Seed

A seed is a part of a flowering plant involved in reproduction. It consists of three major parts: the embryo, endosperm, and testa. The embryo is produced when male and female elements are combined during reproduction. It will eventually grow into a new plant. The endosperm is a collection of stored food the young plant will use as it begins to germinate, or grow. The testa is a tough outer layer that protects the embryo and endosperm from damage by outside factors.

Two kinds of seed plants exist. Gymnosperms are plants that produce naked seeds. The most common type of gymnosperms are conifers, cone-bearing trees and shrubs such as firs, hemlocks, junipers, larches, pines, and spruce. Angiosperms are plants whose seeds are enclosed in a protective structure called the fruit. Angiosperms are also known as flowering plants because they produce flowers in which seeds are produced and in which they develop.

Seed production

Seeds are produced when pollen is released from the male (stamen) part of a plant. That pollen comes into contact with the ovules of the female (pistil) parts of a plant. Some kinds of plants contain both male and female organs on the same plant. In that case, self-fertilization can occur when pollen from one part of the plant fertilizes ovules on another part of the same plant.

In most plants, fertilization occurs between two different plants, one of which contains only male flowers and the other only female flowers. This process requires some kind of mechanism by which pollen can be carried between plants. In some cases, movement of air (wind) can bring about this kind of fertilization. Insects and birds can also produce the same result. For example, a bee may visit a male plant in search of nectar. In that search, the bee may rub off pollen onto its body. When the bee then visits a female plant, it may release that pollen onto the ovules of the second plant, making fertilization possible.

The endosperm within a seed is used when the embryo begins to develop. Seeds vary widely in terms of the relative amounts of embryo and endosperm they may contain. For example, members of the orchid family have tiny, dustlike seeds that consist of little more than core embryonic tissues, with very little in the way of energy reserves. In contrast, the gigantic seeds of some coconuts can weigh more than 60 pounds (25 kilograms), most of which is nutritional reserve surrounded by fibrous, protective husk.

Words to Know

Angiosperm: A plant whose seeds are enclosed in a protective structure called the fruit.

Dispersal: Any process by which seeds are spread outward from their parent plant.

Dormancy: A state of inactivity in an organism.

Embryo: The young form of an organism.

Endosperm: A collection of stored food used by a young plant during germination.

Germination: The beginning of growth of a seed.

Gymnosperm: A plant that produces naked seeds.

Pistil: The female reproductive organ in a plant.

Pollination: The transfer of pollen from the male organ of a plant to the female organ.

Self-fertilization: The process in which pollen from one part of a plant fertilizes ovules on another part of the same plant.

Stamen: The male reproductive organ in a plant.

Testa: A tough outer layer that protects the embryo and endosperm of a seed from damage.

Seed dispersal

A seed exists in a dormant (sleeping) state. It begins to germinate, or grow, only when it is deposited in a favorable environment, such as moist, warm ground. The long process by which a seed changes from a tiny embryo into a fully grown plant requires time and favorable conditions. In most cases, young plants have a better chance to survive and grow if they are deposited at some distance from the parent plant. In those cases, they will not have to compete for sunlight, water, and nutrients with their own parents.

There are, however, some important exceptions to this general rule. For example, the adults of annual species of plants die at the end of their breeding season. In those cases the parent plants do not compete with their seeds. Nevertheless, even annual plants tend to disperse their seeds widely.

Methods of dispersal. Plants have evolved a variety of mechanisms to disperse their seeds effectively. In some plant species, seeds are very buoyant, so they can be dispersed over great distances by the winds. Some well-known examples of this kind of plant are the fluffy seeds of the dandelion and fireweed. The seeds of maple trees are also dispersed by the wind. These seeds have a one-sided wing that causes them to swirl propeller-like after they are released from a parent tree. This structure allows maple seeds to be carried by even light breezes some distance from their parent before they hit the ground.

Some plants have developed an interesting method of dispersal, known as tumbleweeding. These plants grow into a roughly spherical shape. After the seeds are ripe, the mature plant detaches from the ground surface and is then blown about by the wind, shedding its seeds widely as it tumbles along.

The seeds of many other species of plants are dispersed by animals. Some seeds have structures that allow them to attach to the fur or feathers of passing animals, who then carry the seeds some distance away from the parent plant before they are deposited to the ground. One example of this mechanism is burdock, whose spherical fruits have numerous hairs with tiny hooked tips that stick to fur. This fruit also sticks to human clothing, and was the botanical model that inspired the invention of Velcro™, a sticky, synthetic fastening material.

