Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants (angiosperms). Typically, they are structured in four sets, called whorls, around the end of a stalk. These whorls include: calyx, modified leaves; corolla, the petals; androecium, the male reproductive unit consisting of stamens and pollen; and gynoecium, the female part, containing style and stigma, which receives the pollen, and ovary, which contains the ovules. When arranged in groups, with our without modified leaves (bracts), they are known collectively as an inflorescence. Flowers can be described systematically using both formulae and diagrams.