III.

Types of Fruits

Fruits vary widely in physical appearance, but their basic structure is the same. The ripened ovary wall that forms the fruit is composed of three layers, which can be thick or thin, dry or moist, fused or separate. The outermost layer is called the exocarp; the middle layer is the mesocarp; and the inner layer, the endocarp. These three regions together are called the pericarp. A peach that has been cut in half clearly reveals these layers: the soft, fuzzy skin is the exocarp, the juicy, meaty part of the peach is the mesocarp, and the pit, which encases the seed, is the endocarp.

The vast variety of colorful and interesting fruits on earth poses a challenge for scientists who seek to classify them. One widely used classification system divides fruits into fleshy and dry types. Generally speaking, if the exocarp and mesocarp of a fruit are juicy or moist at maturity, the fruit is classified as fleshy. A dry fruit is characterized by a dry exocarp and mesocarp, which is often thin and compressed.

A.

Fleshy Fruits

Differences in flower structure result in several types of fleshy fruits. The fruit may form from a flower with a single pistil—the most common case—or it may develop from a flower with several pistils. The ovary may have a single compartment, or carpel, which houses the ovule or ovules. Or the ovary may consist of two or more carpels, each of which may contain one or more ovules.

A drupe develops from an ovary with a single carpel and is characterized by an edible exocarp and mesocarp and an inedible, hard endocarp, or pit that encloses a single seed. Cherries, peaches, apricots, and plums are examples of drupes. Almonds also are classified as drupes, but in almonds, the fleshy exocarp and mesocarp wither at maturity, and the endocarp, or shell, is cracked to obtain the edible, meaty, seed. Coconuts are drupes with a very fibrous, inedible exocarp and mesocarp. They are unusual in that they contain both a liquid and solid endosperm. The watery fluid often referred to as coconut milk is used as a drink, and the solid endosperm, or whitish coconut meat, is an important food in the tropics.

A berry develops from an ovary containing one or more carpels. Each carpel contains one or more ovules, so berries typically contain more than one seed. Examples include grapes and gooseberries. A tomato also is classified as a berry. Cut in half, a tomato displays distinct sections, each representing a separate carpel with many seeds.

There are several different types of berries. A true berry has a relatively soft pericarp with a thin exocarp or skin. Examples of true berries include the tomato, pepper, eggplant, grape, and persimmon. A pepo is a berry with a comparatively thick exocarp, or rind—cantaloupe, watermelon, pumpkin, cucumber, and squash are pepos. A hesperidium is a berry with a leathery skin containing oils. All citrus fruits, including oranges, lemons, and grapefruits, are hesperidiums. Some fruits with the word berry in their names, such as raspberry and strawberry, develop differently and are not really berries at all.

Most fruits consist of just the mature ovary and its seeds, but in some fruits other flower structures are also part of the fruit. A pome is a fleshy fruit composed of the mature ovary along with other flower parts. These flower parts can include the petals, the colorful leaflike parts of the flower; the sepals, the small, green leaflike structures at the base of the petals; and the receptacle, the enlarged tip of the flower stem to which the flower is attached. Depending on the species, some or all of these flower parts grow and expand around the ovary forming a pome. Apples, pears, and quinces are pomes. An apple cut in half shows the enlarged, whitish, edible parts of the sepals and petals.

In flowers with more than one pistil, the pistils are adjacent and the ovary of each pistil develops into a tiny fruit, or fruitlet. The clumped fruitlets form a fruit, such as a raspberry, called an aggregate fruit. Each little bump on a raspberry represents the ripened ovary from one pistil.

In plants such as the pineapple, several flowers are clustered on one stem. Although the ovaries develop individually, all the fruitlets combine into a single larger fruit called a multiple fruit. Other multiple fruits include mulberries and Osage oranges.

B.

Dry Fruits

Dry fruits are classified by whether they remain intact at maturity or open to release seeds. There are several types of intact, dry fruits. In samaras, the pericarp is light, relatively thin, and partly or completely fused to the seeds. It enlarges slightly, forming one or two small wings that aid in wind dispersal. Maples, ashes, and elms produce beautiful samaras that can be seen twirling slowly in gusts of wind. Nuts, on the other hand, have a relatively hard, heavy pericarp. Examples include chestnuts, hazelnuts, and acorns. In achenes, the seed is loosely attached to the pericarp, and the pericarp can be separated from the seed, as in sunflowers. Buckwheat and buttercups also produce achenes. A caryopsis, or grain, is a fruit in which the pericarp is tightly fused to the seed. A schizocarp is a twin fruit that separates into two one-seeded mericarps, types of fruit that often have tiny oil tubes in their walls. Examples include caraway and dill, whose fruits are harvested for their flavorful oils.

Dry fruits that open at maturity fall into several categories. Legumes, such as peas and beans, are one-chambered pods that split apart along two seams, exposing the seeds that lie within. Siliques and silicles split along two seams but contain two chambers. Siliques are more than three times longer than they are wide, while silicles are shorter. Broccoli, cabbage, and wallflower produce siliques, while dollar plants and alyssum produce silicles. A follicle, on the other hand, splits along one seam only. Milkweed, columbine, and larkspur produce follicles. Capsules split open in several ways, often along or between three or more seams, or between the top and bottom halves, as in primrose. Certain poppies produce capsules with rows of pores that release seeds when the capsule is shaken by the wind.