Another mechanism by which seeds are dispersed by animals involves their encasement in a fleshy, edible fruit. Such fruits are often brightly colored, have pleasant odors, and are nutritious and attractive to herbivorous (plant-eating) animals. These animals eat the fruit, seeds and all. After the fruit passes through the animal's digestive system, the seeds are dispersed at some distance from the parent plant.

The seeds of many plants with this sort of animal-dispersal strategy actually require passage through the gut of an animal before they will germinate. Some familiar examples of species that develop animal-dispersed fruits include the cherries, tomatoes, and watermelon.

Seed germination

After seeds have been dispersed into the environment, they may remain in a dormant state for some time, until appropriate cues are sensed for germination. Such clues include sufficient water, oxygen, and an appropriate temperature. Interestingly enough, the seeds of many species will not germinate even under favorable conditions. For example, seeds produced and dispersed just before the beginning of a cold season might actually experience the right conditions for germination for a short period of time. However, they would probably not survive if they germinated at once. A period of dormancy enables the seeds to wait out the cold season, and to begin growth when conditions are more favorable for the mature plant, in the springtime. It allows seeds a better chance of surviving unfavorable conditions and developing successfully into plants.

Germination begins with an increase of metabolic activity within the seed (that is, organic compounds are broken down to produce energy). The first visible sign of germination in angiosperms is generally an enlargement of the seed. That enlargement is caused by an intake of water from the environment. The seed's covering may wrinkle and crack at this

time. Soon afterward, the embryonic root emerges from the seed and begins to grow down into the soil. At about this time the shoot also emerges and grows upward out of the soil.

Uses of seeds

Seeds are used by humans for a number of purposes. The most important of those uses are as foods. Some seeds are eaten directly, while other are used to manufacture flour, starch, oil, alcohol, or some other edible products. Some examples of such seeds include those of wheat, rice, maize, sorghum, barley, peanut, soybean, lentil, common pea, common bean, coconut, walnut, pecan, and sunflower.

Many other seeds are eaten with their fruits, although it is generally the encasing fruit walls that are the sought-after source of nutrition. A few examples of edible fruits include those of the pumpkin or squash, bell pepper, apple, sweet cherry, strawberry, raspberry, and sweet orange.

[See also Plant; Reproduction ]

Seed

A seed contains the embryo plant from which a new plant will develop. It also contains food storage tissue and a protective coat. The creation of a seed depends on pollination (the transfer of pollen containing male sex cells to the pistil containing female sex cells) and fertilization (the process in which an egg cell and a sperm cell unite to form one cell). Seeds are produced by more than 250,000 different types of plants. Seeds also vary greatly in size and the methods by which they are dispersed.

The first seeds are thought to have developed about 360,000,000 years ago, marking a major step in the evolution of land plants. These early seeds were gymnosperms or "naked seeds" since they were totally exposed. Over time, however, seeds came to develop a coating (called angiosperms) that protected the seed. This coating proved so successful that angiosperms are now the dominant type of plant. A seed is formed sexually when a grain of pollen containing male sperm fertilizes an ovule containing the female egg. This fused egg cell begins to divide rapidly and grows a structure called the embryo. This part of the seed will become the new plant. It contains, in miniature, the radicle or embryonic root that will grow into a primary root after germination (sprouting). It also has the beginnings of a shoot and early leaves, called cotyledon. After sprouting, the seed lives off its endosperm or stored food until it develops roots and leaves to make its own food.

Seed dispersal, or the methods plants have developed to scatter their seeds away from the parent plant, are extremely varied and interesting. In the simplest cases, seeds ripen on the plant, fall to the ground, germinate, and grow into a new plant. While this may seem like a good method, it soon creates an overcrowded situation where seedlings are competing for the same resources as the parent plant. Plants, therefore, have developed strategies and methods to disperse their seeds some distance from the parent plant. One method is the creation of fruit that is the ripened ovary of the plant and that surrounds the seed itself. Fruit can be hard and dry or fleshy and soft. Many plants enclose their seeds in tasty, attractive fruit to encourage an animal to eat it. Once eaten, the seeds pass through the animal's digestive tract and are deposited elsewhere in the animal's waste—often far away from the parent plant. The fruit of berry seeds, for example, are typically small, thin-skinned, and brightly colored with no smell. It is the fruit's bright color that attracts birds that then eat the fruit. Fruits that appeal to mammals are less colorful but often have a pleasant smell. Others have their nutritious seed enclosed in a hard shell (like a nut) that a rodent will chew through and eat. Sometimes rodents bury nuts to eat later. The ones that are not dug up will germinate and grow into a new plant. Some fruits are meant to be eaten, but contain toxins that make an animal regurgitate (vomit) them elsewhere allowing the fruit's seeds to be dispersed in a new location.

