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Fruits

Fruits

Fruits are produced only by flowering plants (angiosperms). Following pollination of the flower, the fertilized ovules develop into seeds while the surrounding ovary wall forms the fruit tissue, or pericarp.

Types of Fruits

The botanical definition of a fruit is an organ that contains seeds, protecting these as they develop and often aiding in their dispersal. This may be at odds with everyday usage of the word "fruit." Botanically, pineapples, oranges, and apples are fruits, but so too are "vegetables" like tomatoes and cucumbers. The pods that contain peas and beans are fruits, as are the dry, inedible structures that bear the seeds of many wild plants.

MAJOR FRUIT TYPES
Type Definition Examples
SIMPLE From a single pistil  
DRY INDEHISCENT At maturity dry; does not split open  
Achene Close-fitting pericarp surrounding a single seed Sunflower
Grain Close-fitting pericap fused to a single seed Corn, wheat
Nut Thick, woody pericarp surrounding a single seed Walnut, hazelnut
DRY DEHISCENT At maturity dry and splits open  
Legume Pod that splits along two opposite sides Beans, peas
Capsule Fruit opening by several splits or pores Cotton, poppy
Schizocarp Fruit splitting into 1-seeded segments Dill
FLESHY Mostly fleshy at maturity; do not usually split open
Drupe 1- to 2-seeded; the innermost pericarp layer, stony and enclosing the seed(s) Plum, peach
Berry 1- to many-seeded; no stony innermost layer of pericarp Tomato, grape, (all citrus fruit are berries of a special type termed a hesperidium)
AGGREGATE Formed by fusion of several separate pistils of one flower Raspberry, cherimoya
MULTIPLE Formed by fusion of several separate pistils of several grouped flowers Pineapple, fig

There are many ways to classify fruits, but the simplest distinction is between fleshy and dry fruits. Fleshy fruits are made of living cells and are often juicy and sweet (oil-rich olives and avocados are exceptions). Dry fruits at maturity are made up of dead cells and are divided into those that split open (dehiscent fruit) and those that do not (indehiscent fruit). Within these broad categories many specialized fruit types are recognized. Nuts (for example, hazelnuts and pecans) are one such category, defined as dry, indehiscent fruits with a hard, stony wall. Everyday usage of the word "nut" is, however, quite different, and includes cashew nuts and peanuts (which are actually seeds not fruits).

True fruits are formed by the ovary, which is the lower region of the pistil and the female sex organ of the flower. Sometimes the bulk of the fruit is not derived from the ovary but from some other part(s) of the flower. Such fruits are termed false fruits or accessory fruits. Strawberry is a good example of this. The fleshy tissue people consider the fruit is derived from the receptacle (the swollen tip of the flower stalk), and the true fruits are the tiny, seedlike achenes on the surface.

Apart from strawberry, all the fruit types discussed so far are simple fruits derived from single pistils. In contrast to simple fruits are aggregate and multiple fruits, which are formed from many pistils and, in turn, many ovaries. Aggregate fruits like raspberries and blackberries are formed from the several ovaries of a single flower. Multiple fruits like pineapples and mulberries develop from the fusion of the ovaries of several flowers. Interest-ingly, some fruits (such as banana) develop without seed formation, a phenomenon termed parthenocarpy.

Dispersal

Fleshy, edible fruits serve as food for animals. Animals in turn spread the enclosed seeds of the fruits they eat and so disperse what will be the next generation of that plant. The coconut provides a good example of a fruit adapted for dispersal by water. Its corky, buoyant outer layer allows this fruit to be carried great distances by ocean currents before the seed within germinates on the seashore. Many dry, dehiscent fruits split explosively, flicking their seeds into the air where they are carried by the wind. Some fruits may have spines for attachment to animal fur, whereas others are winged or feathery for wind dispersal.

Economic Importance

Many fleshy fruit are major food crops of great economic importance. Prime areas of cultivation may be far removed from the original "home" of that particular plant; for example, Citrus species like orange are native to Asia, as are apples. Fruits, like other types of produce, comprise living tissue and require special handling and storage to ensure optimal quality for the consumer. Ripening of fruit involves a range of processes that ultimately make the fruit more attractive for consumption, such as color change, softening, sweetening, and aroma production.

