Seed is a part of a plant by which it propagates. Specifically, a seed is the ripened ovule with the surrounding parts.
Plant seeds may have different sizes and shapes. The smallest seeds have such flowers as orchid and begonia. They are the size of fine dust. Petunia seeds are also quite small. The largest seed has the double coconut palm of the Seychelles Islands. The fruit of this unusual palm weighs up to 40 pounds and has one or two seeds in it. It is not the same as the common coconut, though the common coconut has a large seed structure too.
Though plant and flower seeds vary greatly, they all have three basic parts: miniature plant (embryo) in the inside, food supply (endosperm) in the middle, and a protective coating (seed coat or testa). The embryo has one or more cotyledons or seed leaves. Plants with one cotyledon are monocotyledons or monocots. Those whose seeds have two are dicotyledons or dicots. Plants that have more than two cotyledons are polycotyledons or polycounts. Below the cotyledons there is the hypocotyl, this is the part that will grow downward to form the root of the plant. Above the cotyledons, there is the epicotyl, which will grow upward to form the stem. The endosperm surrounds the embryo. In some seeds, such as the members of the pea family, the endosperm is absorbed by the cotyledons, causing them to be greatly enlarged.
Some plants such as ferns and mosses do not have seeds. Instead of seeds, they have spores to grow new plants. A spore is just one single cell and has to go through a long growth process before producing a new plant. A seed has already a tiny plant developed in it.
Seeds may have additional parts of the plant around them. Apple seeds are in a thick, juicy covering. Melon seeds are surrounded by sweet, watery flesh. Peas and beans grow in pods that may be thin or thick. Anything that contains the seed is called a fruit.
Fruits and the seeds they contain are formed from a flower or blossom of a plant. A flower has two necessary parts – the pistil and the stamens. The pistil has special cells called the ovules. The stamens form a powdery mass of cells called pollen. For the seed to form the pollen must reach the ovules in the pistil, where the two cells join and grow into an embryo. The pollen grain develops a long tube that grows down into the pistil and carries a sperm nucleus to the egg nucleus of the ovule. The sperm nucleus and egg nucleus unite in a process called fertilization and form the embryo plant within the seed. Pollen may come from the same flower or from a flower on another plant of the same species. Each pollen grain and each ovule contains one half of the genetic material in a normal plant cell. When they are joined, the two parents’ characteristics are present in the resulting seed. Half of the characteristics come from the pollen parent and half from the ovule parent.
How seeds grow and develop
The pollen lodges on the stigma of a flower. Then the pollen tube grows down through the pistil into the ovary and the egg cell, or ovule inside the ovary. When the contents of the pollen tube enter the ovule, the flower is said to be fertilized.
Important changes begin to take place. The ovary or seed case may turn into a fleshy pulpy fruit, into a dry pod, capsule or nut. The wall of the ovule hardens and becomes a protective coat called the testa. Inside the testa is an embryo or young plant. Now the ovule is called a seed. Therefore the seed is defined as “the ripened ovule of a flowering plant.” The seed then goes into a period of rest. It grows again or germinates after it has been planted.
The types of seed plants whose seeds are protected inside an ovary are called angiosperms (enclosed seeds). Some seeds lie exposed on the surface of a scale, such plants are called gymnosperms (naked seeds).
The pine tree is an example of a gymnosperm. On the surface of each scale of the female cone are two cavities and each contains an ovule. In spring the scales spread open to receive the windblown pollen from the male cones on the same tree. When a pollen grain falls between two scales it sends out a pollen tube and fertilizes an ovule. The scales are then close to protect the ripening ovule or seed. In late fall or winter the cone dries up, the scales open again and the seeds germinate.
Some seeds need a rest period before the miniature plant of the embryo is ready to grow. Other seeds must grow within a few days or the tiny plant dies. Some seeds will start to grow even while they are still attached to the parent plant. Other seeds can remain dormant for several years or, as in the case off some lotus seed, for several centuries. Seeds require proper temperature, air, and moisture conditions to grow. Some need light and others need darkness. When all of the conditions are correct, a seed begins to grow. When the growth begins it is called sprouting or germination.
In most seeds, a tiny white point pushes through the seed coat. This point (the hypocotyls) turns down toward the soil and starts to form branches that grow into roots. Then the seed leaves begin to grow. They are not like the regular leaves of the plant. Some are round, others are lobed or oval. For example, the two halves of a bean are two seed leaves. After a bean is soaked in water for several hours, it can be split open and a miniature plant can be seen between the two halves.
Only a small number of seeds grow and develop into plants. Many seeds die from lack of moisture, others die from its overage. Some seeds are eaten by animals. Many birds live on seeds that they gather from plants or on the ground. Squirrels live almost entirely on nuts and other seeds. Many animals use seeds for much of their food supply. Many times seeds sprout but die because they have landed in a place unsuitable for growth. Thousands of maple and elm seeds fall along the gutters of streets or even the rain gutters on roofs. They sprout but die when the leaves and dirt in the gutters dry out in summer.