Animals also disperse seeds by carrying both fruit and seeds on their bodies. Certain plants produce seeds with sticky surfaces, while others have barbs or hooks that become attached to an animal's fur or feathers and are then taken away to fall elsewhere to the ground. Some plant species develop extremely light seeds that are easily blown great distances by the wind. The fluffy coverings of dandelion or cottonwood seeds are good examples of wind dispersal. Other plants use the wind more mechanically and have developed seeds with wings (like the sycamore tree) or appendages that act as parachutes. Tumbleweed is an example of an entire plant becoming loose and blowing away with the wind. Other plants develop seeds that can float on water and even have a waterproof coat, such as the coconut. Other mechanical means of dispersal are used by plants that produce capsules that explode when ripe, resulting in the scattering of seeds. Many such plants, like the popular impatiens, grow little curled capsules that build up tension as they develop. When they finally dry out in early fall, they split and abruptly uncurl, flinging their seeds as far as possible from the parent plant.

Many seeds require a period of dormancy or inactivity before they are able to germinate. The plant usually assures that such a seed will not sprout as soon as it ripens by giving it a hard coat or by including a growth-inhibiting hormone in the seed. Some seeds even require a period of freezing temperatures before they will germinate. Seeds are usually very dry and therefore need a great deal of water before they can sprout. By making sure that a seed stays dormant for a certain time, the plant ensures that it will only germinate under the best possible conditions, and not prematurely on a warm fall day. Some seeds can remain dormant for weeks, others months, and still others for years.

Seeds are a major source of food for both animals and human beings. Many species, such as cereal grains (corn, nuts, oats, rice), are a staple for people throughout the world. Many oils are obtained from seeds (sunflower, corn, peanut, and linseed), and are also used to make other products. Two of the most popular seeds consumed today—the cocoa bean and the coffee bean—give us chocolate and coffee.

[See alsoPlant Anatomy; Plant Reproduction; Plants; Reproduction, Sexual ]

seed / sēd/ • n. 1. a flowering plant's unit of reproduction, capable of developing into another such plant. ∎  a quantity of these: grass seed you can grow artichokes from seed. ∎ fig. the cause or latent beginning of a feeling, process, or condition: the conversation sowed a tiny seed of doubt in his mind. ∎ archaic (chiefly in biblical use) a person's offspring or descendants. ∎  a man's semen. ∎  (also seed crystal) a small crystal introduced into a liquid to act as a nucleus for crystallization. ∎  a small container for radioactive material placed in body tissue during radiotherapy.2. any of a number of stronger competitors in a sports tournament who have been assigned a specified position in an ordered list with the aim of ensuring that they do not play each other in the early rounds: he knocked the top seed out of the championships.• v. 1. [tr.] sow (land) with seeds: the shoreline is seeded with a special grass. ∎  sow (a particular kind of seed) on or in the ground. ∎ fig. cause (something) to begin to develop or grow: severance payouts that help seed their new businesses. ∎  place a crystal or crystalline substance in (something) in order to cause crystallization or condensation (esp. in a cloud to produce rain).2. [intr.] (of a plant) produce or drop seeds: mulches encourage many plants to seed freely. ∎  (seed itself) (of a plant) reproduce itself by means of its own seeds: feverfew will seed itself readily.3. [tr.] remove the seeds from (vegetables or fruit): stem and seed the chilies.4. [tr.] give (a competitor) the status of seed in a tournament: Jeff Tarango, seeded five, was defeated by fellow American Todd Witsken. PHRASES: go (or run) to seed (of a plant) cease flowering as the seeds develop. ∎  deteriorate in condition, strength, or efficiency: Mark knows he has allowed himself to go to seed.

seed The structure in angiosperms and gymnosperms that develops from the ovule after fertilization. Occasionally seeds may develop without fertilization taking place (see apomixis). The seed contains the embryo and nutritive tissue, either as endosperm or food stored in the cotyledons. Angiosperm seeds are contained within a fruit that develops from the ovary wall. Gymnosperm seeds lack an enclosing fruit and are thus termed naked. The seed is covered by a protective layer, the testa. During development of the testa the seed dries out and enters a resting phase (dormancy) until conditions are suitable for germination.

Annual plants survive the winter or dry season as seeds. The evolution of the seed habit enabled plants to colonize the land, since seed plants do not depend on water for fertilization (unlike the lower plants).

seed Part of a flowering plant that contains the embryo and food store. It is formed in the ovary by fertilization of the female gamete (see pollen). Food may be stored in a special tissue called the endosperm, or may be concentrated in swollen seed leaves (cotyledons). Seeds are the unit of dispersal of angiosperms and conifers. See also fruit; germination