Fruits can be classified, based on the nature of the pericarp, into two groups: fleshy and dry. Fleshy fruits, in turn, are classified into several types, including drupes, berries, pomes, hesperidia, and pepos. Dry fruits are also subdivided into several categories, including follicles, legumes, capsules, achenes, nuts, samaras, schizocarps, and caryopses.
The three most familiar types of fleshy fruits are drupes, berries, and pomes. A drupe is a fleshy fruit that contains a single seed surrounded by a hard, bony inner wall of the pericarp (called the endocarp). The middle and outer walls of the pericarp (called the mesocarp and exocarp, respectively) are juicy and often sweet. Drupes include all the pitted fruits, such as cherries, plums, peaches, and olives. A berry typically has several seeds, and the pericarp is fleshy throughout. Familiar examples include tomatoes, eggplants, and grapes. A pome is a fleshy fruit, often with many seeds, that has a thick layer of accessory tissue immediately surrounding the pericarp. The accessory tissue is generally juicy, sweet, and often edible. Representative pomes include apples and pears.
Two other fleshy fruits, the hesperidium and the pepo, are characterized by a leathery rind. Hesperidia, also known as a citrus fruits, have rinds rich in aromatic oils surrounding a juicy interior composed of wedge-shaped segments that have a sugary, acidic sap. Hesperidia include oranges, grapefruits, lemons, and limes. The pepo has a tough exocarp which is either smooth or variously sculptured and normally contains many seeds. Examples include cucumbers, cantaloupes, and squash.
Some common dry fruits are follicles, legumes, capsules, achenes, nuts, samaras, schizocarps, and caryopses. Follicles are podlike fruits that open up along one side, revealing numerous seeds. Examples include milkweed pods and the aggregate follicles of magnolias. In contrast, legumes are podlike fruits that open up along two lines, releasing several seeds. They are produced by many members of the legume family, such as peas, beans, and peanuts.
The capsule opens along three or more lines or by pores at the top of the fruit. Lilies and poppies are good examples of plants that produce capsules. Achenes each contain a single seed that is free inside the cavity, except for an attachment at one end called the funiculus, and are typically small. A good example is a sunflower “seed.” Anut is similar to an achene, except that the pericarp is hard and fibrous and is derived from a compound ovary. Representative nuts include acorns, hazelnuts, and hickory nuts. A samara is a modified achene that has part of the pericarp flattened to form a wing. Examples of plants that produce samaras include ashes and elms.
Maples have a winged fruit, called a schizocarp, which is often mistaken for a samara. Close observation reveals that the schizocarps come in attached pairs that later split into single-seeded portions. Schizocarps, which are generally not winged, also occur in the parsley family, where they may split into more than two parts. Finally, a caryopsis is a single-seeded fruit whose seed coat is fused to the pericarp. Caryopses are produced only by plants in the grass family and include the familiar grains wheat, corn, rice, and oats.
The fruits listed above commonly fall under the category of simple fruits. In other words, they are identifiable as individual structures. Other plants produce fruits in dense clusters, and these are termed either aggregate fruits or multiple fruits. Aggregate fruits are produced by a single flower that has numerous pistils. One example is a raspberry, which is an aggregate of drupes which are often referred to as drupelets because of their small size. Another is the strawberry, not a berry in the botanical sense, which is an aggregate of tiny achenes attached to the surface of a swollen, juicy, receptacle (originally the base of the flower where all the flower parts were attached). In contrast, multiple fruits are produced by clusters of small flowers, each of which produces a single fruit. Representative multiple fruits include mulberries, figs, and pineapples.