Physiologically, fleshy fruit fall into two categories: climacteric and nonclimacteric. Climacteric fruit can be picked mature but unripe and then stored for extended periods at low temperature before being ripened and sold. Such fruit include mangoes, bananas, papayas, avocados, and tomatoes. Special methods for handling such fruits allow tropical fruits grown thousands of miles away to be on sale weeks later in supermarkets in temperate regions with no apparent loss of quality. Ripening of climacteric fruit is triggered by the gaseous plant hormone ethylene, and this is exploited by shippers to artificially induce fruit ripening. In several fruit crops, including tomato, it has been possible to use genetic engineering to knock out ethylene production thus preventing ripening and extending the shelf life of the fruit.

Nonclimacteric fruits such as grapes, citrus, and strawberries do not respond dramatically to ethylene as is the case of climacteric fruits. These fruits ripen only while still attached to the parent plant and so cannot be picked early and stored for later ripening.

see also Angiosperms; Flowers; Hormones, Plant; Pollination and Fertilization; Seeds

C. M. Sean Carrington

Bibliography

Simpson, Beryl Brintnall, and Molly Conner Ogorzaly. Economic Botany: Plants in Our World. New York: McGraw-Hill, 1986.

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Fruits

Fruits

The fruit is the mature ovary and its associated parts. This unique covering of the seeds of flowering plants gives this group of plants their name: angiosperms . Fruits are formed by the enlargement and maturation of the pistil . Common examples of fruits include apples, oranges, grapefruits, lemons, grapes, peaches, plums, cherries, pineapples, and pears. Commonly, when people think about fruits, they think only of the fleshy and flavored fruits. In the botanical sense, however, any flowering plant that produces seeds also produces a fruit that contains the seed. Using the botanical definition, a number of so-called vegetables are actually fruits, including green peppers, tomatoes, cucumbers, squash, beans, and even such grains as corn, rice, and wheat. Fruits may also be hardened, such as walnuts, pecans, acorns, and coconuts. Horticulturalists define fruit crops as those that bear fleshy and flavored fruits. These fruit crops are grown on trees that require years of cultivation , but in some plants of the mustard family, the entire life cycle of the plant may be as brief as a month, including fruit formation.

Fruits are formed from the ovary alone in many plants, but in other plants, adjacent tissues that are not part of the pistil may become part of the fruit. These nonpistillate tissues form accessory parts of the fruit. In strawberries, the receptacle, which holds the tiny pistils, forms the fleshy part of the fruit. Pears and apples are two related examples of fruits formed by a floral cup located next to the ovary.

Function

The function of the fruit in flowering plants is to protect the seeds and to facilitate their dispersal. Fleshy fruits are usually edible and are dispersed after being eaten. Color changes in fruit often signal ripeness and make ripe fruit easy to see. When fruits are eaten, the seeds pass through the gut mostly unaffected. In fact, in some cases, the partial breakdown of the seed coat stimulates germination. Excreted seeds are surrounded by nutrients (including minerals and simple organic compounds in the dung) along with a substrate for early growth. Of course, this strategy fails if the seeds are actually digested. As a plant defense mechanism, some plants produce chemicals that can make an animal ill if too many are digested. Some of the plants modified by man no longer have such chemical defenses.

Dry fruits are characteristic of seeds dispersed by the wind and other natural agents or animals. These fruits may have barbs, hooks, or a sticky surface that catches the coat of a passing animal and disperses the seeds. Winged fruits or those with tufts of hair are designed to catch the wind (for example, dandelions). Fruits of seaside plants often float and are resistant to water damage. The occurrence of coconut palms on seashores around the world is evidence of the success of this strategy. A fibrous husk traps air and conveys the hard seed to the high-water mark on a beach where the plant can become established. Some plants, particularly those of the California chapparal, thrive after fire. Fire-adapted fruits not only withstand fire temperatures, but may need them to trigger seed release.

Dry fruits often have specific adaptations for seed release. Many dry fruits are dehiscent, forming openings at specific locations in the wall. How the fruit opens determines whether seed release occurs slowly or all at once. Some dry fruits are indehiscent and do not open at all prior to seed germination.

