Germination is the resumption of growth of the embryo plant inside the seed.
- proper temperature
- the presence of adequate water
Water is always needed to allow vigorous metabolism to begin. It is also sometimes needed to leach away a germination inhibitor within the seed. This is especially common among desert annuals. The inhibitor is often abscisic acid (ABA).
- a preceding period of dormancy (often).
The seeds of many temperate-climate angiosperms will germinate only after a prolonged period of cold. An inhibitor within the seed (probably abscisic acid - ABA) is gradually broken down at low temperatures until finally there is not enough to prevent germination when other conditions become favorable. This mechanism is of obvious survival value in preventing seeds from germinating during an unseasonably warm spell in the autumn or winter.
- Correct photoperiod (often).
Germination in a Dicot
Fig. 18.104.22.168 Germination of a bean seed
- The primary root emerges through the seed coats while the seed is still buried in the soil.
- The hypocotyl ("below the cotyledons") emerges from the seed coats and pushes its way up through the soil. It is bent in a hairpin shape - the hypocotyl arch- as it grows up. The two cotyledons protect the plumule- the epicotyl ("above the cotyledons") and first leaves - from mechanical damage.
- Once the hypocotyl arch emerges from the soil, it straightens out. This response is triggered by light. Both red light, absorbed by phytochrome and blue light, absorbed by cryptochrome can do the job.
- The cotyledons spread apart exposing the epicotyl with the apical meristem at its tip, and two primary leaves
- In many dicots, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
The above image (courtesy of the Pittsburgh Plate Glass Co.) is a time-lapse photograph showing three stages in the germination of a bean seed.
Germination in Monocots
Fig. 22.214.171.124 Coleoptile of an oat seed
When grass seeds like corn (maize) or oats (shown here) germinate,
- The primary root pierces the seed (and fruit) coverings and grows down.
- The primary leaf of the plant grows up. It is protected as it pushes up through the soil by the coleoptile - a hollow, cylindrical structure.
- Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it.
The coleoptile of grass (e.g., oat) seedlings has been a favorite experimental object for studing phototropism.