What is seed dormancy ? How many types of dormancy are there?

Seed dormancy or rest is the internal or innate inhibition of germination of otherwise normal or viable seed even when present under most favourable conditions required for its germination. Dormancy is of several types. The important ones 1 are as follows :
(a) Immature embryo : The embryo is not fully developed at the time of seed shedding. The period of seed dormancy corresponds to the period required by the embryo for its complete development, e.g. Ginkgo biloba.
(b) After ripening: After shedding, the seeds of barley, wheat require an interval of ripening before they attain the power of germination. This period is need to produce the necessary growth hormones.
© Impermeability of seed coats : The seed coats are impermeable to water, gases or chemical e.g., Chenopodium, Apple, Xanthium respectively.
(d) Tough seed coats : The seed coats are hard and provide mechanical resistance to the growth of embryo.
(e) Inhibitors : They are chemical substances which do not allow the seeds to germinate. They may be present inside the fruits, embryo, endoperm or seed coat, e.g., Cucurbita, Iris.
(f) Excessive salts : In Atriplex the seeds contain a high
concentration of solutes which do not allow the embryo to resume its growth.