Although they are not technically part of the flower the bracts serve an important role in the pollination of the plant. They act as kind of a beacon for pollinators that might otherwise miss the small white flowers as they fly by.
Daisies are another type of plant with a similar structure. What is commonly called a daisy flower is actually a cluster of small flowers (called the Capitulum) surrounded by bracts. In this image the bracts are white and the capitulum is yellow.
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