What is the function sepals in the unopened flower?

Sepals, which form the first part of the flower termed as calyx, are those small green petals that nourish and protect the flower while it still is enclosed in a bud and has yet not developed into a full-fledged flower.

In a simple flower's structure, there are rings of leaves around it. The outermost circle consists of five green, leaf-shaped sepals, which are guarding the flower. Inside the sepals is another ring of colourful petals, which are usually heart-shaped and have flaps at their bases. This is the area where nectar is produced to attract bees and other insects.

Both the sepals and petals are attached at their bases to the receptacle. A receptacle is the swollen end of the flower stalk, appearing like a cone in the midst of a flower. Most flowers are built in this fashion, however the variations in colors, shapes and sizes varies with every different kind of flower.