**3.1 Exogenous dormancy**

*Exogenous dormancy* is caused by conditions outside the embryo and is often broken down into three subgroups [19–22]:

i.**Physical dormancy;** this is caused by impermeability of layers of macrosclereld cells and mucilaginous outer cells to water. The movement of water is restrained by hardened endocarp of the seeds. This happens when seeds are impervious to water or gas exchange. Legumes are a good example of physically inactive seeds since they have a low moisture content and the seed coat prevents them from absorbing water. Water can be absorbed by chipping or splitting the seed coat or other coatings. Impermeability is frequently generated by an exterior cell layer made up of macrosclereid cells or a mucilaginous cell layer. A stiffened endocarp is the third source of seed coat impermeability. During the latter stages of seed development, seed coverings that are impervious to water and gases form.

