The male reproductive structures in flowering plants are called the stamens and the female structures are the pistils. In a stamen, there is the anther at the top and the stalk or filament, which holds it up. Inside the anther, pollen grains are produced. These contain sperm nuclei and are adapted to be transferred to female sex organs via wind, water, insects, birds. The pistil has three parts--the stigma, style and ovary. The stigma is sticky and during pollination, pollen grains stick to the stigma and begin to germinate forming a pollen tube. Inside the pollen tube, sperm nuclei travel down to the ovary through the style. Once inside the ovary, a double fertilization occurs in tiny structures called ovules. One sperm nuclei fertilizes an egg inside an ovule and forms an embryo plant. The other sperm nuclei fertilizes two polar bodies, to form endosperm, or a food supply for the embryo plant. Both the embryo and endosperm inside the ovule become the seed and the ovary develops into a fruit.