When a pollen grain lands on or is carried to the receptive tissue of a pistil known as the stigma, the flower has been pollinated. But this is only the first step in a complicated process that, if successful, leads to fertilization. The pollen grain contains two cells-a generative cell and a tube cell. The generative nucleus divides to form two sperm nuclei. The tube cell grows down into the pistil until it reaches one of the ovules contained in the ovary. The two sperm travel down the tube and enter the ovule. There, one sperm nucleus unites with the egg. The other sperm nucleus combines with the polar nuclei that exist in the ovule, completing the process known as double fertilization. These fertilized nuclei then develop into the endocarp, the tissue that feeds the embryo. The ovule itself develops into a seed that is contained in the flower's ovary (which ripens into a fruit). In gymnosperms, the ovule is exposed (that is, not contained in an ovary), and the pollen produced by the male reproductive structures lands directly on the ovule in the female reproductive structures. Fertilization in conifers can be slow in comparison to flowering plants-the pollen nuclei of pines, for example, take as long as 15 months to reach the ovule after landing on the female cone. And there are variations: In the ginkgo, the ovules fall off the tree and pollination occurs on the ground.