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True dormancy or inherent (or innate) dormancy is caused by conditions within the seed that prevent germination even if the conditions are favorable. [7] Imposed dormancy is caused by the external conditions that remain unsuitable for germination [8] Seed dormancy can be divided into two major categories based on what part of the seed produces dormancy: exogenous and endogenous. [9]
Many plants and seeds recognize this and enter a dormant period in the fall to stop growing. The grain is a popular example in this aspect, where they would die above ground during the winter, so dormancy is favorable to its seedlings but extensive domestication and crossbreeding has removed most dormancy mechanisms that their ancestors had. [6]
The seedlings of some flowering plants have no cotyledons at all. These are said to be acotyledons. The plumule is the part of a seed embryo that develops into the shoot bearing the first true leaves of a plant. In most seeds, for example the sunflower, the plumule is a small conical structure without any leaf structure. Growth of the plumule ...
Some seeds germinate when the soil is cool 28–40 F (-2 - 4 C), and some when the soil is warm 76-90 F (24–32 C). Some seeds require exposure to cold temperatures (vernalization) to break dormancy. Some seeds in a dormant state will not germinate even if conditions are favorable.
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Growing degree days (GDD), also called growing degree units (GDUs), are a heuristic tool in phenology. GDD are a measure of heat accumulation used by horticulturists , gardeners , and farmers to predict plant and animal development rates such as the date that a flower will bloom, an insect will emerge from dormancy, or a crop will reach maturity.
The seeds of many Australian plants and plants from southern Africa and the American west require smoke or fire to germinate. Some plant species, including many trees, do not produce seeds until they reach maturity, which may take many years. Seeds can be difficult to acquire, and some plants do not produce seed at all.
However, smaller seeds can be produced in larger quantities which has the potential to produce more offspring and have better chances of some of the seeds dispersing into suitable habitat. [3] This seed size-number trade off [10] has led to the evolution of a wide range in size and number of seeds in response to environmental selection pressures.