Despite the tremendous energy that plants put into flowering, reproduction really only begins when a pollen grain comes to rest on a flower’s stigma. If the pollen grain is compatible with the host flower, then pollen germination begins. In the case of many plants, the gametes are self-incompatible, meaning that the pollen must come from another plant.
Each individual grain of pollen consists of two cells. After transfer, one of these cells elongates, forming a long microscopic tube that penetrates down through the length of the style, growing closer to the ovules located at the base of the pistil. Elongation of this pollen tube is rapid, and is fueled by nutrients and hormones supplied by the pistil.
Eventually the pollen tube enters a tiny pore in the ovule which terminates at an unfertilized egg. While one cell of the pollen grain forms this pollen tube, the second cell divides to form two sperm cells. The first of these sperm cells travels down the pollen tube, completing fertilization of the waiting egg, and forming the zygote. The second sperm cell also travels down the pollen tube and combines with another cell in the ovule to form a food storage tissue called endosperm. Together the zygote and endosperm form the embryo—a miniature plant. As the embryo matures, it remains enclosed in the ovary, which slowly enlarges to form the completed seed.
When pollen is not transferred to the receptive stigma, no seed is formed. Many flowers, such as apples, may contain multiple ovules. If only a little pollen is transferred between flowers, not all of the eggs may become fertilized. The result is fewer seeds with correspondingly smaller fruit. These problems can occur when pollination vectors, such as bees, are absent.