Why are daisies and sunflowers considered composite flowers?

Daisies and sunflowers are considered composite flowers because what appears to be a single flower is actually a compact inflorescence—a head containing many individual flowers, or florets.1

In members of the Asteraceae family, these heads can contain more than 1,000 individual florets. The structures that look like petals on a daisy or sunflower are not individual petals in the traditional sense; they are actually specialized ray flowers located in the outermost rows of the head. These ray flowers possess a modified, flat, and elongated corolla that mimics the appearance of a single petal.12

The center of these heads typically consists of disk flowers, which have regular, tubular corollas. Because the entire structure is a collection of numerous distinct flowers arranged to function and appear as one, they are referred to as composite.12