|B & T World Seeds|
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The germination of seeds is dependent on both internal and external conditions. The most important external factors include: temperature, water, oxygen and sometimes light or darkness. Different plants have seeds with distinctive requirements for successful germination. Germination requirements often depend on the individual seed variety and are closely linked to the ecological conditions of the plant's natural habitat. For some seeds, future germination responses are affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.
Stratification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian, African, and some Mediterranean and American native plants), or soaking in a body of water for a long period of time. Others have to be passed through an animal's digestive tract to weaken the seed coat and enable germination.
Many live seeds have dormancy, meaning they will not germinate even if the environment has sufficient water and warmth for the seed to germinate. Dormancy factors include conditions affecting many different parts of the seed, from the embryo to the seed coat. Dormancy is broken or ended by a number of different conditions, and is caused by internal or external and sometimes both factors. Environmental factors like light, temperature, fire, ingestion by animals, are conditions that can end seed dormancy. Internally seeds may be dormant because of plant hormones, which affect cell growth and prevent germination. One plant hormone that is a common dormancy inducing chemical is absciscic acid, while the production and application of the hormone gibberellin can break dormancy and induce seed germination.
In some definitions, the appearance of the radicle marks the end of germination and the beginning of "establishment", a period that ends when the seedling has exhausted the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress. The germination index can be used as an indicator of phytotoxicity in soils. The mortality between dispersal of seeds and completion of establishment can be so high, that many species survive only by producing huge numbers of seeds.
In agriculture and gardening, germination rate is the number of seeds of a particular plant species, variety or particular seedlot that are likely to germinate. This is usually expressed as a percentage, e.g. an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions. Germination rate is useful in calculating seed requirements for a given area or desired number of plants.
The part of the plant that emerges from the seed first is the embryonic root, termed radicle or primary root. This allows the seedling to become anchored in the ground and start absorbing water. After the root absorbs water, the embryonic shoot emerges from the seed. The shoot comprises three main parts: the cotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs between plant groups.
In epigeous (or epigeal) germination, the hypocotyl elongates and forms a hook, pulling rather than pushing the cotyledons and apical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air. Beans, tamarind, and papaya are examples of plant that germinate this way.
Another way of germination is hypogeous (or hypogeal) where the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose. Peas, and Wheat, are among many hypogeal plants. Germination also starts with one tiny seedling and then begins to sprout.
In monocot seeds, the embryo's radicle and cotyledon are covered by a coleorhiza and coleoptile, respectively. The coleorhiza is the first part to grow out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. There, it stops elongating and the first leaves emerge through an opening as it is.