How seeds are scattered
Every seed has its own way of traveling. Some may travel only a few inches, others may travel many miles. If they all fell to the ground directly beneath the parent plant, they would be too crowded and too shaded to grow. Seeds need good soil and plenty of space and sunshine if they are to develop into strong and healthy plants.
Many flower seeds are adapted to fly in wind currents. Dandelions, milkweeds, cattails, thistles, and asters have seeds with fluffy little parachutes. They drift through the air on the slightest breeze if the air is dry. On damp days the parachute stays closed. Such seeds may travel many miles on their parachutes.
Some seeds are enclosed in dry husks equipped with one or two propeller blades. The maple, ash, and ailanthus trees have such seeds. They twist and turn in the air and may sail a short distance from the parent tree. Other plants have winged seeds. Among them are catalpa, birch, elm trees, and the trumpet creeper. The seeds of the orchids are so fine and light that they blow about like dust.
The long stiff beards of the grains and grasses act like kite tails. Some plants break away from the soil in autumn. The entire plant rolls width the wind, scattering its seeds over the countryside, such a plant is called a tumbleweed.
Some plants that grow in water or near it may have buoyant waterproof coverings that let them float. Thus many tropical islands have been planted with coconuts brought to them by the ocean tides.
There are plants that scatter their seeds by exploding. The pod bursts and forcibly shoots the seeds in all directions. The examples of such plants are wood sorrel, jewelweed, witch hazel, bergamot, and pansy.
Because seeds have their own food supply and are protected by a coat, they may be carried on great distances. Distribution occurs in a variety of ways by air (wind), water, explosion, and animals.
Seed distribution by animals occurs in several ways. Some seeds have stickers or hooks on them, or they are contained in fruits that have stickers or hooks. These seeds are carried by fastening to the animal’s fur or to the people’s clothes. Thus they may be carried many miles from the parent plant. Cocklebur and burdock have hooked fruits that enclose several seeds. The seeds of the beggar-ticks are flat with two hooked barbs on one end. The wild avens have seeds with many little bristles that fastens to the clothes.
Some plants have sticky seeds. For example: the mistletoe seeds are inside the sticky white berries. When birds eat the berries, the seeds cling to their bills and feet and are carried to trees and other places where the birds perch.
Berries and juicy fruits are eaten by birds and animals. Generally, the seeds contained in them do not digest. Instead, they pass through the animals unharmed and are deposited in the animal excrement away from the place where they originally grew. Poison ivy, grapes, mulberries, and many other plants are often found growing under trees and other places where birds roost. Manure from cattle farms spreads seeds of clover, grass, and grain to new areas. Squirrels and chipmunks carry seeds and fruits such as acorns, nuts, and grain which they bury in the ground. Many of these are forgotten and grow into new plants.
Seeds are frequently transported to new places when they are carried into ships, planes, cars, and trucks on the clothes and baggage of passengers. People also spread seeds by throwing away apple cores, peach pits, grape seeds, and plum stones. Garbage and waste material from packing is another mode of transporting seeds.
Fragile material from Europe and East is often packed in straw or hay that contains seeds. Cleanings from grain shipped from distant places also contain some other kinds of seed. Commercial seed, although carefully cleaned, may also contain small amounts of these traveling seeds. Seeds are also carried along in a material, such as mud, that is stuck on the carrier.
Carrying the pollen from one flower to another is called pollination. Bees are the commonest carriers of pollen. Flies, humming-birds, moths, butterflies, wasps, water, and wind are also pollen carriers. When a man pollinates flowers to improve its color, size, shape, and other characteristics, this process is called plant breeding.
Usage of seeds in the world
Seeds are tasty, this is a nutritious food for people. Seeds are the most important foods in the world. They include wheat, corn, rice, barley, rye, oats, buckwheat, millet, sesame, peanuts, beans, lentils, peas, and coconuts. Other important seeds for food are: nuts, sunflower, melon, mustard, caraway, coriander, poppy, celery, anise, black pepper, and nutmeg.
Some seeds may contain a lot of starch, such as rice, wheat, and oats. Other seeds have a great amount of oil, such as sesame, soybeans, peanuts, cottonseed, and various nuts, including coconuts, pecans, walnuts, brazil nuts, and filberts. Common cooking oils come from cottonseed, corn, safflower, and peanuts. In the East sesame and soybeans supply oil for cooking and baking. Coconuts and palm seed oils are used for soap and industrial purposes. Oils from some seeds are used in paints and varnishes.
Commercial seed growing is an important industry. The wholesale value of seeds sold for planting in gardens and farm fields is about 100 million dollars annually. Seeds shipped in interstate commerce must meet certain standards set by the federal Department of Agriculture. Packages must bear labels that give the percentage of seed guaranteed to germinate, the percentage of weed seed present, and other information to protect the buyer. Samples of imported seeds must be tested by the Department before they can be released for sale.