Fruit

I

Introduction

Fruit, seed-bearing structure of a flowering plant. A fruit is actually a ripened ovary, a component of the flower’s female reproductive structure. Fertilization of the egg, or female sex cell, within the ovary stimulates the ovary to ripen, or mature. Depending on the type of plant, the mature ovary may form a juicy, fleshy fruit, such as a peach, mango, apple, plum, or blueberry. Or it may develop into a dry fruit, such as an acorn, chestnut, or almond. Grains of wheat, corn, or rice also are considered dry fruits. Certain foods commonly termed vegetables, including tomatoes, squash, peppers, and eggplant, technically are fruits because they develop from the ovary of a flower.

Fruits are vital to humans. Worldwide, over 475 million tons of fruit are produced each year, with China, India, Brazil, and the United States counted among the top producers. Corn, wheat, rice, and other grains were staple foods in early civilizations and are still a basic part of the human diet worldwide. Ten out of the eleven most important food crops in the world today are grains. Many of these dry fruits serve as the raw materials for important industries since they are processed into bread, beverages, and alcohol. Grains also are used as feed for livestock. Today the livelihood of countless farmers is directly related to the raising of these crops or the raising of the cows, pigs, sheep, and other animals that feed on them. Over 1.8 billion metric tons of the major grains—corn, wheat, and rice—were produced worldwide in 2002.

Many species of mammals, birds, and insects rely on fruit as an essential component of their diet. Fruits also play a critical role in dispersing seeds, increasing the likelihood that at least some will land in an environment favorable for germination, or sprouting, which helps to perpetuate the plant species. Birds, for example, help distribute seeds when they feed on berries. The seed within the berry passes through the bird’s digestive tract and is deposited at a location where, if conditions are favorable, it will flourish. Most of the 250,000 known species of seed-producing plants have survived because their fruits serve as the vehicles that help spread seeds.

Fruits come in a wide range of sizes, shapes, and colors. The tiny floating plants known as water-meal are only 1 to 2 mm (0.04 to 0.08 in) wide and have very small flowers. Their miniature ovaries develop into fruits that are barely visible. Watermelons, on the other hand, may be more than 60 cm (24 in) long and weigh more than 20 kg (45 lb). Jackfruits, found in the tropics, can weigh more than 40 kg (90 lb). Diverse fruit shapes abound, including round grapefruits, oval eggplants, elongated bean pods, and star-shaped star anise. Pale to intense shades of orange, red, yellow, and even purple can be found in fleshy fruits, while dry fruits display many tones of brown.

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II

Fruit Formation

Fruits develop from the ovaries of flowers. As the reproductive organs of plants, flowers typically contain both female and male structures. The female part of the flower is located at its center and is called the pistil. Often resembling a tiny vase, the pistil is composed of three regions—a sticky or feathery stigma at the top, a narrow neck called the style in the middle, and the round or oval-shaped ovary at the base. The ovary contains one or more whitish ovules, each of which contains an egg. If the egg is fertilized, the ovule under most conditions matures to become a seed. At maturity, the ripened ovary, or fruit, encloses the ripened ovules, or seeds. The fruit and seeds are typically hundreds or even thousands of times larger than the ovary and ovules from which they develop.

The male parts of the flower are called the stamens. Each stamen consists of a thin stalk, called the filament, capped by the anther, which is a chamber where pollen is produced. Flowers typically have several stamens arranged in various patterns around the pistil.

The process of fruit formation begins with pollination, the transfer of pollen from the anther of one plant to the stigma of another, which prepares the plant for fertilization. Once pollen is deposited on the stigma, fertilization may occur if certain conditions are favorable. For instance, the pollen grain must land on the stigma of a flower of the same or a very closely related species, and the pollen and stigma must be at the right stage of development. The flowers of some plants, such as peas, can be fertilized from their own pollen (known as self-pollination), but most species require pollen from different plants (called cross-pollination) for fertilization to take place.

If conditions are favorable, chemicals in both the stigma and the pollen grain trigger events leading to fertilization. The pollen grain germinates on the stigma and produces a pollen tube that grows through the stigma and into the style. Two sperm cells develop within the lengthening pollen tube.

The pollen tube grows through the ovary and enters the ovule, where it releases the two sperm cells. One sperm fuses with the egg, which accomplishes fertilization. The fertilized egg grows into an embryonic, or immature, plant, which is fed by the endosperm, a starchy food that develops from the union of the second sperm with two structures in the ovule. As the fertilized egg develops, hormones are released that cause tissues of the ovule to enlarge. A seed is formed that consists of the ripened ovule, the enclosed embryonic plant, and the endosperm. In many species, the endosperm also provides some of the energy used by the embryonic plant for germination.

Ovules produce hormones—especially ethylene—as they develop into seeds. These hormones cause the ovary surrounding the ovule to expand and ripen into a fruit. Thus, fruit and seed development is carefully synchronized. As fruits and seeds develop, the flower’s petals, stigma, style, and stamens typically dry up. The flower is no longer needed since it has produced the seeds that will grow into new plants.

In nature, a few types of flowers do not require pollination and fertilization to produce fruits. These fruits are termed parthenocarpic, and they do not contain seeds (see Parthenogenesis). A navel orange is an example of a naturally occurring parthenocarpic fruit. Parthenocarpic fruit can be artificially produced by treating a flower with hormones. Seedless tomatoes are produced in this way.

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.