Flowers first evolved between 150 and 190 million years ago, in the Jurassic. Plants with flowers replaced non-flowering plants in many ecosystems, as a result of flowers' superior reproductive effectiveness. In the study of plant classification, flowers are a key feature used to differentiate plants.

The structure of a flower, termed its morphology,[13] can be considered in two parts: the vegetative part, consisting of non-reproductive structures such as petals; and the reproductive or sexual parts. A stereotypical, or complete.

The sepals, collectively called the calyx, are modified leaves that occur on the outermost whorl of the flower. They are leaf-like,[19] in that they have a broad base, pores, green pigment, and may have analogous outgrowths from the stem. Sepals are often waxy, tough, and grow quickly to protect the flower as it develops.