Zinnia, sunflower, yarrow, black eyed susan, goldenrod, ageratum, cosmos, and many other flowers are in the family Composite (kom-POS-uh-tee), that is, composed of ray flowers, and disk flowers.  The center of the zinnia is made up of disk flowers.  With a magnifying glass, you can see that the small yellow parts in the center are actually tiny flowers.  The ray flowers are what we incorrectly call petals. The seeds of the zinnia is found attached to the dried ray flowers when they are pulled outward from the center.