Fruit - the ripened ovary of an angiosperm flower.

Fruits, like flowers, are the unique aspects of reproduction in angiosperms. They protect the enclosed seed, and aid in their dispersal.

1. After we’ve discussed fertilization, we need to talk about seeds, and the way seeds are dispersed for eventual germination.
1. Flowering plants produce a young plant embryo complete with stored nutrients in a compact package: the seed that develops after fertilization has occurred.
2. Development of the embryo following fertilization is possible because of the constant flow of nutrients from the parent plant into the developing seed.
3. A mature seed consists of an embryonic plant and nutrients stored in either endosperm or cotyledons.
4. Endosperm is the nutritive tissue that surrounds the embryonic plant in a seed.
5. Cotyledons are embryonic leaves that not only have stored nutrients, they are the first leaves to emerge from the seed and, thru photosynthesis, begin manufacturing carbohydrates to feed the developing plant. Cotyledons often absorb endosperm just prior to germination.
2. The seed dispersal vehicle for a flowering plant is the fruit. The word “fruit” has different meanings to different people. It carries the connotation of a sweet, soft plant product. Thus, fruits are usually regarded as a dessert food, whereas vegetables are usually considered part of the main course meal. However, the botanical definition of a fruit is quite specific – it is the ripened ovary that usually contains the seeds. Not all fruits are sweet and soft, and many are not good to eat at all. Vegetables are considered to be the edible portion of the vegetative plant body – stem, roots, leaves, flowers.
1. A fruit is a seed container derived from an ovary and any tissues that surround it.
2. As such, fruits are products of flowers and therefore only occur in flowering plants.
3. Angiosperms (flowering plants) have a remarkable diversity of fruit types. Fruits are classified on the basis of characteristics of the mature ovary tissue – e.g., whether the fruit is fleshy or dry, whether the ovary is fused to other kinds of tissues. They are also classified on the basis of whether they are dehiscent or indehiscent (the process of splitting open at maturity). In the former, the pericarp splits open to release the seeds (i.e. the seed is the unit of dispersal). In the latter, the pericarp encloses the seed so that the entire fruit disperses (the whole fruit is the dispersal unit).
4. The function of fruit is two-fold:

i. to protect the developing seed

ii. to aid in the dispersal of the seed

Pericarp – fruit wall which has developed from the ovary wall. In fleshy fruit, the pericarp is comprised of 3 layers:

exocarp - outer layer, or skin

mesocarp - middle layer, often the fleshy part

endocarp - inner layer that surrounds the seed.

In dry fruits, these three layers are indistinguishable within the pericarp.

Simple fruits - derived from the ovary of a single carpel or several fused carpel, but always from a single gynoecium. They may be fleshy or dry.

Fleshy fruit - when ripe, the pericarp of fleshy fruits is often soft and juicy. Seed dispersal of these fruits is accomplished when animals eat the fruit, walk off a bit, and defecate out the seeds. Examples:

Berry

Hesperidium

Drupe

Fruits developing from flower parts other than the ovary are referred to as accessory fruits (pomes, pepos - squash, melon, cucumber).

Dry dehiscent fruits (Dehiscence refers to a fruit splitting at maturity and releasing its seeds. These fruits often have a pericarp that is tough, woody, thin, or papery. Wind often acts as the dispersal agent for seeds of these fruits). Examples:

Follicles

Legumes

Capsules

Capsule that opens by pores - poricidal capsule. E.g. poppy, iris, orchid

Capsule that splits longitudinally thru the locules - loculicidal capsule.

Capsule that splits longitudinally along seam - septicidal capsule. E.g. cotton

Dry, indehiscent fruits (Indehiscent fruits do not split open. They use other means of dispersing its seeds)

Achenes

Samaras

Grains (caryopsis)

Nut - hard, dry, (stony pericarp) indehiscent fruit, usually with a single seed. E.g. hazel nut, chestnut, acorn. Peanuts, cashews, almonds are not really nuts, but seeds. Peanut the seed of a legume fruit, almonds the seed of a drupe.

2. Aggregate and multiple fruits

a. Aggregate fruits - these fruits develop from a single flower with many separate pistils (carpels), all of which ripen at the same time. A cluster of small, fleshy fruits originating from a number of separate pistils of a single flower. E.g. clustered drupelets of raspberries, blackberries.

b. Accessory fruit - fleshy fruit developing from a succulent receptacle rather than the pistil. The ripened ovaries are small achenes on the surface of the receptacle. E.g. strawberries

c. Multiple fruits - result from the fusion of ovaries from many separate flowers of an inflorescence. Fruit formed from several separate flowers crowded on a single axis. E.g. figs, pineapples, mulberries.

Dispersal

Just as the structure and shape of the flower is a way to facilitate pollination, the structure and shape of a fruit is a way to facilitate dispersal of the seed.

Since reproductive success is a major factor driving evolution, there has been selection for species that have effective seed dispersal mechanisms. Each of these, in different ways, increases the probability that a few of the many seeds produced by a plant will land on an appropriate substrate. The agents of seed dispersal are wind, water, explosive ejection, and animals. Even though it is the seed that must be dispersed, the structure of the fruit can be modified for dispersal or as in the case of tumbleweeds, the whole plant can take part in dispersal.

