|Structure and Types of Fruit|
When most people think of a fruit, what typically comes to mind is a juicy, edible object, such as an apple, orange, or banana. To botanists, however, fruit includes many plant-derived structures, such as grains, nuts, and many vegetables.
In essence, a fruit is an enlarge dovary, often with some accessory tissue, that develops after a flower has been pollinated. After pollination, seed development begins, and soon the peripheral parts of the flower fall away, leaving the immature fruit. The fruit subsequently enlarges and then ripens to maturity. It is then often edible.
Almost all fruits have a general structure that consists of an outer layer called the pericarp. The pericarp, in turn, encloses the seed or seeds. Usually there is a space between the seed and the pericarp, called a locule. The pumpkin is a good illustration of this structure, with orange rind as the pericarp, the hollow space within the locule, and the seeds inside the locule.
There are many different kinds of fruits. Some, such as cherries, tomatoes, and apples, have fleshy, juicy pericarps. Others, such as peanuts, milkweed pods, and acorns, have dry pericarps. The variability in fruits represents different seed dispersal strategies. In some plants, the seeds are dispersed while still enclosed within the fruit.
Seeds in fleshy fruits are often dispersed by animals that eat the fruit and then either discard the seeds or later defecate them. Other fruits have barbs or hooks that catch on to fur or feathers and then travel with an animal until they are removed or drop off. Still other seeds, such as those of maple and ash, have “wings” for wind dispersal.
A few, such as those of the coconut palm and many sedges, have fruits that float and are dispersed by water. In most instances, the fruit merely opens, and the seeds drop onto the ground. Some plants, such as witch hazel and the touch-me-not, produce fruits that open explosively and can disperse their seeds great distances.
Fruits can be classified, based on the nature of the pericarp, into two groups: fleshy and dry. Fleshy fruits, in turn, are classified into several types, including drupes, berries, pomes, hesperidia, and pepos. Dry fruits are also subdivided into several categories, including follicles, legumes, capsules, achenes, nuts, samaras, schizocarps, and caryopses.
The three most familiar types of fleshy fruits are drupes, berries, and pomes. A drupe is a fleshy fruit that contains a single seed surrounded by a hard, bony inner wall of the pericarp (called the endocarp).
The middle and outer walls of the pericarp (called the mesocarp and exocarp, respectively) are juicy and often sweet. Drupes include all the pitted fruits, such as cherries, plums, peaches, and olives. A berry typically has several seeds, and the pericarp is fleshy throughout. Familiar examples include tomatoes, eggplants, and grapes.
A pome is a fleshy fruit, often with many seeds, that has a thick layer of accessory tissue immediately surrounding the pericarp. The accessory tissue is generally juicy, sweet, and often edible. Representative pomes include apples and pears.
Two other fleshy fruits, the hesperidium and the pepo, are characterized by a leathery rind. Hesperidia, also known as a citrus fruits, have rinds rich in aromatic oils surrounding a juicy interior composed of wedge-shaped segments that have a sugary, acidic sap.
Hesperidia include oranges, grapefruits, lemons, and limes. The pepo has a tough exocarp which is either smooth or variously sculptured and normally contains many seeds. Examples include cucumbers, cantaloupes, and squash.
Some common dry fruits are follicles, legumes, capsules, achenes, nuts, samaras, schizocarps, and caryopses. Follicles are podlike fruits that open up along one side, revealing numerous seeds.
Examples include milkweed pods and the aggregate follicles of magnolias. In contrast, legumes are podlike fruits that open up along two lines, releasing several seeds. They are produced by many members of the legume family, such as peas, beans, and peanuts.
The capsule opens along three or more lines or by pores at the top of the fruit. Lilies and poppies are good examples of plants that produce capsules. Achenes each contain a single seed that is free inside the cavity, except for an attachment at one end called the funiculus, and are typically small. A good example is a sunflower “seed.”
