Germination is the process by which a seed begins its development into a mature plant. Germination begins with an increase of metabolic activity within the seed. The first visible sign of germination in angiosperms (flowering plants) is generally an enlargement of the seed, due to intake of water from the environment. The seed's covering may wrinkle and crack at this time. Soon afterward, the embryonic root (called the radicle) emerges from the seed and begins to grow down into the soil. At about this time the shoot (plumule) also emerges, and grows upward out of the soil.
In most species, the food reserves that provide fuel for the seed's development are contained in the fleshy part of the seed. In some seeds, this fleshy part is divided into two seed leaves, or cotyledons. Seeds having two seed leaves are said to be dicotyledonous; those having only one are monocotyledonous. In some plants, the growth of the shoot carries the cotyledons above the soil into the sunlight, where they become more leaf-like in appearance while continuing to provide sustenance for the growing plant. Germination that follows this pattern is called epigeal germination. In other species the cotyledons remain underground; this is known as hypogeal germination.
Germination requires the presence of suitable environmental conditions, including sufficient water, oxygen, and an appropriate temperature. However, in many species the onset of germination is preceded by a period of metabolic inactivity, known as dormancy. While dormant, seeds will not germinate even under favorable conditions, but eventually they break their dormancy and begin to develop. The processes in the seed by which dormancy is broken are known as after-ripening. Dormancy serves to give seeds a better chance of surviving unfavorable conditions and developing successfully into plants. For example, seeds produced and dispersed just before the beginning of a cold season might not survive if they germinated at once. Dormancy enables them to wait out the cold season, and to begin growth when conditions are more favorable for the mature plant, in the springtime. Typical dormancy periods of seeds vary widely from species to species (and even within the seeds of a given species), as do the mechanisms by which dormancy is broken.