Exogenous dormancy is imposed on the seed by factors outside the embryo like the fruit or seed coverings. This may involve a physical, mechanical or chemical factor.
The most common exogenous dormancy is caused by impermeability of the seed coat due to a layer of palisade-like macrosclereid cells.
In order to get these seeds to imbibe water, they must first have this outer layer of cells naturally eroded or treated by the process of scarification.
Hardseededness is not a common form of dormancy. It occurs in only a few plant families.
These families include the Fabaceae, Malvaceae, Cannaceae, Geraniaceae, and Convolvulaceae.
Crop plants within these genera have been bred over many years to not display this type of dormancy.