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Fruit
A fruit is the structure that encloses one or more seeds and to some extent assists in the dissemination of the seeds. Many botanists consider a fruit to be a mature ovary, and therefore do not consider cones to be fruits (for more on cones, see the discussion of gymnosperms in "Plant Reproduction," page xx). In the more advanced fruits discussed below, a carpel surrounds the seed, which is therefore no longer naked. This type of fruit is found in the angiosperms, or flowering plants (see Plant Reproduction).
Terms for Describing a Fruit [Illustration]
The fruit is a very complex structure composed of a number of different parts. Some of the more common terms used for describing fruit are:
- Carpel: The unit of an ovary formed from one highly modified leaf. Simple pistils have a single carpel. Compound pistils have several to many carpels (often three or five) fused together.
- Dehiscent: Split, allowing the seeds to fall out and be dispersed.
- Endocarp: The inner layer of the pericarp (fruit wall). The endocarp can be hard and bony as in peaches or soft as in grapes.
- Epicarp or exocarp: The outer layer of the pericarp (fruit wall). The epicarp can be tough and leathery as in citrus or soft as in cherries.
- Fruitlet: The part of a fruit that becomes dispersed in schizocarpic fruits. The fruitlet contains one or more seeds surrounded by part of the ovary wall. Dill "seeds" are actually fruitlets.
- Indehiscent: A fruit that does not split open. For example, a berry.
- Locule: A chamber or cavity of a fruit.
- Mesocarp: The middle layer of the pericarp (fruit wall).
- Pericarp: The fruit wall, often composed of three layers: epicarp, mesocarp, and endocarp.
- Pit: The stony endocarp of a peach or other drupe.
- Sarcocarp: Any internal fleshy layer of a fruit.
- Schizocarp: A fruit that forms from one ovary but breaks into two or more fruitlets, each containing seeds.
- Seed: Mature ovules composed of a seed coat, endosperm (starch or oil used to nourish the developing embryo), and embryo.
- Segment: A division or portion of a fruit. Usually these correspond to the locules.
- Septum: A partition between two fused carpels.
- Stone: The hardened endocarp of a peach or other drupe.
- Valves: The parts of the pericarp (fruit wall) that are separated at dehiscence.
General Types of Fruit [Illustration]
One way to understand fruits is to look at how they develop. In the simplest developmental pattern, a single pistil develops into a single fruit. Three fruit types display this pattern: simple, rhexocarpic, and schizocarpic. A simple fruit is dispersed as a whole (for example, a berry); a rhexocarpic fruit splits open and the seeds are shed from the fruit (for example, a capsule); and a schizocarpic fruit splits into separate fruitlets, which are dispersed (for example, a dill fruit splits into two fruitlets).
In addition to this basic type of fruit are compound fruits (also called aggregate fruits), which develop from more than one pistil in a single flower (such as raspberries and strawberries), and multiple fruits, which develop from more than one pistil in more than one flower (such as a pineapple; cut open a pineapple and the remnants of each of the many flowers that contributed to the fruit become visible).
Another way to look at fruits is to distinguish between those that are formed solely from the ovary and those that are formed from the ovary as well as other flower parts.
A pericarpium is a fruit formed solely from the ovary. Some of the more common types of pericarpium include:
- Achene: A simple, one-seeded fruit in which the pericarp (fruit wall) is attached to the seed. A nut (in some uses of the term) is an achene with a hardened pericarp. A common achene is that of the sunflower (Helianthus annuus).
- Bacca (also called berry): A simple, fleshy fruit that does not dehisce. A common berry is the grape (Vitis vinifera). Berries are usually dispersed by animals that eat them. The seeds pass through their bodies and are excreted.
- Capsule: A rhexocarpic (dehiscent) fruit formed from more than one carpel. Rhododendrons have capsular fruit. There are several types of capsule, which are distinguished by the type of dehiscence they exhibit: in a septicidal capsule, dehiscence is along the septa (the area where the carpels or locules are fused to each other) separating the carpels. In a loculicidal capsule dehiscence is via slits formed in the outer wall of the locules (or cavities). In a poricidal capsule, dehiscence is via pores, small holes in the side of the capsule.
