Parts of a Flower and Their Functions (With Diagram) The four main parts of a flower are the petals, sepals, stamen, and carpel (sometimes known as a pistil). If a flower has all four of these key parts, it is considered to be a complete flower. If any one of these elements is missing, it is an incomplete flower. Complete Rose Hibiscus Tulip Incomplete Holly Corn Squash Grasses Perfect vs. Imperfect