A pollen tube is a tube formed after germination of a pollen grain. The pollen tube carries the male gametes into the ovule (egg). In flowering plants each pollen tube penetrates the tissues of the stigma and style on its way to the ovary. After entering the ovary, each pollen tube grows toward an ovule, passing down a micropyle to the nucellus and finally penetrating the nucellus tissues into the embryo sac where it ruptures at the tip, releasing two male gametes, one of which fuses with the egg nucleus. The second male gamete fuses with the central fusion nucleus; the resulting nucleus, called endosperm, surrounds the developing embryo and provides food for its growth.