A nut is similar to an achene, except that the pericarp is hard and fibrous and is derived from a compound ovary. Representative nuts include acorns, hazelnuts, and hickory nuts. A samara is a modified achene that has part of the pericarp flattened to form a wing. Examples of plants that produce samaras include ashes and elms.
Maples have a winged fruit, called a schizocarp, which is often mistaken for a samara. Close observation reveals that the schizocarps come in attached pairs that later split into single-seeded portions. Schizocarps, which are generally not winged, also occur in the parsley family, where they may split into more than two parts.
Finally, a caryopsis is a single-seeded fruit whose seed coat is fused to the pericarp. Caryopses are produced only by plants in the grass family and include the familiar grains wheat, corn, rice, and oats.
The fruits listed above commonly fall under the category of simple fruits. In other words, they are identifiable as individual structures. Other plants produce fruits in dense clusters, and these are termed either aggregate fruits or multiple fruits.
Aggregate fruits are produced by a single flower that has numerous pistils. One example is a raspberry, which is an aggregate of drupes which are often referred to as drupelets because of their small size.
Another is the strawberry, not a berry in the botanical sense, which is an aggregate of tiny achenes attached to the surface of a swollen, juicy, receptacle (originally the base of the flower where all the flower parts were attached). In contrast, multiple fruits are produced by clusters of small flowers, each of which produces a single fruit. Representative multiple fruits include mulberries, figs, and pineapples.
Development and Maturation
|Fruit Development and Maturation|
Two hormones are particularly implicated in fruit formation: auxin and gibberellin. Many fruit growers routinely spray their plants with auxin to induce the formation of seedless, or parthenocarpic, fruits.
Fruit ripening is an important process that must occur properly in order for the seeds to be effectively dispersed. In fleshy fruits, such as tomatoes, cherries, apples, oranges, and bananas, fruit ripening involves several important changes in the pericarp that make the fruit more visible and palatable to a potential animal disperser.
Perhaps the most visible change is in the color of the fruit. Immature fruits are green because of the presence of the pigment chlorophyll in the cells of the outer layer. Potential dispersers fail to notice the immature fruit because it blends in with the surrounding leaves.
As fruits ripen, the chlorophyll breaks down, and other colors, such as orange, yellow, red, or blue, become evident. Those colors are the result of pigments that either are present in the unripe fruit and masked by the chlorophyll or develop as the fruit ripens. The texture and chemical composition of the pericarp change as well.
Most fruits soften as they ripen, a result of the degradation of the cell walls in the pericarp. At the same time, starches or oils in the pericarp are chemically transformed into simple sugars such as fructose. That change causes the fruit to become better-tasting, more digestible, and thus more attractive to a hungry animal.
The physiology of fruit ripening has been well studied. Fruits such as grapes, citrus fruits, and strawberries ripen gradually. Others, such as tomatoes, apples, and pears, exhibit a transitional event called climacteric, which is marked by a dramatic increase in the rate at which oxygen is absorbed by the fruit, followed by a rapid change in the color and physical nature of the pericarp.
Studies of climacteric fruit have shown that the onset of ripening can be delayed by storing the fruit at low temperatures or in an atmosphere devoid of oxygen. On the other hand, climacteric can be induced by exposing the fruit to ethylene, a plant hormone. Interestingly, ethylene is produced by ripening fruits, and thus a ripe fruit promotes the development of any unripe fruits nearby.
Finally, many plants drop their fruits at some point after they become ripened. Botanists use the term abscission to refer to the dropping process. Fruit abscission, like leaf abscission, occurs when a layer of cells at the base of the pedicel (the stem that attaches to the plant) become weakened. Studies have shown that abscission is influenced by two hormones: ethylene and auxin.
Ethylene promotes abscission of fruits in many plant species, such as cherries, blueberries, and blackberries. Auxin, on the other hand, has effects that vary depending on the dose: Low concentrations promote fruit retention, while high doses cause fruits to drop.