seen an involucre. Defined as a ring or whorl or bracts immediately below the flowers, it is usually a ring of bracts (leaf-like structures that are often showy). The important thing is that the bracts are in a ring and immediately below the flowers. So the bracts that surround the the tiny flowers of a poinsettia are bracts, and do perform the function of attracting pollinators, but they do not form an involucre (although the euphorbia family is one that frequently has involucres and the greenish yellow bracts around many euphorbias ARE involucres). The commonest involucres are in the Apiaceae, the carrot family, where leafy bracts are found where the flowers originate at the base of the unbel. Usually green and not that decorative, the inflorescences of astrantias (above) are a colourful exception.
The ‘flowers’ of the daisy family (Asteraceae) have leafy bracts at the base, enclosing the flowers in bud, forming an involucre and in xerochrysum (above) they are coloured and papery.