Fruits are enjoyed by many different animals, but they serve an important function for plants. A fruit is the mature ovary of a flower. Fruits protect seeds and aid in seed dispersal. Fertilization triggers hormonal changes that initiate the transformation of the ovary into fruit. In the absence of fertilization, flowers generally wither and do not develop into fruit. In seed-bearing plants, the structure that gives rise to female reproductive cells is called the ovule. The ovules usually develop into seeds as the ovary wall develops into the pericarp. In a typical fleshy fruit, the pericarp consists of three layers. The outer layer is called the epicarp, the edible middle portion is the mesocarp, and the innermost hard seed covering is the endocarp. In some fruits, such as pea pods, the pericarp is dry at maturity. In other fruits, such as peaches, the pericarp remains fleshy. Fruits can be categorized according to how they develop from carpels—the ovule-bearing parts of flowers. A fruit formed from one carpel, or fused carpels of a single flower ovary, is called a simple fruit. An aggregate fruit, by contrast, develops from multiple separate carpels of a single flower. A multiple fruit, on the other hand, is formed from the many carpels of the multiple flowers that make up an inflorescence. In accessory fruits, such as pears, organs other than the ovary largely contribute to fruit formation. Overall, a mature fruit allows a plant to propagate by protecting its seeds from environmental stressors and aiding in their dispersal.