Understanding Seeds: Types, Development, and Importance

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Introduction to Seed

Seed, a reproductive structure produced by plants as a means of giving rise to new plants similar to the parents. Not all plants produce seedsonly those that bear flowers or cones. And even some of these plants can reproduce by other means, such as by bulbs and runners.

Seeds vary greatly in size, shape, and color. Orchid seeds are scarcely larger than specks of dust, while a coconut seed can weigh as much as 50 pounds (23 kg). The size of a seed does not indicate the size of the plant that will grow from it. For example, a redwood that is 300 feet (90 m) tall grows from a seed that is only 1/6 of an inch (4 mm) long, but a lima bean seed that is one inch (2.5 cm) long produces a plant only two feet (60 cm) tall.

Some seeds are round, some are flat, and some are irregularly shaped. Some have a ridged surface; others are smooth. Seed color varies from species to species, as well as among plants of the same species. Some seeds have a characteristic shading or patterning. It is often possible to identify the species of plant from which the seed comes by the external appearance of the seed.

Economic Importance

Various seeds are rich in fats, proteins, or carbohydrates (chiefly starch and sugar). The grains of wheat, rice, corn, rye, and barley are important sources of food. (Botanically, a grain, also called a kernel, is a one-seeded fruit consisting almost entirely of the seed.) Coffee beans and cacao beans are seeds used to make beverages and flavorings. Cardamom, nutmeg, and mustard seed are used as spices in cooking. Many nuts are rich in calcium and iron. (A nut is a fruit consisting of a seed covered with a hard shell.) Some seeds, however, are harmful to humans and may be fatal if eaten in sufficient quantity. For example, the seeds of apples and cherries contain a poisonous substance containing cyanide.

Rye grain is an important source of food.

Many useful oils are obtained from seeds. The seed of the flax plant yields an oil used in making paints, varnishes, soft soap, and printer's ink. Soybean oil, extracted from the seed and then refined, is used in many different food products, chiefly margarines and shortenings. Corn kernels furnish an oil used in cooking. Cellulose fibers on the surface of the cotton seed are used in making cotton cloth.

Seeds are raised commercially for use by farmers in planting crops and by gardeners in planting fruits, vegetables, flowers, and lawns. In the United States, seeds marketed in interstate commerce must meet certain requirements set by the U.S. Department of Agriculture. Each package of seeds must be labeled with the percentage guaranteed to germinate, (sprout) and the amount of weed seeds and other impurities.

The federal government and the large seed companies have extensive research facilities where new plant varieties are developed, and existing ones are improved. Such research often results in the development of seeds that give rise to crop plants better able to resist such conditions as drought and disease. To help them grow and develop these resistant crops, researchers often obtain seeds from plant gene banks, facilities throughout the world where seeds of almost every plant species are stored.

Structure

A seed consists of three parts: the embryo, the food-storage tissue, and one or two protective coverings.

Embryo

The embryo, the young plant in the early period of development, consists of one or more cotyledons, the epicotyl, and the hypocotyl. The cotyledon, also called the seed leaf, is a modified leaflike structure that digests and stores food. The epicotyl gives rise to the plumule, which becomes the stem and leaves of the mature plant. The hypocotyl gives rise to the radicle, or root tip, from which the roots of the mature plant develop.

Botanists classify angiosperms (flowering plants) according to the number of cotyledons present. If only one cotyledon appears in the embryo, as in corn, the plant is called a monocotyledon, or monocot. If two cotyledons are present, as in beans, the plant is called a dicotyledon, or dicot. Monocots and dicots differ in structure. For example, monocots usually have parallel veins in their leaves; dicots usually have branching veins. Gymnosperms (nonflowering plants that typically bear cones), such as pines and spruces, usually have several cotyledons. They are not classified according to the number of cotyledons because the seeds of all gymnosperms develop into structurally similar plants, regardless of the number of cotyledons present.

Food-storage Tissue

In plants with one cotyledon and in gymnosperms, the food-storage tissue is a specialized structure, the endosperm. In plants with two cotyledons, the cotyledons absorb the endosperm before the seed germinates. The cotyledons then serve as the food-storage tissue.

Protective Covering

In gymnosperms and a few angiosperms, the protective covering consists of only the seed coat (or testa), which protects the embryo from excessive moisture, rapid changes in temperature, and injury. In most angiosperms, a second protective covering, the tegmen, forms on the inside of the seed coat.

Development

In angiosperms, sperm carried in pollen grains fertilizes the egg contained in the part of the flower called an ovule. (Ovules are enclosed in the part of the flower called an ovary.) After fertilization, the ovule develops into a seed and the ovary grows and matures. The mature ovary is commonly known as a fruit.

The seeds of gymnosperms also develop when sperm fertilize the eggs contained in the ovules. These ovules, however, are not inside ovaries; instead, they are typically borne on the scales of a cone. Because the seeds of gymnosperms are not enclosed in an ovary, these seeds are sometimes called naked seeds.

Dormancy and Germination

A seed remains in a dormant, or resting, stage before the plant embryo begins to develop into a seedling, a young plant. This dormancy may last only a few days, or it may last several years, depending on the species of plant. During this time, the embryo uses little or no oxygen, and its life processes are considerably reduced. Some seeds remain dormant until environmental conditions enable them to resume growth. Other seeds remain dormant because the embryo has not developed fully enough to germinate; the resting stage allows the embryo to mature completely. Seeds hundreds of years old are occasionally found that will germinate when provided with proper conditions, such as heat and moisture.

Seed germination is the resuming of growth of the plant embryo. Many factors, such as water, oxygen, and light, influence the time at which the seed will germinate. A specific temperature range and a suitable soil acidity are also necessary. Auxins and other plant hormones must be produced inside the seed to stimulate growth. Finally, food must be available for the embryo to grow and develop.

If the foregoing conditions are present, the seed will absorb water and begin digesting available food for the energy it needs to resume growth. The embryo grows through a continuous process of cell division and cell enlargement. Soon the seed coat ruptures because of the absorption of water and the pressure of the expanding embryo, and the seedling breaks through the ground. As soon as the seedling is able to make its own food by photosynthesis, germination is complete. The seedling continues to grow into the mature plant. A mature plant is capable of reproductionthat is, of producing a new generation of seeds.

Dispersal

Seeds are scattered widely by wind, water, and animals. Tumbleweeds break off near the ground in the autumn when their seeds are ripe. Their seeds are then scattered when the wind tumbles the plant along the ground. Some seeds are buoyant and are carried by water currents from place to place. Some weeds are covered with hooks, spines, or barbs that enable them to adhere to clothing and animal fur and feathers. Birds and other animals eat many kinds of fleshy fruits; in many cases the seeds pass through their digestive tracts unchanged and are deposited far from where the fruit was consumed. Squirrels often bury nuts some distance away from the tree that bore them. Many of the nuts are never found by the squirrels, and the seeds they contain may eventually develop into new plants.