Fruit Development and Ripening

The formation of fruits is typically triggered by sexual fertilization. Many changes occur in the flower accompanying fertilization, including the loss of petals, anthers, and stigma, modification or shedding of the sepals , development of the ovules into seeds, and formation of the embryo and endosperm within the seed. As part of fruit formation, the walls of the ovary surrounding the seeds are stimulated to resume cell division and to expand. The differences in the size and shapes of fruits are limited to some extent by the structure of the flower, and to an even larger degree by later patterns of growth.

Some fruits do not form seeds, such as bananas and seedless grapes. Fruits produced without seeds are examples of so-called parthenocarpic fruits, in which ovules (precursors of seeds) are formed but do not successfully fertilize.

Plant hormones play an important role in the formation and maturation of fruit. Fleshy fruits grow and thicken in response to hormonal growth signals emitted by fertilized seeds. In strawberries, for example, seed formation is highly successful except for the tip of the fruit, which is poorly developed. Where seeds are underdeveloped, so is the fruit. The stimulus for fruit production in this plant can be replaced by a plant hormone known as auxin, which is often produced by developing seeds. Fruit maturation and the development of fruit color are triggered by a later-occurring hormonal signal, produced by the gas ethylene. For grocery stores, fruit is often picked before becoming ripe because unripe fruit is not as easily bruised. To ripen the fruits for sale, a human-made gas related to ethylene is used after harvest, causing the immature fruit to develop its characteristic color and texture.

Once the fruit is ripe, the pedicel or stem that holds the fruit begins to seal itself off from the plant, under the influence of the hormone abscisic acid (ABA). When this hormone is produced, fruit drop is stimulated. To prevent fruit drop, another hormone called cytokinin can be used to inhibit the production of ABA and delay overripening and fruit drop. Oranges and other citrus crops can be harvested yearlong by inhibiting fruit drop and senescence through the application of a cytokinin. Citrus fruits, which normally mature in the winter, can thus be harvested year round.

A careful examination of the fruit reveals how the tissues change during development. In citrus fruits, like grapefruits and oranges, the bulk of the fleshy fruit is formed by small juice sacs, which originate from small hairs lining the inside of the pistil. These juice sacs are simply hairs that swell at different positions along their length, filling the fruit. The nature of these hairs can be seen by gently teasing a few sacs from the center of the fruit. The fleshiness of the tomato fruit is the result of the swelling of the placenta, a tissue that connects the seeds to the walls. Frequently, the ovary wall itself forms most of the fruit, but the exact region of thickening differs in each plant group. In squash, cucumbers, and pumpkins for example, the middle of the ovary wall grows thicker than the inner and outer layers, whereas in grapes, the inner wall grows thicker, and in watermelons, the outer wall is particularly thick. In dry fruits, the thickening of the ovary wall is sometimes accompanied by cell hardening, which is caused by chemical changes in the cell walls. These hardened cells form the walls of nuts and other hard fruits. In dry fruits, the walls of the fruit are no longer living.

Types of Fruits

Fruits consist of three major types, depending on whether they are formed from a single flower with fused or unfused multiple simple pistils or from multiple flowers: (1) simple fruits consist of one simple or fused pistil, in which the pistil forms the simple fruit; (2) aggregate fruits consist of many unfused pistils as part of a single flower; and (3) multiple fruits consist of many flowers on the same floral stem fusing together during growth. Fruits formed with large areas of nonpistillate parts in the flower are known as accessory fruits, a term that may be used in combination with these other terms.

