A seed is a fertilized, matured ovule consisting of an embryo or miniature undeveloped plant and food reserves enclosed in a protective coating. It is the result of the sexual reproduction in gymnosperm and angiosperm plants.
The seeds of some plants have cotyledons (or seed leaves) which act as the source of stored reserved food for the embryo upon germination.
Definition
Seeds are the mature fertilized ovules of flowering plants, gymnosperms and angiosperms. They contain an embryo or miniature undeveloped plant and food reserves enclosed in a protective seed coat. They are the biological way of reproduction for all flowering plants.
A seed contains a hilum, micropyle and cotyledons. Cotyledons are seed leaves that act as source of stored reserve food materials for the embryo during germination. They can be present on one end (monocotyledonous seeds) or both ends of the embryonal axis (dicotyledonous seeds). They are also known as aleurone layer.
A seed coating protects the cotyledons and embryo from physical, mechanical and temperature-related damage. It is a thick and leathery tissue. In some seeds such as beans, gram and peas, the endosperm is removed and they are known as non-endospermous seeds.
Origin
Seeds are the fruit of sexual reproduction and contain DNA from both male and female parents. They contain an embryo and a reserve of food protected by a skin called the seed coat. The embryo is a rudimentary plant from which a new one can grow, and the nutrient supply comes from tissue within the seed called endosperm.
During fertilization, male pollen grains germinate on the stigma and develop into long, slender tubes. These tubes travel down through tissue in the style, to the ovule, where they unites with the two polar nuclei of the egg.
One of the sperm nuclei fuses with the egg, forming a zygote. The other sperm cell is absorbed or aborted. The result is a seed with two parent plants’ DNA. The evolution of seeds is thought to have greatly contributed to the rapid spread and dominance of angiosperms over terrestrial flora.
Characteristics
Seed plants (spermatophytes) are the most common plants grown in gardens and on farms. They evolved to produce seeds that can wait for the right conditions to sprout into new plants. These characteristics help them dominate biological niches on land, including forests, grasslands and hot and cold climates.
All seed plants have a food reserve in their seeds, called endosperm, to fuel the growth of the embryo and the new plant. This nutritive tissue is surrounded by the protective seed coat.
Some seeds have physical dormancy barriers that must be broken by soaking or scarifying (scratching the seed surface). Others have internal chemical conditions, such as abscisic acid or gibberellin, that prevent germination until those factors are removed. These chemicals can be broken down by cold/moist stratification or leaching.
Uses
Seeds are essential for the reproduction of many plants, including grasses, trees and a wide variety of shrubs and flowers. They are also a vital source of food for animals and humans.
Depending on the type of plant, seeds come in different shapes and sizes and are covered by a protective coat or husk. Inside the seed is an embryo and a supply of nutrients. The embryo is made of cotyledons and radicle, and the nutrients are stored in a region called the endosperm.
Some seeds are sown to grow new plants, while others are used for medicinal purposes or as a food source. The neem tree, for example, produces a compound that has insect-repelling properties. This and other seed compounds are being studied for their potential to be incorporated into pharmaceuticals.
Sources
Tree seeds can be collected from farmland or natural forest, or sourced through vegetative propagation (taking cuttings or using grafting). ICRAF has developed a toolkit for managing seed sources and collecting tree seeds.
Seeds form following sexual reproduction in flowering plants and are the means of dispersal of plant species. They have the potential to travel very long distances—whether in the wind, by water currents or clinging to animals (e.g. birds) that carry them.
Seeds typically contain an embryo, food reserves and protective outer coverings. The embryos of dicot seed plants have two cotyledons while monocots have one. Local heirloom seed growers and groups are an excellent source of a wide range of varieties, and often sell them for an affordable price. They are also available at farmers markets and through online searches.
