Fruit

I

Introduction

Fruit, mature ovary in flowering plants, together with all inseparably connected parts of the flower. In strict botanical usage, the meaning may be restricted to the ovary alone. Commonly the term “fruit” is often restricted to succulent, edible fruits of woody plants, to melons, and to such small fruits as strawberries and blackcurrants. In nature, fruit is normally produced only after fertilization of ovules has taken place, but in many plants, largely cultivated varieties such as seedless citrus fruits, grapes, bananas, and cucumbers, fruit matures without fertilization, a process known as parthenocarpy. In either case, the maturation of the ovary results in the withering of stigmas and anthers and enlargement of the ovary or ovaries. Ovules within fertilized ovaries develop to produce seeds. In unfertilized varieties, seeds fail to develop, and the ovules remain their original size. The major service performed by fruit is the protection of developing seeds. In many plants, fruit also aids in seed distribution.

II

Structure of Fruit

As the ovary matures, its wall develops to form the pericarp, which is divided into three layers. The outermost, the exocarp, is usually a single epidermal layer. The extent of the middle layer, the mesocarp, and the inner layer, the endocarp, varies widely, but in any single type of fruit one of the layers may be thick, the others thin. In fleshy fruits the pulpy layer is usually the mesocarp, as in peaches and grapes. The seed or seeds, which lie immediately within the pericarp, in some cases constitute the entire edible portion of the fruit. For example, the hard outer husk of a coconut is the complete pericarp, and the edible part inside, including the “milk”, is the seed.

In typical cases, the fruit is simply the ripened ovary, as in the pea pod; but in apples it includes the ovary and receptacle (other fused floral parts), in strawberries it is an aggregation of small individual fruits, called achenes, on a fleshy receptacle, and in pineapples it is a development of an entire infructescence, or cluster of fruits.

III

Types of Fruit

Fruit is classified by several characteristics, the most significant being the number of ovaries. A simple fruit is a single ovary, developed from the pistil of a single flower, which may be single or compound; an aggregate fruit is composed of many ovaries attached to a single receptacle; a multiple, or collective, fruit is formed from the coalesced ovaries of an entire inflorescence. Simple fruits are further subdivided into two categories, dry or fleshy. Ovary walls that develop into simple fruits are succulent when young, but as they mature, those of dry fruits lose most of their moisture, whereas those of fleshy fruits increase in size and moisture capacity. Dry fruits that dehisce, or split, when ripe are called dry dehiscent fruits; those that do not are indehiscent fruits.

Among the dry dehiscent fruits, a pod, or legume, characterizes most of the legumes. The shell of the pod is the pericarp, and the beans or peas inside it are the seeds. Dehiscence occurs along the sutures or seams of the two edges, the seeds being attached to the ventral or lower suture. A few leguminous plants have pods that do not dehisce but break at maturity; a pod of this type is termed a loment. A follicle, found in the peony and in delphinium, has two sutures like a pod but opens only along one of them. A capsule, unlike a follicle, contains more than one seed chamber, or fused carpels. When capsules split down the middle of each chamber, as in lilies, their dehiscence is termed loculicidal. When the dehiscence occurs at the lines of fusion of the chambers, as in the azalea, it is called septicidal. Poppy capsules open by pores, the dehiscence known as poricidal. Capsules of plantain split along a circular horizontal line, so that a “lid” comes off the top; this type of dehiscence is termed circumscissile. A siliqua, characteristic of the cabbages, is a two-chambered dry fruit that dehisces along two sutures, leaving the exposed seeds clinging to a thin, membranous partition. A siliqua is less than three times as long as it is broad; when more than three times as long as broad, it is called a silicula.

Most indehiscent fruits develop a single seed for each ovary. The pericarp of these fruits is so closely invested around the seed that the entire fruit assumes the appearance of a seed. The true grain, or caryopsis, characteristic of grasses, is little more than a seed with a thin, membranous pericarp inseparable from it. The achene, such as the “grain” of buckwheat, is sometimes called a naked seed because of its thin, separable pericarp. The samara, or key fruit, typified by the fruits of elms, sycamores, and ashes, has a wing-like outgrowth of the ovary wall that aids in dispersal by wind. The typical fruit of the Apiaceae and the mallows, the schizocarp, has more than one seed. Unlike other dry indehiscent fruits, it splits into single-seeded portions at maturity. The nut exemplified by acorns and hazelnuts is a single-ovaried fruit with an extremely hard pericarp.

All fleshy fruits are indehiscent, the pulpy parts remaining attached to the seeds during dispersal. The true berry, typified by the tomato, blackcurrant, and gooseberry, possesses seeds dispersed throughout the fleshy mesocarp and endocarp. The exocarp is a thin skin. Many fruits, such as strawberries and raspberries, are commonly, but incorrectly, called berries. Two specialized types of berry, the hesperidium and the pepo, include valuable commercial fruits. All citrus fruits, including oranges, lemons, and grapefruits, are hesperidia, having leathery rinds composed of exocarp and mesocarp, and juicy sections of endocarp. The pepo is the characteristic fruit of the cucumber family, Cucurbitaceae, including cucumbers, pumpkins, melons, and gourds. The outer layer of the pepo is receptacle tissue covering the exocarp; the pulpy portion of the fruit is mostly endocarp and mesocarp. The remaining type of fleshy fruit, the pome, has a pericarp limited to the so-called core and the inner fleshy portion of the fruit, as in apples, pears, and quinces. The other portion of the fleshy part of a pome is tissue developed from the fusion of the other floral parts and the ovary. The drupe is the stone fruit of such plants as plum, cherry, olive, peach, and almond. (The familiar edible almond, incorrectly called a nut, is the dried stone of a large drupe.) The single seed is surrounded by a stony endocarp; the fleshy portion is mesocarp. A small drupe occurring as part of a larger cluster is usually called a drupelet.

The constituent fruitlets of most aggregate and multiple fruits can be recognized as belonging to the same classification as simple fruits. The aggregate fruitlets of blackberries, raspberries, and dewberries, for example, are drupelets, and those of strawberries are achenes. It is not the fruit—that is, achenes—of strawberries that are eaten, but the fleshy receptacle. In the pineapple, on the other hand, the separate fruitlets cannot be classified as types of simple fruit; the multiple fruit is a mass of fused ovaries growing from the central axis of the pineapple.

IV

Food Value

Fruits are eaten raw or cooked, dried, canned, or preserved. Carbohydrates, including starches and sugars, constitute the principal nutritional material. Citrus fruits, tomatoes, and strawberries are primary sources of vitamin C, and most fruits contain considerable quantities of vitamin A and vitamin B. Vitamin content is sharply reduced in storage and shipping of fresh fruits, but is maintained efficiently in frozen fruit. The jam-making quality of many fruits is due to pectin, an important carbohydrate constituent. In general, fruits contain little protein or fat. Exceptions are avocados, nuts, and olives, which contain large quantities of fat, and grains and legumes, which contain considerable protein. Although the edible portions of fruits have a small ash (inorganic) content, fruits supply an important part of the mineral matter necessary in human diet. Dried or evaporated fruits contain much more nutritional material in proportion to their bulk than do fresh fruits, because of concentration by evaporation. See Nutrition, Human.

See also Crop Farming: Fruit Crops; Horticulture; Orchard; Plant Breeding; Pruning.