The wall of an ovary of mature fruit, in which the seed is fully developed, has three layers. The outer layer is called the exocarp, the middle layer is known as the mesocarp, and the inner layer is the endocarp. The three layers together are called the pericarp.
Botanists classify fruits into two main groups: (1) simple fruits and (2) compound fruits. A simple fruit develops from a single ovary, and a compound fruit develops from two or more ovaries.
Simple fruits are by far the largest group of fruits. They are divided into two types, depending on whether their pericarp is fleshy or dry.
Fleshy simple fruits include most of the seed-bearing structures that are commonly called fruits. The three main kinds are: (1) berries, (2) drupes, and (3) pomes.
Berries have an entirely fleshy pericarp. Botanists classify bananas, blueberries, grapes, green peppers, muskmelons, oranges, tomatoes, and watermelons as berries. Some berries, including watermelons and muskmelons, have a hard rind. Such fruits are called pepos. Other berries, including the citrus fruits, have a leathery rind. They are called hesperidiums. Raspberries, strawberries, and most of the other fruits commonly known as berries are actually compound fruits.
Drupes have an exocarp that forms a thin skin. The endocarp develops into a stone or pit, and only the mesocarp is fleshy. Such fruits include apricots, cherries, peaches, and plums.
Pomes are fleshy fruits with a paperlike core. Apples and pears are pomes.
Dry simple fruits include the pods of the bean plant, the milkweed, the pea plant, and the locust tree; the grains of the corn, rice, and wheat plants; and nuts. Botanists regard nuts as single-seed fruits with a hard pericarp called a shell. The seed is the edible part. Acorns, chestnuts, and hazelnuts are true nuts. But many so-called nuts are classed otherwise by botanists. For example, almonds are the seeds of drupes.
Compound fruits consist of a cluster of ripened ovaries. There are two main types of compound fruits, aggregate fruits and multiple fruits. Aggregate fruits develop from single flowers, each of which has many ovaries. Blackberries and raspberries are aggregate fruits. The strawberry is a special type of aggregate fruit. Each "seed" in a strawberry is actually a complete fruit. The flesh surrounding the seeds develops from the base of the flower rather than from the ovaries. Multiple fruits develop from a cluster of flowers on a single stem. Figs, mulberries, and pineapples are multiple fruits.