The seeds of most wild and cultivated plants of temperate climates will not germinate immediately after ripening. At maturity they enter a state of dormancy that varies in length, depending upon the species, from a few weeks or months to several years or even centuries.
Botanists recognize two categories of dormancy: exogenous and endogenous.
Exogenous Seed Dormancy
Exogenous (external) dormancy is caused by forces acting on the seed from the outside. Exogenous dormancy is caused by one or more external physical factors:
Physical dormancy occurs when the seed coat is impermeable to water and/or gases. Under natural conditions, the permeability of the seed coat is gradually increased by freezing, thawing, and the action of microorganisms within the soil until water and oxygen can finally penetrate the seed coat and awake the embryo within.
Some seeds must be scarified (scraped or scratched) before they will germinate. Tiny impermeable seeds are often blown over abrasive surfaces that scratch (scarify) the seed coat thereby permitting water absorption and germination.
Mechanical dormancy occurs when the seed coat is too thick and rigid to allow the germinating embryo to expand. Mechanical dormancy can be overcome by the acid in animal guts, microbial action in warm, moist environments, and fire.
Chemical dormancy is the result of the presence of chemical inhibitors in the walls of fruits or in the seed coat. These inhibitors often break down upon the drying of fruits or seeds. They may also be washed away if the fruit or seeds are covered by water.
Endogenous Seed Dormancy
Endogenous (internal) dormancy is caused by conditions within the embryo itself, and is also often broken down into three categories: physiological dormancy, morphological dormancy and combined dormancy,
Physical dormancy prevents embryo growth and seed germination until certain chemical changes occur. Physical dormancy make be broken in a number of ways depending on the species.
- Drying may excess moisture from the embryo and trigger germination.
- The presence or absence of light may trigger germination.
- Warm or cool temperatures may trigger germination.
Morphological dormancy is a situation in which the embryo is not mature when the seed is released. Upon the maturation of the embryo, the seed can undergo germination.
Combined dormancy is found in some seeds that have both physiological and morphological inhibitors delaying germination.
Dormancy is Not Universal and Varies in Length
Dormancy in seeds is common but not universal. The seeds of some plants, especially those that mature early in the growing season, will germinate as soon as they have attained full size or when they drop from the parent plant.
The most extreme case of absence of dormancy is found in the seeds of the red mangrove, a low tropical or subtropical tree growing on seacoasts or along tidal rivers. The fruit contains a single seed, in which the embryo begins to grow while the fruit and seed are still attached to the parent tree.
The seedling remains attached to the parent tree for some months, but finally detaches and falls into the mud or shallow water beneath. Many seedlings fall vertically downward in such a way that they stick into the mud upright, literally planting themselves.
Seeds vary greatly in the length of time during which they will retain their viability and ability to germinate. The seeds of some species are viable for only a few days or month; those of others for years, decades, or even centuries.
Seeds of a tropical tree legume germinated after 158 years in dry storage. Seeds of a lotus, a member of the water lily family, found in the soil of a former lake bed in southern Manchuria, germinated in spite of being about 1,000 years old. Magnolia seeds removed from Japanese archeological digs and dated at over 2,000 years old not only germinated but in their 10th year of life, set their first flower buds.
However, the champion survivors must be the grass seeds found frozen in the Alaskan permafrost. When thawed, they germinated in spite of the fact that carbon dating estimated them to be about 10,000 years old.
Thus it is clear that plant seeds not only travel geographically, but that they also travel temporally through time as a result of dormancy.
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