FRUIT CATEGORIES AND COMMON EXAMPLES
Major and Minor Categories of Fruit Types Common Examples
Simple fruits Fleshy fruits Berry (multi-seeded fruits with rind or skinlike covering) Typical berry (fruits with skinlike covering) Grape, tomato, gooseberry, cranberry
(develop from one pistil and often include surrounding [accessory] ovary tissues)
Pepo (fruits with inseparable rind) Cucumber, pumpkin, squash
Hesperidium (fruits with separable rind) Orange, grapefruit, lemon
Drupe (single seeded with thin skin) Peach, plum, cherry, olive
Pome (multi-seeded fruit formed from floral tube [inferior ovary]) Apple, pear, quince
Dry fruits Dehiscent fruits (fruits that split at maturity) Legume (single pistil forming two slits) Peas, beans, locust
Follicle (single pistil forming a single slit) Milkweed, columbine, larkspur, magnolia
Capsule (compound pistil opening variously) Poppy, purslane, iris, Saint-John's-wort, morning glory
Nondehiscent fruits (fruits that do not naturally split at maturity) Grain (caryopsis; one-seeded with inseparable covering) Corn, wheat, oats, rye, barley
Achene (one-seeded with separable covering) Sunflower, lettuce, buckwheat
Samara (winged achene) Ash, maple, elm, birch
Nut (one-seeded, hard covered fruit with large embryo) Chestnut, walnut, hazelnut, acorn, beechnut
Aggregate fruits (develop from one flower with multiple separate pistils) Strawberry, raspberry, blackberry
Multiple fruits (develop from a flower cluster, multiple flowers of an inflorescence) Pineapple, mulberry, osage orange, fig

Simple fleshy fruits are divided into three major types. Berries are multi-seeded fruits covered by a thinner skin (as in tomatoes) or a thickened rind (as in cucumbers). Some berries may be further divided into subtypes, including the pepo, characteristic of the cucumber family (e.g., cucumbers, squash, and pumpkins), and the hesperidium, characteristic of the citrus family (e.g., oranges, grapefruits, and lemons). Pomes are also multi-seeded fruits, but their fleshy body consists of largely nonfloral (accessory) parts. Since their body is not just pistil tissue, pomes can be regarded as accessory simple fruits. In pomes, the outer wall develops from the floral cup or hypanthium of the flower, as in apples, pears, and quinces. Drupes are single-seeded fruits that may contain a leathery or stonelike seed. Peaches and plums are examples of fruits with rock-hard seeds at their center, commonly classified as stone fruits.

Simple dry fruits include two types of fruits. Dehiscent dry fruits are those that normally open during the maturation process, releasing their seeds. Frequently, a line of dehiscence forms the opening in the fruit. Legumes are formed from single pistils that have two slits or lines of dehiscence on either side of the fruit. Legumes include peanuts and beans, and are characteristic of the bean family. Follicles are dry fruits, often with vertical slits, which have a single dehiscence line. Capsules are formed from compound pistils and open through a variety of mechanisms. In poppies, these fruits have small pores at the top of their fruits. In contrast, irises form fruits that open along the suture lines of the compound pistil, splitting into their component pistils. The position of these openings is used to establish further subtypes (not mentioned here).

Nondehiscent dry fruits are those that do not normally open to release their seeds. Four types are commonly found. Grains, or caryopses, are small, one-seeded fruits that have fruit walls that are fused to the seed and are therefore inseparable, as in corn. Achenes are single-seeded indehiscent fruits in which the seed and fruit are readily separated, as in sunflowers. Samaras are winged fruits, such as those of maple, ash, and elm, which are readily dispersed by wind. Nuts are one-seeded fruits as well, but are characterized by their hard covering and often large and meaty embryos, as in walnuts, chestnuts, and acorns.

Aggregate fruits develop from single flowers with multiple separate pistils. Common examples composed mainly of pistillate tissues include raspberry and blackberry. The fleshy region of the strawberry originates from the receptacle of the former flower. Therefore, in addition to being an aggregate fruit, it is also called an accessory fruit.

Multiple fruits consist of the fused flowers of whole inflorescences (or flowering stalks). The most common of the multiple fruits is the pineapple, although the mulberry, Osage orange (or bois d'arc), and fig are also commonly encountered multiple fruits.

see also Fruits, Seedless; Grains; Reproduction, Sexual; Rosaceae; Seed Dispersal; Seeds.

Scott D. Russell

Bibliography

Nicholson, B. E., C. Geissler, John G. Vaughan, Elizabeth Dowle. The Oxford Book of Food Plants, 2nd ed. Oxford University Press, 1998.

Schery, R. W. Plants for Man. New York: Prentice-Hall, 1959.

Simpson, B. B., and M. C. Ogorzaly. Economic Botany. New York: McGraw-Hill, 1995.

Whiteman, K. Fruits of the World: A Comprehensive Guide to Choosing and Using. Hermes House, 2000.

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