SEED (sed) is the part of a plant that is developed for producing new plants. Specifically, a seed is the ripened ovule and its surrounding parts.
Seeds have a variety of sizes and shapes. The smallest are those of some orchids and begonias. They are the size of fine dust. Petunia seeds are also quite small. The largest seed is that of 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, although the latter also has a large seed Structure.
Although seeds vary greatly, all have three basic parts: the miniature plant (embryo) on the inside, a supply of food (endosperm) for the tiny plant, and a protective coating (seed coat, or testa). The embryo has one or more cotyledons, or seed leaves. (Plants whose seeds have one are monocotyledons, or monocots. Those whose seeds have two are dicotyledons, or dicots. Those having more than two are polycotyledons, or polycots.) Below the cotyledons is the hypocotyl, which is the part that will grow downward to form the root of the plant. Above the cotyledons 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, 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 the tiny plant already developed in it.
Every seed has some 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, they would be too crowded and too shaded to grow they must find good soil and plenty of space and sunshine if they are to develop into strong, healthy plants.
How Seeds Are Scattered
Many seeds are adapted to riding 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. 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 fruits. 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 the catalpa, birch, and 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. Certain plants break loose from the soil in the autumn. The entire plant rolls before the wind, scattering its reeds over the countryside. One such plant is called a tumbleweed.
The fruits of plants growing in or near the water may wear buoyant, waterproof coverings that let them float. Many a tropical island has been planted with coconuts brought to it by the ocean tides (see Coconut Palm).
There are fruits that scatter their seeds by literally exploding. The pod bursts open and forcibly shoots the seeds in all directions. Wood sorrel, jewelweed, or touch-me-not, witch hazel, bergamot, and pansy are familiar examples.
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 the 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 seed to form, the pollen must reach the ovules in the pistil, where the two cells join and grow into the 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.
Carrying the pollen from one flower to another is called pollination. Bees are the commonest carriers of pollen, but flies, humming-birds, moths, butterflies, wasps, water, and wind are also carriers. When man pollinates flowers to improve flower color, size, shape, and other characteristics, the process is called plant breeding.
Commercial Seed Growing
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 which 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. The seeds of water plants are able to float. Some plants, at a touch, shoot their seeds with explosive force. Man scatters the seeds of food plants in fields and gardens.
Botany of the Seed, the origin and growth of seeds in plants are explained in the article on Flower. It tells how pollen lodges on the stigma of a flower, how the pollen tube grows down through the pistil and 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 or 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 the embryo, or young plant. Now the ovule is called a seed. A botanist defines a seed, therefore, 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.
Two Kinds of Seeds Plants whose seeds are protected inside an ovary are called angiosperms (“enclosed seeds”) – Some seeds lie exposed on the surface of a scale. Plants with such seeds are called gymnosperms (“naked seeds”).
The pine tree is an example of a gymnosperm (see Flower). On the surface of each scale of the female cone are two cavities, each containing an ovule. In the 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 then close to protect the ripening ovule, or seed. In late fall or winter the cone dries up, the scales again open, and the seeds Germination.
Some seeds need a rest period before the miniature plant of the embryo is ready to grow. Other seeds must grow within a matter or 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. The beginning growth is called sprouting, or germination. In most seeds a tiny white point pushes through the seed coat. This point (the hypocotyls) turns downward toward the soil and starts to form branches that grow into roots. The seed leaves then begin to grow. They are not like the regular leaves on the plant. Some are round, others are lobed, and a few are oval. The two halves of a bean, for example, are two seed leaves. After a bean is soaked in water for several hours, it can be split open and the miniature plant 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 too much. 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. Man and other 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 the summer. In order that some of the seeds will grow into new plants, nature has various methods that ensure a wide distribution of the seed.
Because seeds have their own food supply and are protected by a coat, they may be carried great distances. Distribution occurs in a variety of ways by air (wind), water, explosion (expulsion), and animals. Seed distribution by air occurs with fine seeds that are light and therefore easily carried by the wind. Larger seeds have special structures that aid the wind. Dandelion and milkweed seeds have fluffy hairs that spread.
Seed distribution by animals takes place 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 about by fastening into the fur of animals or on the clothing of people. 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 has seeds with many little bristles that catch on clothing. The seeds of some plants are sticky. Mistletoe seeds, for example, are contained in 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 clothing and baggage of passengers. Man also spreads 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 the Orient 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.
The stored food in seeds is a tasty, nutritious food for man. Some of the most important foods of the world are seeds. They include wheat, corn, rice, barley, rye, oats, buckwheat, millet, sesame, peanuts, beans, lentils, peas, and coconuts. Other seeds useful as food are nuts, sunflower, melon, mustard, caraway, coriander, poppy, celery, anise, black pepper, and nutmeg.
The stored food of seeds may contain starch, such as rice, wheat, and oats. Other seeds have a great deal 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 Orient sesame and soybeans supply oils for cooking and baking. Coconuts and palm-seed oils are used for soap and industrial purposes. Oils from some seeds are used in paints, varnishes.