Dispersal by wind

Examples: ash or maple (samara like a helicopter), dandelion (plumes like a parachute), orchids (small seeds fine as dust), evening primrose (wooly hairs on seed), tumbleweed (whole plant tumbles).

Generally, fruits and seeds adapted for wind dispersal are relatively lightweight and have an outer coat that either is winglike or has various projections to catch the wind. Seed dispersal by tumbleweeds is the result of an adaptation by a whole plant whereby the stem separates from the roots after the seeds are mature. As the plant is blown about by the wind, the seeds drop out

Dispersal by water - Evolutionary selection pressure has favored the development of adaptations to water-dispersed seeds by species that live near the water. Some fruits are adapted to water dispersal by virtue of the fact that they contain trapped air, so they can float.

Examples: inflated sacs around the seeds, enabling seeds to float. Others may have waxy material coating the seed which, temporarily, prevents them from absorbing water while they are afloat.

Some fruits have thickened or spongy pericarps which absorb water only very slowly, a dispersal adaptation to ocean currents. The best known of the ocean-dispersed plants is the coconut palm, whose lg. fruits have been carried hundreds of kilometers throughout the tropical seas of the world.

Mechanical ejection - fruits of some families mechanically eject seeds, sometimes with considerable force.

In filarees or geraniums, each carpel of the fruit splits away and curls back from the central axis. Each fruitlet has a single seed which is pointed at its base. At the other end is a long, slender beak, which is sensitive to changes in humidity. At night, when the humidity increases, the beak is relatively straight, but in the sun it coils up like a corkscrew, literally drilling the pointed seed into the ground as it does so, and effectively planting it in the process. Geranium beak: increased humidity at night: straight beak. Decreased humidity by day: coiled beak.

As the fruit of witch hazel (Hamamelis) dries, the buildup of stresses and strains bursts the pericarp along the sutures, releasing the stress suddenly. This results in the explosive ejection of the seeds.

Dispersal by animals - Both fleshy and dry fruits, as well as some seeds, are adapted to animal dispersal. At the time of maturity, a fruit signals is maturity by its color (red rather than green), by its texture (soft rather than hard), by its fragrance, and by its taste (sweet rather than either starchy or acidic). [There is no advantage to a plant if its fruit is eaten when the seeds are immature].

As the embryo in the seed matures, hormonal signals are released that initiate ripening. The skin color changes from green to a color that contrasts with the vegetative color of the plant, one in which animals cannot fail to notice. A characteristic aroma that indicates ripeness is often present. Because the food contained in the flesh cannot be used by the plant embryo or the developing seedling, it appears that the fleshy fruit evolved solely as part of a seed dispersal mechanism by animals.

1. Many fruits and seeds catch in or adhere (hitchhike) to the fur or feathers of animals and birds. Fruits of bedstraw (Galium) and cocklebur (Xanthium) are covered with small hooks that catch in fur.

2. Large capsules of unicorn plants (Proboscidea) have curved extensions that catch in the fetlock of a deer or other ambulatory animal that happens to step on the fruit, with the seeds being scattered as the animal moves along.

3. Some species of plants, such as mistletoe, have seeds that are coated with a viscid substance that sticks to a bird’s beak when the fruits are eaten. The bird’s response to this is to fly to another tree and wipe its beak against the bark of a branch, a process that dislodges the mistletoe seed and leaves it adhering to the bark. Since mistletoe is a parasitic epiphyte, this mechanism of seed dispersal ensures that at least some seeds end up in the correct environment.

Critical thing here - in order for this strategy for fruit dispersal to be successful, i.e. to transport the seed to a distant and favorable site for germination, the fruit must be eaten or carried away without damaging the seed. The animal that eats the fruit must not also eat the seed. It must either defecate the seed, or toss the seed away when finished eating the more attractive fruit. Plants often accommodate this necessity by having seeds that are especially small - difficult to crush, easily passing thru an animal’s gut - or unpalatable.

From a plant’s perspective, a damaged seed is useless in the plant’s “objective” to invest itself into the succeeding generation.

Seeds contained in edible fruits must possess adaptations that allow them to survive ingestion by an animal. This may involve possession of a hard seed coat or a slim layer around the seed, so that the seed escapes the grinding action of the teeth. Once in the stomach, the seed receives further protection against digestion from the outer layer of the seed. After passage thru the remainder of the digestive system the seed is left behind in the animals feces. Here the seed is in a warm, moist environment that contains all of the mineral nutrients necessary for its growth. The tomato is a good example of this type of dispersal mechanism.

Humans - Humans are by far the most efficient transporters of fruits and seeds. Noxious weeds and plant diseases, as well as valuable food and medicinal plants, have been carried from one continent to another by explorers and travelers over the past few hundred years in particular. Most countries now have strict regulations barring the importation of plant materials, except by special permit, and some plants are not allowed across borders under any circumstances.