Sexual Propagation
In nature, sexual propagation begins when water, wind, insects, birds, or small mammals carry pollen randomly between plants ( see Pollination ). In flowering plants, this transfer of pollen enables the male sex cells, or sperm, of one flower to fertilize the female sex cell, or egg, of a second flower (see Fertilization ). The egg is located at the base of the flower in a structure called an ovule, found within the ovary. Depending on the species, an ovary contains one, several, or many ovules. The ovaries of peach and avocado flowers, for example, have one ovule, while those of watermelon and cantaloupe have many. As the fertilized egg (or eggs) within the ovule begins to develop into an embryonic plant, it produces a variety of hormones that stimulate the outer wall of the ovule to harden into a seed coat. Other biochemical changes in the ovule produce a starchy substance that will be used as a food supply. In this way, the ovule ripens into a seed—a structure containing an embryoni