[10] There have been a number of classification schemes developed to group different dormant seeds, but none have gained universal usage.
Dormancy occurs because of a wide range of reasons that often overlap, producing conditions in which definitive categorization is not clear.
[11] Many garden plants have seeds that will germinate readily as soon as they have water and are warm enough, though their wild ancestors had dormancy.
Physical dormancy is the result of impermeable layer(s) that develops during maturation and drying of the seed or fruit.
[14] Physical dormancy has been identified in the seeds of plants across 16 angiosperm families including: Physical dormancy has been recorded in a few species of Cycadales with thick sclerotesta and slow embryo development, for example Cycas revoluta and Zamia floridana, but not in any of the other groups of extant gymnosperms.
[18][19] Generally, physical dormancy is the result of one or more palisade layers in the fruit or seed coat.
[14] Specialised structures, which function as a "water-gap", are associated with the impermeable layers of the seed to prevent the uptake of water.
[16] Breaking physical dormancy involves the disruption of these specialised structures within the seed, and acts as an environmental signal detector for germination.
[clarification needed] Other water-gap structures include carpellary micropyle, bixoid chalazal plug, imbibition lid and the suberised "stopper".
[12] For example, the high and fluctuating temperatures during the dry season in northern Australia promote dormancy break in impermeable seeds of Stylosanthes humilis and S.hamata (Fabaceae).
Indigenous people recognizing that the weight of Abrus precatorious seeds remains stable have used them as a weighing unit (Ratti).
[24] Mechanical dormancy when seed coats or other coverings are too hard to allow the embryo to expand during germination.
Secondary dormancy occurs in some non-dormant and post dormant seeds that are exposed to conditions that are not favorable for germination, like high temperatures.
The mechanisms of secondary dormancy are not yet fully understood but might involve the loss of sensitivity in receptors in the plasma membrane.