With seed catalogs arriving, gardeners are planning their vegetable and flower gardens. This is an excellent opportunity to learn about seeds.
Seeds contain embryos and stored nutrition (endosperm). They also have a seed coat that provides protection. They are the dominant method of reproduction for plants that have evolved to exploit it.
Seeds are mature fertilized ovules of flowering plants and gymnosperms that contain the embryo and food it will need for germination and growth. They are usually enclosed in a protective outer covering called a seed coat or testa. Seeds may be stored inside the ovary of the plant in the case of angiosperms, or, as in orchids and some other plants with minute seeds, they may be free-standing. In either case, they are surrounded by a reserve material, called endosperm or perisperm, which functions as food for the embryo.
A seed is also a term for a person, idea, or thought that can grow and become more important. For example, a tennis player might be seeded number one in the world based on his or her record and ability to win tournaments. The word can also refer to a place or event where people are ranked according to their abilities, such as the playoffs in sports.
Scientists studying the genetics of ovule and seed development (particularly those genes involved in the development of the seed coat) are beginning to understand one of the evolutionary factors that enabled gymnosperms to develop seeds.
Before seed plants evolved, most land plants like ferns and liverworts reproduced by sending out spores that landed in moist soil to develop into whole new plants. These spores were not as durable or well-adapted for long-distance dispersal, however, so they could not become dominant in the biological niches occupied by seed plants today.
The evolution of seeds resulted in a more robust way for plants to reproduce, which allowed them to develop the more diverse and complex plant lineages that now dominate today’s terrestrial habitats. The first seed-bearing plants emerged at least 365 million years ago during the late Devonian Period. Seed plants are the most diversified group of vascular plant species, and they occur in a wide range of landscapes and climates.
Seeds serve several important functions: They protect and nourish the embryo or baby plant, they disperse plants to new locations, and they provide a period of dormancy during unfavorable conditions.
A seed contains an embryonic plant, a food reserve called the endosperm, and a protective covering known as a seed coat. The endosperm is bulky and stores food for the developing plant; it is separated from the embryo by a layer of protein called the aleurone layer. Some seeds are non-endospermic; these include grasses, millets, palms and Brazil nuts.
The seed’s ability to grow depends on many factors, including the length of its physiological dormancy, its response to a period of moist chilling (or stratification), and internal biological processes that control DNA damage and transcription during germination. Seeds with a shorter physiological dormancy are more likely to germinate and produce mature plants. They also tend to grow more quickly than sporlings from the same parent, owing to larger food reserves in their endosperm and cotyledons.
Seeds are an important means of reproduction for flowering plants. They contain an embryo or miniature undeveloped plant and food reserves enclosed within a protective covering. Some seeds are dispersed by animals (birds, rodents, mammals, reptiles, fish) that eat and carry them away, or by water currents. Other seeds are dispersed by abrasion or by wind, snow, or rain.
Seeds may also have medicinal properties. Chamomile seeds, for example, are used to make a tea with calming effects. Other seeds are pressed for oil production. Many seeds are used as fertilizers for agriculture. Others are used in laboratories to produce genetically modified plants that are resistant to pests or disease. Seeds can be stratified to break down physiological dormancy; this process, called moist chilling, involves adding moisture and exposing the seeds to fluctuating temperatures. The resulting spores are more likely to germinate. This method is often more successful than soaking and freezing seeds. A fungus in the soil can destroy seeds before they emerge from the ground, so seeds are usually stored in an insect-free environment.