The structure that protects the developing reproductive parts of the flower. In dicotyledons it normally consists of two distinct whorls, the *calyx
and the *corolla
. In many monocotyledons (such as the tulip) these whorls are not differentiated and the individual perianth units are then termed *tepals
. The perianth units tend to be simple, separate, and inserted spirally in the more primitive families, such as the Magnoliaceae and Papaveraceae. In the more advanced families, such as the Labiatae and Compositae, they are often fused and inserted in whorls. The opening and closing of some flowers is controlled by temperature and light, these stimuli being perceived by the perianth segments.