Germination is the growth of seeds into seedlings. It starts when seeds become active below ground, and ends when the first foliage leaves appear above ground. A seed consists of an embryo and
its food supply, surrounded by a testa (seed coat). The embryo is made up of one or two cotyledons (seed leaves) attached to a central axis. The upper part of the axis consists of an epicotyl, which has a
plumule (embryonic shoot) at its tip. The lower part of the axis consists of a hypocotyl and a radicle (embryonic root). After dispersal from the parent plant, the seeds dehydrate and enter a period of dormancy. Following this dormant period, germination begins, provided that the seeds have enough water, oxygen, warmth, and, in some cases, light. In the first stages of germination, the seed takes in water; the embryo starts to use its food supply; and the radicle swells, breaks through the testa, and grows downward. Germination then proceeds in one of two ways, depending on the type of seed. In epigeal germination, the hypocotyl lengthens, pulling the plumule and its protective cotyledons out of the soil. In hypogeal germination, the cotyledons remain below ground and the epicotyl lengthens, pushing the plumule upward.