- Drupe: A simple, fleshy, indehiscent fruit with one or more internal stones. The stones are an adaptation to animal dispersal; they protect the seed during its passage through the animal's digestive tract. Some common examples of drupes are peach (Prunus persica) and mango (Mangifera indica).
- Follicle: A dry, dehiscent fruit that develops from one carpel and opens through only one slit. Magnolias have follicular fruit.
- Hesperidium: A fleshy, berry-like fruit with a tough rind, such as orange, lemon, and other citrus. In citrus fruits the juice is formed in each wedge in hairs.
- Legume: A dry dehiscent fruit formed from a single carpel, which usually opens along two sides. This type of fruit is found only in the bean family, for example, the bean (Phaseolus vulgaris).
- Samara: A simple, dry, indehiscent fruit with wings. A typical samara is the fruit of elms (Ulmus species). Maples (Acer species) are more complex. They are schizocarpic; each winged portion is a fruitlet.
An anthocarp is a fruit formed from the ovary and attached floral parts that have undergone marked development after fertilization in order to aid in the dissemination of the seeds.
Some common types of anthocarp:
- Anthecium: A grass fruit in which a single spikelet with fertile florets breaks apart above the glumes (the bracts at the base of the spikelet). There are several other types of grass fruit that will not be covered here.
- Cypsela: A fruit with wings or bristles at its tip. A typical cypsela is the fruit of a dandelion (Taraxacum officinale).
- Glans: A fruit in which the receptacle, pedicel, or peduncle are enlarged. A typical example is the acorn in oaks (Quercus species), in which the acorn cap is formed from bracts below the flower.
- Pome: A fruit in which the hypanthium (a cup formed by the fusion of the sepals, petals, and stamens) becomes enlarged and fleshy. The typical pome is the apple (Malus domestica).
Fruit and Seed Dispersal
Fruits and seeds usually move away from the parent plant in some way. The mechanism by which this is done is called dispersal. There are six common means of dispersal:
- Anemochory: Dispersal by wind. The dandelion (Taraxacum officinale), with its fluffy parachutes, is easily dispersed by wind over long distances.
- Autochory: Dispersal by physical expulsion, often explosively. The fruit of the jewel-weed (Impatiens capensis) explodes, shooting its seeds some distance.
- Endozoochory: Dispersal through animal ingestion and excretion. Many seeds cannot germinate and grow without first passing through the gut of an animal. The acid washing or physical grinding of the seeds helps them germinate. Bearberry (Arctostaphylos uva-ursi) will not germinate without first passing through a bird's digestive tract.
- Epizoochory: Dispersal by attachment to fur or feathers. Plants such as burdock (Arctium minus) have barbs on their fruit, which become attached to passing animals and fall to the ground when the animals clean themselves.
- Hydrochory: Dispersal by water. Seeds of rushes (Juncus species) are light and easily carried by water. They also often have tails at each end, which subsequently help them become lodged in mud or other debris.
- Myrmecochory: Dispersal by ants. Myrmechochory is a very important but often overlooked form of dispersal. Violets and many other spring wildflowers form small bodies attached to the seed, the sole purpose of which is to provide food for the ant. The ant takes the whole seed to its nest and then removes the food body. In the process, the seed is dispersed.
Surface Features
Some features can be found on virtually any part of a plant -- namely, surface features. These generally are hairs, called trichomes in botany. To describe the hairiness of a leaf, for example, botanists describe the trichomes, their abundance, and orientation, collectively called the indumentum. The following are common terms used to describe plant surfaces:
- Glabrous: Without any hairs or other surface covering.
- Glabrescent: Becoming glabrous over time. In other words, the hairs fall off as the leaf ages.
- Pubescent: Downy. With short soft hairs.
- Hispid: With stout, stiff hairs.
- Hirsute: With rough, coarse hairs.
- Tomentose: Wooly. With long, soft, matted hairs.
- Villous: Shaggy. With long, soft, unmatted hairs.
- Strigose: With long, stiff, appressed hairs.
- Stellate: With stellate hairs (hairs that branch and look like small stars).
- Viscid: Sticky.