The calyx is the outermost whorl of a flower, which is comprised of leaf-like structures and protects the flower while it is developing. The calyx is usually green, and it is located just beneath the corolla, which is the next whorl of the flower. The calyx is made up of sepals, which are the functional units of the calyx. The sepals are often the same number as the petals, and they can be either separate or marginally fused, forming a tube with terminal lobes or teeth. The calyx plays an essential function for the flowering plant by protecting and preventing the drying out of the developing flower, and it continues to support fruit growth after the flower has bloomed.

In summary, the calyx is the protective leafy structure that encases a flower bud, and it is made up of sepals that protect the developing flower and support fruit growth after the flower has bloomed.