In developed seeds metabolic activities are generally very slow. But at the time of germination, the metabolic activities in seeds increase and they grow into new plants under favourable conditions of growth. This is called seed germination. Seed germination is the return of metabolic activities and growth by the seed tissue to give rise to a new plant by the development of the embryo.
Some seeds do not germinate immediately after dispersal even if suitable conditions of growth are provided. In this period growth of the seeds remains suspended and it is said to be in the rest or dormant stage. This phenomenon is called dormancy of seeds. It may occur due to immature embryo, hard or impermeable seed coat, and presence of inhibitors like abscissic acid.
In flowering plants two types of germination are found. They are:
In epigeal germination, hypocotyl elongates and cotyledons come out above the soil surface. Examples: seeds of pumpkin, mustard, tamarind, and french bean.
In hypogeal germination, the epicotyl elongates and cotyledons remain below the soil surface. Examples: Most monocots seed like rice, wheat, maize, and coconut.
In seed germination, the first step is the imbibition or absorption of water by seed. Then the seed swells and the seed coat ruptures. Through the ruptured seed coat the radicle comes out from one end of embryonic axis. This radicle gives rise to root system. From the other end of embryonic axis the plumule elongates and develops as the shoot of the plant.
Seed germination requires five factors:
The seed must swell up to rupture its seed coat. A ripe seed contains very low quantity of water. So for swelling to cause rupture of seed coats supply of adequate water is essential. Biochemical reactions required for growth and development of the seedling require water.
For germination of seeds a particular temperature is required. The degree of temperature required varies from species to species. Warmth accelerates chemical reactions inside.
Oxygen is required in breaking down reserve food of seed and release energy for metabolism of growth of the embryo.
In most of the seeds light is not an essential factor for germination. But in some cases like lettuce and tobacco light is absolutely essential.
Besides the above external factors, hormones also control germination of seeds.
Gibberellins can induce germination in some cases even in complete darkness. Auxin, Cytokinins and Ethylene can break dormancy in many seeds and initiate germination. In some seeds Abscissic acid inhibits germination process.