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Fruits

Plant Sciences

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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