When the Spanish conquistadors marched into Mexico City, formerly known as Tenochtitlan, the highly civilized Aztec Indians living there were growing zinnias in their gardens. Most of today's many varieties and hybrid strains of Zinnia are derived from three wild Zinnia species, all from Mexico. The one at the right is Zinnia angustifolia.
As the picture shows, Zinnia flowering heads are composed of two types of flower: disk flowers in the center with yellow corolla lobes in this species, and; ray flowers, which in this species comprise the pinkish outside "petals."
One feature making a Zinnia a Zinnia is that the green scales, or involucral bracts, below the flowering head are broad, rounded, and they overlap. You might recall that the bracts below marigold flowers are typically united along their edges, forming a kind of cup.
At the right the flowering head is broken apart. Now you can see how the disk flowers are packed together in the center, anchored on a conical to cylindrical receptacle -- the receptacle being the white, pitted, golfball-like hump at the center in the bottom part of the picture. If you look very closely and halfway know what you're looking for, you can see that wedged between the flowers are long, slender, cellophane-like items often known as chaffy scales. A better view of a chaffy scale is provided below. Chaffy scales help protect the flowering head's developing flowers and fruits, and their presence is very important to note if you're trying to determine whether you have a Zinnia or not. Many common, similar looking flowers don't have these chaffy scales.
At the left, the expanded white items at the bottom of each flower are immature fruits, or achenes. Yet another identification feature for Zinnias, at least for the cultivated forms, is that at the top of the immature fruits there is no pappus -- no hairs that later will become the "parachute" atop the seed, and no spines that later will help the seed stick to animal fur.