In these photos, the flower (left) has a gynoecium of several distinct, green carpels. These ripen into a cluster of separate achenes (right). As the achenes mature, they turn brown and fall from the plant (bottom).

In the bottom photo, note that 2 achenes are shown in longitudinal section to show the single seed in each. The achene wall is formed from the carpel wall and is free from the seed coat.

Most (but not all) achenes come from a single distinct carpel.