Flowers are the sexual reproductive structures in flowering plants or angiosperms. Some flowers have both male stamens and female pistils within the same flower and it is possible for self-pollination to occur. Other flowers are either male with only stamens, or females, with only pistils. These rely on wind, insects, birds, etc. to help transfer pollen from the top of the stamen, or anther, to the top of the pistil or stigma, thus accomplishing pollination. Flowers give rise to fruits and seeds. The bottom of the female pistil, or ovary, grows after fertilization and gives rise to the fruit. The seeds inside are a result of the sperm nuclei inside the pollen grain uniting with an egg cell to form the zygote and the other sperm nuclei fusing with two polar cells from the female ovule to form the endosperm(food) inside the seed. Flowers have the calyx or outermost whorl consisting of sepals which enclose the rest of the flower in the bud stage. The corolla is composed of petals which aids in attracting animals for pollination purposes. The next whorl is the stamens--these have the anther at the top and the stalk below. Pollen is produced by meiosis in the anther. The final innermost whorl of the flower is the carpels which form a holloow structure called an ovary. It contains ovules which into turn give rise to egg cells. Different species of flowers have different numbers and arrangements of stamens, pistils, petals and sepals.