Germination

Germination is how a seed comes back to life. A mature seed is very dry and almost inactive, sometimes for years. Germination begins when the seed absorbs water, a step called imbibition, and in the strict sense it ends when the radicle, the embryonic root, pushes out through the seed coat; growth of the seedling follows. The word is also used for spores and pollen grains that start to grow.

Water uptake runs in three phases: rapid initial soaking, a plateau, then renewed uptake as the embryo begins to grow. Meanwhile cells repair their DNA and membranes, restart respiration and protein synthesis, and enzymes break down starch, protein and oil stored in the endosperm or cotyledons to feed the embryo. Germination also needs oxygen and a suitable temperature, and many small seeds need light. Dormant seeds will not germinate even in good conditions until something breaks the dormancy, such as a cold spell, fire, a period of dry storage or wear on a hard seed coat. Two hormones act as a switch: abscisic acid maintains dormancy, while gibberellins promote germination.

The emerging shoot of a bean is bent into a protective hook as it pushes through soil, while a grass shoot is sheathed in a coleoptile.