It is the lower most whorl of the flower, consisting of a number of sepals. These are usually green, somewhat resembling small leaves. Below the calyx, in some plants, there may be presence of epicalyx which is regarded as bracteoles rather than parts of a flower.
In the bud stage, flower is protected by the calyx. Like symmetry of flower, it may be regular or irregular. If the members are free from one another, it is termed as polysepalous or dialysepalous, as seen in mustard.
If the sepals are wholly or partially united with one another forming a tube like structure, it is called gamosepalous. Regular gamoscpalous calyx may be tubular, bell shaped or urceolate, etc. Irregular gamosepalous calyx is generally bilabiate.
In some, the sepals may be colored like petals and are called petaloid. These attract insects for pollination.